Phy Notes Manhatten 3

8/14/2019 Phy Notes Manhatten 3

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Manhattan Press (H.K.) Ltd. 2001

Chapter 18Chapter 18

ElectrostaticsElectrostatics


ElectrostaticsElectrostatics 18.118.1 Electric ChargesElectric Charges

18.218.2 Different Charging MethodsDifferent Charging Methods

18.318.3 Relation between ElectricRelation between Electric

Current and Electric ChargesCurrent and Electric Charges

18.418.4 Electric FieldsElectric Fields

18.518.5 Electrostatic Hazards andElectrostatic Hazards and

ApplicationsApplications


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Section 18.1Section 18.1

Electric ChargesElectric Charges


Electric ChargesElectric Charges

Charges in atomCharges in atom

Conservation of chargesConservation of charges


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??

18.1 Electric charges (SB p. 2)

Electrostatics Whycanthecombattract

thehairsaftercombing?


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Charges charges in atom

Atoms consist of nucleus and electrons

-

-

-

-

-

+ proton

neutron

electron

++

++

electrons

nucleus

Charges in atom18.1 Electric charges (SB p. 3)


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Nucleus consists of protons and

neutrons

neutron

proton++

++

-

-

-

-

-

+ proton

neutron

electron

nucleus



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proton carriespositive charge

neutron has nocharge

electron carries

negative charge

and circulatesaround nucleus--

-

-

-

+ proton

neutronelectron

++

++

nucleus

Protons, neutrons and electrons



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Electrical neutrality

Apositivechargeand

anegativecharge

canceleachother

whentheymeet.The

atomissaidtobeelectrically

neutralNormally,anatom

hasanequal

numberofelectronsandprotons(i.e.

carriesnonetcharge)



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Neutralization

-

-

-

-++

++

proton 4 1.6 1019 Celectron 4 1.6 1019Cnet charge 0

charges

neutral



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-

-

-++

++

Positive ions

-

lost electron

-

proton 4 1.6 1019 Celectron 3 1.6 1019C

net charge 1.6 1019 C

charges

carriespositive charge



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Negative ions

-

-

-

-++

++

-

-

gain electron

proton 4 1.6 1019 Celectron 5 1.6 1019C

net charge 1.6 1019 C

charges

carries

negative charge



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When a body loses a certain amount ofcharge, another body will gain the same

amount of charge at the same time

--

-

-++ ++

-

--

-

-++ ++

-

transfer

of electron

Conservation of charges

Conservation of charges18.1 Electric charges (SB p. 4)


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Different Charging MethodsDifferent Charging Methods


Different Charging MethodsDifferent Charging Methods

Charging insulatorsCharging insulators

Charging conductorsCharging conductors


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Conductors and insulators

Conductors Insulators

silver

copper

aluminium

iron

earthhuman

bodies

water

wood

18.2 Different charging methods (SB p. 4)


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Charging

electrically neutral

charging

positively charged negatively charged

or

+++

++

++

+++

--

--

--

--

-

The process of converting an electricallyneutral object to a charged object

Charging insulators18.2 Different charging methods (SB p. 4)


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Discharging

electrically neutral

discharging

positively charged negatively charged

or

++

++

++

+

+++

--

--

--

-

-

-

The process of converting a charged object toan electrically neutral object



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Charging by friction - negatively

charged

polythene rod

Rubbing a polythene rod

with a neutral dry cloth

Electrons are transferred

from the cloth to the rod



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Charging by friction - positively charged

acetate rod

Rubbing an acetate rod

with a neutral dry cloth

Electrons are transferred from

the acetate rod to the cloth



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Experiment 18AExperiment 18A

Charging by friction

Intro. VCD Expt. VCD

http://c/Documents%20and%20Settings/09088427A/Local%20Settings/Temp/Temporary%20Directory%2021%20for%20manhattan%20PHY.zip/Video/18a.avihttp://c/Documents%20and%20Settings/09088427A/Local%20Settings/Temp/Temporary%20Directory%2021%20for%20manhattan%20PHY.zip/Video/18head.avi


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Experiment 18A

Like charges repel

rubbed by

fingers

two strips carry like charges

and repel each other

two strips carry like charges

and repel each other


Ch i i l18 2 Diff h i h d (SB 6)


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rubbed with a

piece of

woollen cloth

two balloons carry like

charges and repel each other

two balloons carry like

charges and repel each other


Experiment 18A

Like charges repel

Ch i i l t18 2 Diff t h i th d (SB 6)


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Experiment 18A

Unlike charges attract

two balloons

are rubbed

against each

other

two balloons carry unlike

charges and attract each other

two balloons carry unlike

charges and attract each other




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Class Practice 1Class Practice 1

If a polythene strip and an acetate strip are rubbed

with a dry cloth, explain what happens when the

strips are brought close together.The strips ____________ each other because the

polythene strip and the acetate strip are charged

____________ and _______________ respectively.Ans

wer

positivelynegatively

attract




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- - - - - -

+ + + + + +

induced

charges

neutralpaper scrap

Why can a charged object

attract a neutral object ?

- - - - - -

+ + + + + +

+ + + + + + + + + +

a positively charged ruler

paper scrap is

attracted upwards




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What will happen to the water flowing from a tap

if a charged rod is brought close to it?

The water ___________________ (will beattracted / will be repelled / will not be affected)

by the charged rod. It is because

________________________________________

______________________

will be attracted

water molecules become polarized. The

attractive force between the rod and water

molecules is stronger than the repulsive force

between them. Ans

wer


Ch i d t18 2 Diff t h i th d (SB 8)


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Experiment 18BExperiment 18B Charging by EHT power supply

Expt. VCD

Charging conductors18.2 Different charging methods (SB p. 8)

E.H.T. power

supply

metal strips

insulating rods

Charging conductors18 2 Different charging methods (SB p 9)
http://c/Documents%20and%20Settings/09088427A/Local%20Settings/Temp/Temporary%20Directory%2021%20for%20manhattan%20PHY.zip/Video/18b.avi


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_ +Unlike charges

+

+

+

+

+

+

+

+-

-

--

-

-

-

-

two strips attracttwo strips attract




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Like charges_

-

-

--

-

-

--

-

-

-

-

-

-

--

two strips repeltwo strips repel

_




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Experiment 18CExperiment 18C Charging by sharing

Expt. VCD


foam board

Van de Graaff

generator

http://c/Documents%20and%20Settings/09088427A/Local%20Settings/Temp/Temporary%20Directory%2021%20for%20manhattan%20PHY.zip/Video/18c.avi


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Experiment 18C

Van de Graaff

generator

The hairs stand

on their ends!

-

-

--

-- - -

-

-

-

-------

--

- ---

---

--

---

-

--




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A metal sphere is charged by sharing of

charges




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Sharing of charges between two spheres of

different sizes

conductor less charges more charges




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__

_

_

_

__

_

_

_

the sphere

acquires anegative net

charge

Charging by induction


electrons flowelectrons flow

positivelycharged

metal rod

metal sphere

insulated stand



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EarthingAll charges ofthe conductor

willmovetotheearth



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Section 18.3Section 18.3Section 18.3Section 18.3

Relation between ElectricRelation between ElectricCurrent and Electric ChargesCurrent and Electric Charges

Relation between ElectricRelation between ElectricCurrent and Electric ChargesCurrent and Electric Charges

18 3 Relation between electric current and electric charges (SB p 14)


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Experiment 18DExperiment 18DElectric current and electrostatic charges

Expt. VCD

18.3 Relation between electric current and electric charges (SB p. 14)

light beam

galvanometer

earthsocket

18 3 Relation between electric current and electric charges (SB p 14)
http://c/Documents%20and%20Settings/09088427A/Local%20Settings/Temp/Temporary%20Directory%2021%20for%20manhattan%20PHY.zip/Video/18d.avi


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Electric current

Electronsremain

stationarywhenthe

re

isnopathforthemt

o

flow.Theyarecalled

electrostaticscharg

es

Theflowof

electronsiscalledelectriccurrent

18.3 Relation between electric current and electric charges (SB p. 14)


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Electric FieldsElectric FieldsElectric FieldsElectric Fields

18.4 Electric fields (SB p. 15)


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ExperimentExperiment18E18EDifferent electric field patterns

Expt. VCD


EHT power supply

+ electrodeelectrode

point

electrodecastor oil

semolina

http://c/Documents%20and%20Settings/09088427A/Local%20Settings/Temp/Temporary%20Directory%2021%20for%20manhattan%20PHY.zip/Video/18e.avi


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Electric field lines

Anelectricfieldisa

regioninwhichan

electriccharge

experiencesa

force

Thepatternsformedbysemolina

representtheelectricfieldlines

Electricfieldlinesindicatesthedirectionofelectricfield




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Different electric field patterns




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Positive charge

+

Electricfieldlinesaredirectedfrompositivecharge

( p )

E18.4 Electric fields (SB p. 16)


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Negative charge

Electricfieldlinesaredirectedtowardsthenegativecharge

( p )



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Direction of electric field lines

+

( p )



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Electric field lines

Do not

cross one

another

Do not have

branches

Each point of the

electric field has onedirection only

( p )



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Sketch the electric field lines between theelectrodes in the electric field apparatus shown

below:


Ans

wer

( p )


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Electrostatic Hazards andElectrostatic Hazards and

ApplicationsApplications


Electrostatic Hazards andElectrostatic Hazards and

ApplicationsApplications Electrostatic hazardsElectrostatic hazards

Electrostatic applicationsElectrostatic applications

18.5 Electrostatic hazards and applications (SB p. 17)


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Lightning

- - - --- - - - ---

++

+

+

+ +

Electrostatic hazards



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Lightning conductor

- - - --- - - - ---

++

+

+

+ +-

-

-

install a

lightning

conductor




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Oil tanker

metal chain

connected

to theground




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Aircraft landing

aircraft tyre madefrom conducting

rubber




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Electrostatic nuisance

The screens of

TV sets,

monitors and

plastics attractdust particles

When taking off

woollen clothes,

crackling sound

will be heard

CD, VCD

attract dust

particles easily

On dry days,

touching metallic

doors may get

electric shock




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Electrostatic precipitatorElectrostatic applications



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Electrostatic precipitator

exhaust gas

clean gas

negatively

charged side wall

positively chargedcentral part

high voltage

source

in

out

positively chargedsmoke particles

attract to side wall

Electrostatic applications



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positively

chargedmetal

surface

nozzle of spray gun

Electrostatic sprayingElectrostatic applications

nozzle of spray gun

negatively

charged droplet

nozzle of spray gun

negatively

charged droplet

nozzle of spray gun


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CircuitsCircuits


CircuitsCircuits19.1 Electric Circuit

19.2 Electromotive Force and

Potential Difference

19.3 Ohms law and Resistance

19.4 Simple Circuits

19.5 Electrical Power and Energy

19.6 Domestic Wiring and Electrical

Safety


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Electric CircuitElectric Circuit


Electric CircuitElectric Circuit

Electric currentElectric current

Circuit diagramCircuit diagram

19.1 Electric circuit (SB p. 30)


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59

Electric circuit

Combination of

different

electrical

components

No electric19.1 Electric circuit (SB p. 30)


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60

Open circuit

the switch

is opened

No electric

current flows

through the

circuit. The lightbulb doesnt light

19.1 Electric circuit (SB p. 30)


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61

A closed circuit

the switch

is closed

Electric current

flows through

the circuit. Thelight bulb lights

Electric current19.1 Electric circuit (SB p. 31)


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62

Direction of electric current

conventional current

flow of electrons

electrons

Conventional current



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63

Electric current

t

QI=

=

i.e.

takenTimeflowChargecurrentElectricElectric

current (I)

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

quantity of

charge (Q)

time (t)

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-



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64

Unit of current: ampere (A)

-1

sC1A1 = -1-3 sC10re)(milliampemA1 =-1-6 sC10re)(microampeA1 =

ammeter milliammeter microammeter

Circuit diagram19.1 Electric circuit (SB p. 33)


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65

Circuit symbols of electric components

battery light bulb switch



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66

ammeter voltmeter




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67

resistor rheostat potential divider




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68

Class Practice 1Class Practice 1 The following figure shows an electric circuit.

Draw the circuit diagram for it.Ans

wer


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Electromotive Force andElectromotive Force and

Potential DifferencePotential Difference


Electromotive Force andElectromotive Force and

Potential DifferencePotential Difference

Electromotive force

Potential difference

Relation between e.m.f. and p.d.

Electromotive force

19.2 Electromotive force and potential difference (SB p. 34)


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70

Electromotive force

Q

E=

=

e.m.f.Or

cellthethroughCharge

cellthebysuppliedEnergy

forceiveElectromot

e.m.f.

Electromotive force




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71


Q

EV =

=

Or

pointsthethroughCharge

pointstwobetweenformsotherintoconvertedEnergy

differencePotential

potential difference (V)





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72

Connection of ammeter and voltmeter

A

V

V

I

ammeter

in series

voltmeterin parallel


Relation between e m f and p d



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73

electromotive force = 6 V

V V

I

2 V 4 V

2 V 4 V

potential

difference = 6 V

Relation between e.m.f. and p.d.

6 V


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Ohms law and ResistanceOhms law and ResistanceSection 19.3Section 19.3

Ohms law and ResistanceOhms law and Resistance

Ohms law

Change of resistance with

temperature


dimensions of a wire

Resistor and rheostat

Intro VCD Expt VCD

Ohms law19.3 Ohms law and resistance (SB p. 37)


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75


Ohms law

eureka wires

voltmeter

ammeter

switch

batteryrheostat

Intro. VCD Expt. VCD

E i t 19A R lt

http://../Video/19a.avihttp://../Video/19head.avi


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76

Potential

difference (V) / V

1 2 3 4 5 6

Current (I) / A

potential

difference

current

Experiment 19A Results

0.1 0.2 0.3 0.4 0.5 0.6

IV

Oh l



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77

Ohms law

IV

The potential difference across a conductoris directly proportional to the current passing

through it, provided that the temperature

and other physical conditions remain

unchanged

R i t (R)



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78

Resistance (R)

):symbolohm,:(Uniti.e.

conductorthroughcurrentconductoracrossp.d.Resistance

I

VR=

=

V

I

R

Change of resistancewith temperature

19.3 Ohms law and resistance (SB p. 40)


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79

Experiment 19BExperiment 19BChange of resistance with temperature

12 V d.c. supplyswitch

ammeter

voltmeter

light bulb

Expt. VCD

E i t 19B R lt


http://../Video/19b.avi


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80

Experiment 19B Results

Potential

difference (V) / V

1 2 3 4.5 6 8

Current (I) / A 0.1 0.2 0.3 0.4 0.5 0.6


Current

Do not

obey

Ohms law

T t i t




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81

-

-

-

-

-

-

-

-

-

-

-

-

Temperature , resistance

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

When the temperature increases, the atoms

of the conductor vibrate more violently andhinder the motion of the electrons. Hence,

the resistance increases





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82

l

ammetervoltmeter

battery

Experiment 19CExperiment 19C

Change of resistance with dimensions of wire

eureka wiresof different

thickness

Expt. VCD

Cl P ti 2Cl P ti 2



http://../Video/19c.avi


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83


A uniform metal wire has a length of 2 m and aresistance of 10 .If the wire is cut into two halves, what would be the

resistance of 1-m wire?

ByRl, the resistance = ______________________

If the two 1-m wires are twisted to form a thicker wire,

what would be its resistance?The resistance of the wire would be in the range of

_________ (0-5 / 5-10 / 10-20). Ans

wer0-5

52

10

=


Resistor and rheostat19.3 Ohms law and resistance (SB p. 44)


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84


resistors rheostat

Rheostat



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85

Rheostat

AB

C

By varying the position

of the movable contact

of the rheostat, the

length of the uniform

resistance wire in whichthe current flows is

changed



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86

Class Practice 3Class Practice 3 A student tries to find theresistance of a semiconductor. The data is given below.

(a)(a) Plot a graph of voltage

against current.

(b)(b) Does the semiconductor

obey Ohms law? Explain

briefly.

Voltage / V 0.1 0.2 0.3 0.4 0.5 0.6

Current / mA 0.01 0.02 0.03 0.04 0.08 0.12

No. Because the voltage

is not proportional to

the current

Ans

wer

voltage

current


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Simple CircuitsSimple Circuits


Simple CircuitsSimple Circuits

Combination of resistors

R i t i i

Combination of resistors19.4 Simple circuits (SB p. 48)


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88

Resistors in series



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89

VVVIII

=+==

21

21

( )IIIRRR

RIRIIR

VVV

==+=

+=

+=

2121

2211

21

As

Resistors in series



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90

Resistors in series

R= R1 + R2 + R3 + R4 + ...

Resistors in parallel



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91




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92

21 VVV ==

21

21

21

21

21

or

111

As

RR

RRR

RRR

R

V

R

V

R

V

III

+

=

+=

+=

+=




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93


.........

11111

4321 ++++= RRRRR


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Electrical Power and EnergyElectrical Power and Energy


Electrical Power and EnergyElectrical Power and Energy

Electrical power

Electrical energy

Electric bill

Electrical power

Electrical power19.5 Electrical power and energy (SB p. 53)


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95

( )WWatt,:Unit

takenTime

dtransferreenergyElectricalpowerElectrical

t

EP=

=

Electrical power

Electrical power



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Electrical power

220 V 220 V

each second

consumes 100 J

each second

consumes 60 J

Electrical power



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Electrical power

( )

IVP

V

t

Q

QVEt

QV

tEP

=

=

==

=

Electrical power



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Electrical power

( ) ( )

RIP

IRVIRI

IVP

2=

===

Electrical power



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Electrical power

R

VP

R

VIR

R

V

RIP

2

2

2

=

=

=

=

Electrical power



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Electrical power

VIP=

RIP2

=

RVP

2

=

or

or

Electrical energy

Electrical energy19.5 Electrical power and energy (SB p. 54)


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Electrical energy

( )

( )

==

==

===

R

VPt

R

V

RIPRtI

VIPVIt

PtE

22

22

Electrical energy



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Electrical energy

J103.6kWh1

s6003sJ0001

h1W0001

kWh1kWh1

6

1

=

=

=

=

Experiment 19DExperiment 19D Electrical energy



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Experiment 19DExperiment 19D Electrical energy

kilowatt-hour meter

hair dryer

rotating

disc

to mains

Expt. VCD

Calculate electrical power

http://../Video/19d.avi


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Calculate electrical power

t

EE

t

EP

12

(PowerElectrical

==)

Initial kWh meter reading : E1

Final kWh meter reading : E2

Electrical energy consumed by

the appliance : E2 E1

Time : t

Class Practice 4Class Practice 4 C l h f ll i bl

Electric bill19.5 Electrical power and energy (SB p. 56)


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AppliancePower

/ WPower/ kW

Time/ h

Energyconsumed

/ kWh

Kettle 1500 3Light bulb 60 6

Vacuumcleaner

0.8 5

Iron 3 3

Complete the following table:

Ans

wer

1.5 4.50.06 100

800

1000 1

4

Class Practice 5Class Practice 5 From the electric bill shown, findthe cost of electricity per kWh.


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y p

Cost for two months =__________________

Energy consumed = ___________________________ kWh

Cost per kWh =________________________Ans

wer

$206.88

25974 25734 = 240

862.0$240

88.206=


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Domestic Wiring andDomestic Wiring and

Electrical safetyElectrical safety


Domestic Wiring andDomestic Wiring and

Electrical safetyElectrical safety

Electricity supply

Wiring of electric appliance

Domestic wiring Electrical safety

DC and AC

Electricity supply19.6 Domestic wiring and electrical safety (SB p. 57)


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DC and ACvoltage / V

time / s

direct current (d.c.)direct current (d.c.)

voltage / V

1 cycle

time / s

alternating current (a.c.)alternating current (a.c.)

Live wire (L) and neutral wire (N)



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Live wire (L) and neutral wire (N)

zero

voltage

LN

positive and

negativevoltages appear

alternately

Alternate change of positive and



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Alternate change of positive and

negative voltages in a live wire

L: positive voltage

N: zero voltage

L: negative voltage

N: zero voltage

0.01 s

0.01 s

Wiring of electric applianceWiring of electric appliance

19.6 Domestic wiring and electrical safety (SB p. 59)


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Wiring of electric appliance

plug socket

E

N LN

L

E

plastic coated wire

brown wire is

connected to

the live (L) pin

cartridge

fuse

blue wire is

connected to the

neutral (N) pin

yellow-green

wire is

connected to the

earth (E) pin

SwitchWiring of electric appliance



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S c

FuseWiring of electric appliance



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Installing earth wireWiring of electric appliance



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Installing earth wire

fault occurs

electric current

flows to the ground

No earth wire Wiring of electric appliance



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No earth wirefault occurs

electric current flows to the

ground via the human body

in parallel

Domestic wiringDomestic wiring19.6 Domestic wiring and electrical safety (SB p. 62)


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lightning circuitconsumer unit (fuse box)

kWh

meter

cableN L

main

fuse at

electric

company

to water

heater

to air-

conditioner

earth

high power appliances ring mains

Class Practice 6Class Practice 6 On a Christmas tree, the bulbs are


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connected in series. Each bulb has rated value of 5 V, 5 W.

(a)(a) Calculate the resistance of one bulb.

(b)(b) If each bulb operates at rated value from themains supply (220 V), how many bulbs can be

connected?

(c)(c) What is the total power output at that time?

Answer

=== 55

522

P

VR

44

5

220

bulboneofVoltage

voltageMainsbulbsofNumber ===

W220544outputpowerTotal ==

Do not overload a socket

Electrical safety19.6 Domestic wiring and electrical safety (SB p. 67)


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Replace the worn leads and do not join wires



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p j

Pull out the plug before filling an electric kettle



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p g g

Do not run extension leads into the bathroom



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Do not poke anything into sockets or appliances



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Magnetic Effect of a CurrentMagnetic Effect of a Current


Magnetic Effect of a CurrentMagnetic Effect of a Current

20.120.1 Magnetic EffectMagnetic Effect

20.220.2 ElectromagnetElectromagnet

20.320.3 Force on Current-CarryingForce on Current-Carrying

Conductor in Magnetic FieldConductor in Magnetic Field

20.420.4 Moving-Coil GalvanometerMoving-Coil Galvanometer


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Magnetic EffectMagnetic Effect

Permanent magnetPermanent magnet Magnetic fieldMagnetic field Current-carrying conductorsCurrent-carrying conductors

Permanent magnetPermanent magnet20.1 Magnetic effect (SB p. 87)


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SN

North

pointing to

Arctic

South

pointing to

Antarctic

Everymagnet

hasnorthpole

andsouth

pole

Like poles

Permanent magnet20.1 Magnetic effect (SB p. 87)


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S N N SS N N SS N N S

repelrepel

Unlike poles



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S N NSS N NSS N NS

attracattractt

The earth is like a magnetPermanent magnet20.1 Magnetic effect (SB p. 88)


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Arctic

Antarctic

The earth is like a large magnet

Arctic

Antarctic

N-poles of magnets point to Arctic

Magnetic effectPermanent magnet20.1 Magnetic effect (SB p. 89)


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iron rod

S N

Iron objectsare

magnetized

Which objects can be attracted by magnet?



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N

iron

gold

silver

copper

aluminium




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There are three metalsX

,Y

andZ

.X

is attracted bya magnet no matter which pole of the magnet is

facing it. Ymay be attracted or repelled depending

on the pole of the magnet. The magnet cannot

attractZat all. From these results, decide which ofthem is a piece of iron, a piece of aluminium and a

magnet. Ans

werXis a piece of iron.

Yis magnet.Zis a piece of

aluminium.

Magnetic fieldMagnetic field20.1 Magnetic effect (SB p. 89)


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Amagnetwill

produceamagneticfieldinthe

space

aroundit,justlikea

chargewilles

tablish

anelectricfield

Asmagneticfieldscanexertforcesoneachother,twomagnetscanattractorrepeloveradistance


Magnetic field20.1 Magnetic effect (SB p. 90)


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pMagnetic field of magnet Intro. VCD Expt. VCD

Magnetic field patternsMagnetic field20.1 Magnetic effect (SB p. 91)


134/449


bar magnet


135/449



136/449


Two bar magnets with like

poles facing each other



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A slab-shaped magnet withunlike poles facing each other


Magnetic field20.1 Magnetic effect (SB p. 91)


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For a bar magnet, the magnetic field is the

strongest at its two _____________. The

magnetic field lines are directed form one pole to

_____________ pole. Ans

wer

poles

another

Plotting a magnetic field lineMagnetic field20.1 Magnetic effect (SB p. 92)


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N Scompass

Amagneticfieldlineshowsthe

directionofthemagneticfield

Magnetic field linesMagnetic field20.1 Magnetic effect (SB p. 92)


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Direction:

from north to south

Strength:

higher density of

magnetic field lines,

greater magnetic

field strength

Experiment 20BExperiment 20B

Current-carrying conductors20.1 Magnetic effect (SB p. 92)


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Magnetic effect of current

Expt. VCD

Straight current-carrying wireCurrent-carrying conductors20.1 Magnetic effect (SB p. 94)


142/449


currentpoints

upwards

Straight current-carrying wireCurrent-carrying conductors20.1 Magnetic effect (SB p. 94)


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currentpoints

downwards

Right-hand grip rule for current-carrying

t i ht i



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right hand

field lines

straight wire

current

Current flows upwardsCurrent-carrying conductors20.1 Magnetic effect (SB p. 94)


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right hand

field lines

current


Current flows downwards


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current

Current-carrying flat coilCurrent-carrying conductors20.1 Magnetic effect (SB p. 95)


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Right-hand grip rule for current-carrying

fl il



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flat coil

Magnetic field

points upwards

Magnetic field points

into the paper

Current-carrying solenoidCurrent-carrying conductors20.1 Magnetic effect (SB p. 96)


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Right-hand rule for current-carrying

l id



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solenoid

S i 20 2S ti 20 2S ti 20 2S ti 20 2


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ElectromagnetElectromagnet

Applications of electromagnetsApplications of electromagnets

Electromagnet consists of soft-iron core

d l id

20.2 Electromagnet (SB p. 98)


152/449


and solenoid

soft iron

U-cores

low voltage d.c.power supply

Experiment 20CExperiment 20C Electromagnet Expt. VCD20.2 Electromagnet (SB p. 98)


153/449


soft iron

cores

low voltage

d.c. power supply

Showing direction of magnetic field20.2 Electromagnet (SB p. 99)


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When the power supply is turned on,the needle of the compass shows

the direction of the magnetic field

20.2 Electromagnet (SB p. 99)

Strength of magnetic field


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number of

turns of

wire

Number of turns of wire Magnetic field strength

Direction of magnetic field20.2 Electromagnet (SB p. 99)


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S N

Direction of magnetic fielddepends on direction of current

Applications of electromagnetsApplications of electromagnets20.2 Electromagnet (SB p. 99)


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Electricb

ell Telephonereceiver

Ticker-tapetim

er

Cranein

scrapya

rd

Experiment 20DExperiment 20D Model electric bellExpt. VCD

Applications of electromagnets20.2 Electromagnet (SB p. 100)


158/449


Working principle of a

model electric bell

A, B are in contact

th i it i l d



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model electric bell

to low voltage

d.c. power supplywire

tape

screwbladesupporting block

B A

the circuit is closed

the electromagnet

attracts the blade

the blade bends

downwards

A, B are not in contact

the electromagnet loses its magnetism

the blade rebounds upwards

Telephone receiverApplications of electromagnets20.2 Electromagnet (SB p. 101)


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mouthpiece

earpiece

plate

electromagnet


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Ticker-tape timer



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iron strippaper

tape

dipperdiode

electromagnetcoil

spring

Current

Time

Crane in scrapyardApplications of electromagnets20.2 Electromagnet (SB p. 102)


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Section 20 3Section 20 3Section 20 3Section 20 3


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Force on Current-CarryingForce on Current-Carrying

Conductor in Magnetic FieldConductor in Magnetic Field

Flemings left hand ruleFlemings left hand rule Moving-coil loudspeakerMoving-coil loudspeaker Electric motorsElectric motors

Experiment 20EExperiment 20E

Magnetic force on conductor

Expt. VCD20.3 Force on current-carrying conductor in magnetic field (SB p. 103)
http://../Video/20e.avi


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Magnetic force on conductor

12 V d.c. power supply

Flemings apparatus

When a current flows through the rider,

the rider moves

20.3 Force on current-carrying conductor in magnetic field (SB p. 103)


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the rider moves

12 V d.c. power supply

Flemings apparatus

rider

the rider moves

Flemings left-hand rule

f

Flemings left hand rule20.3 Force on current-carrying conductor in magnetic field (SB p. 104)


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field

force

current

forceForce acting current-

carrying conductor in B-field

Flemings left hand rule20.3 Force on current-carrying conductor in magnetic field (SB p. 104)


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current

field

current

carrying conductor in B-field

Turning effect of a coilFlemings left hand rule



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field

current

force

Class Practice 3Class Practice 3 A copper rod is placed on an opencircuit. When the switch is closed and the resistance of the

rheostat is increased gradually, state and explain the motion


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of the rod when it is placed at positionsAB and CD.

AtAB:

At CD:

Ans

wer

magnetic

field regionThe rod moves to the right

according to Flemings left hand

rule. It moves at a decreasing

acceleration because the current

is decreasing.

The rod does not move

because there is no

magnetic field around it.

Moving-coil loudspeakerMoving-coil loudspeaker



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Structure of moving-coil loudspeakerMoving-coil loudspeaker



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permanent magnetpaper cone

voice coil

back panel of

receiver

speaker

terminals

Working principle of moving-coil

loudspeaker

Moving-coil loudspeaker20.3 Force on current-carrying conductor in magnetic field (SB p. 107)


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loudspeaker

magnetic field

paper

cone

voice

coil

force

force

Electric motorsElectric motors


d ill


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d.c. electric fans

a.c. washing

machine

d.c. drill

a.c. drill

Experiment 20FExperiment 20F

Model electric motor

20.3 Force on current-carrying conductor in magnetic field (SB p. 108)Electric motors


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Model electric motor

Expt. VCD

Working principle of electric motor20.3 Force on current-carrying conductor in magnetic field (SB p. 109)

Electric motors
http://../Video/20f.avi


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rotation

carbon brush

commutator

Rotation of a coil - at the beginning20.3 Force on current-carrying conductor in magnetic field (SB p. 109)

Electric motors


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Rotation of a coil - rotate 9020.3 Force on current-carrying conductor in magnetic field (SB p. 109)

Electric motors


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Electric motors


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Electric motors


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Ways to increase the turning speed

of the coil

20.3 Force on current-carrying conductor in magnetic field (SB p. 110)Electric motors


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Increase the current

Use a stronger magnet Increase the number of turns of the coil

Use a coil with larger surface area

Section 20 4Section 20 4Section 20 4Section 20 4


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Moving-Coil GalvanometerMoving-Coil Galvanometer

Conversion of galvanometer toConversion of galvanometer toammeter and voltmeterammeter and voltmeter

Moving-coil galvanometerElectric motors20.4 Moving-coil galvanometer (SB p. 111)


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A galvanometer Circuit symbol

Experiment 20GExperiment 20G

Model moving-coil galvanometer

20.4 Moving-coil galvanometer (SB p. 111)


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g g

Expt. VCD

Structure of a moving-coil galvanometer20.4 Moving-coil galvanometer (SB p. 111)
http://../Video/20g.avi


185/449


2

1

iron yoke

split pin

straw pointerabout 10 turns of insulated wire

wire wound in a loose

spiral to form a spring

thin rod

magnets

Conversion of milliammeter to galvanometer20.4 Moving-coil galvanometer (SB p. 112)


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to terminals

pointer

fixed soft-iron

cylinder

permanent

magnet

insulating support

lower hairspring

cylinder

support

pointer counter

balance

scale

upper hairspring

zero adjuster

coil

Ways to increase the sensitivity of

moving-coil galvanometer:



187/449


g g

Use a stronger magnet

Use weaker hairsprings Increase the number of turns of the coil

Increase the surface area of the coil

Full-scale deflection (f.s.d.)Conversion of galvanometer to

ammeter and voltmeter



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to terminals


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Structure of ammeter

ammeter

Conversion of galvanometer to




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Rs

IIg

r

I Ig

ammeter

shunt

Resistance of shunt (Rs)

V lt th l t





191/449


Voltage across the galvanometer

= Voltage across the shunt

( )gIII

rgIsR

gII

rgI

sR

sR)gI(IrgI

>>=

=

=

Conversion to voltmeterConversion of galvanometer to




192/449


multipliers

Structure of a voltmeterConversion of galvanometer to




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Rmr

Ig

Ig

voltmeter

V

multiplier

Resistance of a multiplier

Voltage across the voltmeter





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Voltage across the voltmeter

= Voltage across the galvanometer +

Voltage across the multiplier

rgI

VmR

)mR(rgImRgIrgIV

=

+=+=

Class Practice 4Class Practice 4 A milliammeter of resistance100 and full-scale deflection current 10 mA isconverted to an ammeter by using a resistor of

resistance 11.


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(a)(a) Draw a circuit to show how the milliammeter canbe converted to the ammeter.

(b)(b) Find the maximum current that can be measured by

the ammeter.

Answer

A101.0

11.0111

)11)(1010()100)(1010(

)(33

=

=

=

=

I

I

I

RIIrI sgg

Class Practice 5Class Practice 5 A milliammeter of resistance1 000 and full-scale deflection current 10-3 A isconverted to a voltmeter of full-scale deflection voltage

10 V.


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(a)(a) Draw a circuit to show the connection.

(b)(b) Find the resistance of the multiplier and the

voltmeter.

Ans

wer

==

+=

+=

=

+=

+=+=

k1000010

00090001

multiplierofResistanceermilliammetofResistancevoltmetertheofResistance

0009

)0001)(10(10

)(

m

m

3

mgmgg

R

R

RrIRIrIV

Chapter 21Chapter 21Chapter 21Chapter 21


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Electromagnetic InductionElectromagnetic Induction Electromagnetic InductionElectromagnetic Induction 21.1 Induced EMF and Induced Current21.1 Induced EMF and Induced Current

21.2 Generators21.2 Generators

21.3 Transformer21.3 Transformer

21.4 Transmission of Electrical Energy21.4 Transmission of Electrical Energy



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Induced EMF and InducedInduced EMF and Induced

CurrentCurrent

Lenzs lawLenzs law Induced e.m.f. and induced current in aInduced e.m.f. and induced current in a

conducting wireconducting wire

Applications of induced e.m.f. in coilsApplications of induced e.m.f. in coils


Electromagnetic induction

Intro. VCD Expt. VCD21.1 Induced EMF and induced current (SB p. 133)


199/449


light-beam

galvanometer

magnet coil


200/449

Ways to increase the induced e.m.f.:

21.1 Induced EMF and induced current (SB p. 133)


201/449


Move the magnet faster


Increase the number of turns in the coil

Faradays law of electromagnetic induction



202/449


conductor

+ change of

magnetic field

e.m.f.

strength of e.m.f.

rate of change of magnetic field


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Electromagnetic induction - induced current

Lenzs law21.1 Induced EMF and induced current (SB p. 134)


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NS

induced current

Lenzs law



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Lenzs law states that the induced

current always flows in a direction such

that it opposes the change producing it.

repulsive force

repulsive force

To prove Lenzs law



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N

repulsive forcep

attractive force

attractive force

To prove Lenzs law



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S

Flemings right hand rule

motion

Induced e.m.f. and induced

current in a conducting wire



208/449


field

current

Flemings right hand ruleInduced e.m.f. and induced




209/449


motion

current

field

Flemings right hand ruleInduced e.m.f. and induced




210/449


motion

field

current

Ways to increase the induced

e.m.f.





211/449


Move

thewire

faster


212/449

Ways to increase the induced

e.m.f.





213/449


Useastron

ger

magnet

Class Practice 1Class Practice 1 In which direction will theinduced current flow (if any) when the conductor or

magnet is moved in the ways shown below?



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Answer

no current

Class Practice 2Class Practice 2 When a copper rod is moving along a metal frame, an

induced current flows as shown.

(a)(a) In which direction is the copper



215/449


( )( ) pp

rod moving?

(b)(b) Due to the induced current, the

rod is experiencing a force inside the

magnetic field. In which direction

does this force act on the rod?

Ans

wer

induced

current

metal frame

By using Flemings right

hand rule, the copper rod

is moving to the left.

By using Flemings left hand rule or

Lenzs law, the force on the rod acts to

the right. It opposes the motion of the rod.

Applications of induced e.m.f. in coils

Moving-coil

Applications of induced e.m.f. in coils21.1 Induced EMF and induced current (SB p. 140)


216/449


induced e.m.f.induced e.m.f.

microphone

toamplifier

magnet

diaphragm

moving coil


217/449

Applications of induced e.m.f. in coils

Magnetic tape recording and

Applications of induced e.m.f. in coils21.1 Induced EMF and induced current (SB p. 141)


218/449


playback

current coil

magnet

tape

motionmagnetic tape



219/449


GeneratorsGenerators

A coil moving in a magnetic fieldA coil moving in a magnetic field

AC generator (alternator)AC generator (alternator)

DC generator (d.c. dynamo)DC generator (d.c. dynamo)

Bicycle alternatorBicycle alternator

Alternators in power stations and carsAlternators in power stations and cars

A coil moving in a magnetic field

A coil moving in a magnetic field21.2 Generators (SB p. 142)


220/449


When the coil starts to turn

When the plane of



221/449


the current flows from

the right to the left

the coil ishorizontal, the rate of

cutting the

magnetic fieldlines is the

highest

the inducedcurrent is the

maximum

The coil is turned 90When the plane of

the coil is vertical,



222/449


no current


no field linesare cut

no current is

inducedAfter passing the

vertical position,

the induced

current recurs,

but the direction

is reversed

When the plane of


The coil is turned 180


223/449


the direction of

induced current isreversed

the coil ishorizontal,

the rate of

cutting themagnetic field

lines is the

highest

the inducedcurrent is the

maximum

The coil is turned 270When the plane of




224/449


no current

no field linesare cut

no current is

induced

After passing the

vertical position,the inducedcurrent recurs,

but the direction

is reversed


The coil is turned 360


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The coil isturned to the

starting position

The coil isturned

continuously

An alternating

current isproduced

AC generator (alternator)

AC generator (alternator)21.2 Generators (SB p. 144)


226/449


external

circuit

carbon

brush

slip rings

rotation a current

is induced

Induced e.m.f. and the number of revolutions



227/449


Ways to increase the e.m.f.



228/449


Rotate the coil at a higher speed

Increase the number of turns of the

coil

Wind the coil on a soft-iron core

(armature)


Experiment 21BExperiment 21B

Model dynamo

DC generator (d.c. dynamo)21.2 Generators (SB p. 145)


229/449


Expt. VCD

DC generator (d.c. dynamo)

DC generator (d.c. dynamo)21.2 Generators (SB p. 146)


230/449


commutator

carbon

brush

rotation

a current

is induced


231/449

Bicycle alternator

cylindricaldriving

wheel

Bicycle alternator21.2 Generators (SB p. 147)


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output terminals

magnet (rotor)

coil

soft iron

axle

Experiment 21CExperiment 21C

Model bicycle alternator



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Expt. VCD

Display of output of a bicycle alternator



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The graph of output voltage against time for an a.c.

generator is shown below:

21.2 Generators (SB p. 150)


235/449


(a)(a) Find the peak voltage and

the frequency of the output

voltage.

Peak voltage = ___________

Period = ________________

Frequency = _____________Ans

wer

0.02 s

5 V

Hz5002.0

1=

Class Practice 3 (Contd):Class Practice 3 (

Contd):

(b)(b) Draw the new graph when the number of

turns of the coil is trebled.

21.2 Generators (SB p. 150)


236/449


Answer

Alternator in power station

Alternators in power stations and cars21.2 Generators (SB p. 150)


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238/449


TransformerTransformer

Efficiency of a transformerEfficiency of a transformer

Transformer

21.3 Transformer (SB p. 151)


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Mutual induction of a transformer21.3 Transformer (SB p. 151)


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at the instant when

the switch is closed

Mutual induction of a transformer

at the instant when

the switch is closed



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at the instant when

the switch is opened

Experiment 21DExperiment 21D

Simple transformer

Expt. VCD



242/449


Transformer



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p

s

p

s

N

N

V

V

=

Vp NpVs

Ns

A step-up transformer

soft-iron core



244/449


input

voltage

output

voltage

secondary

coilprimary

coil

circuit symbol

(Ns > Np)

A step-down transformer

soft-iron core



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input

voltageoutput

voltagesecondary

coil

primary

coil

circuit symbol

(Ns < Np)

2 5001 000 :

Find the number of turns in the

secondary coil



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220 V

p

s

p

s

N

N

V

V=

10002500

220s =V

550s =V

Experiment 21EExperiment 21E

Winding a transformer



247/449


Expt. VCD

Efficiency of a transformer

Efficiency of a transformer21.3 Transformer (SB p. 155)


248/449


100%

powerelectricalInput

powerelectricalOutputrtransformeaofEfficiency =

100%=pp

ss

IV

IVe

Ideal transformer


IVIV


249/449


ppss IVIV =

s

p

p

s

I

I

V

V=

p

s

s

p

p

s

N

N

I

I

V

V==

Ideal transformer VS Practicaltransformer



250/449


transformer

input

power

output

power

input

poweroutput

power

power

loss

transformer

ideal practical

Reason for energy loss (1)

iron core



251/449


input

voltage

output

voltagesecondary

coil

primary

coil

the coils have

resistance

current in the coil produce

heating effect

electrical energy

converts to heatenergy is lost

Waystomin

imizeenergyloss:

Waystomin

imizeenergyloss:

useconduc

tingwiresof

smallerresis

tance

useathickerwire

iron core

eddy

currentReason for energy loss (2)



252/449


input

voltage

output

voltagesecondary

coil

primary

coil

current in the primary coil

produce magnetic field

induce eddy

current in the

iron coreheating effect of

the iron coreenergy is lost

Waystomin

imizeenergy

loss:

Waystom

inimizeenerg

yloss:

useanironcoremad

eformastac

kof

thinmetalsli

cesthatareinsulatedf

rom

oneano

ther,toincrea

setheresist

ance

andreducethe

eddycurren

t

Reason for energy loss (3)

Alternating current through the transformer



253/449


continuous magnetization and

demagnetization of iron core

the iron core is heated up

energy is lost

Waystominim

izeenergylos

s:

Waystominim

izeenergylos

s:

useasoftironcore,

whichcan

bemagnetize

danddemagnetized

easily,soen

ergylossisre

duced

Class Practice 4Class Practice 4 A transformer is used to stepdown the 220 V mains supply to run an appliance of

110 V, 550 W.

( )( ) If th i il h 1 000 t h t i th



254/449


(a)(a) If the primary coil has 1 000 turns, what is thenumber of turns in the secondary coil?

(b)(b) What is the current drawn by the appliance?

Ans

wer

turns500

220

100

0001

By

s

s

p

s

p

s

=

=

=

N

N

V

V

N

N

A5110

550===

V

PI

(c)(c) It is found that the current drawn from the

i i 2 8 A Wh t i th ffi i f th

Class Practice 4 (Contd):Class Practice 4 (

Contd):



255/449


mains is 2.8 A. What is the efficiency of thetransformer?

Ans

wer

%89

%100616

550

Efficiency

W616

8.2220powerInput

W550powerOutput

==

===

=

VI


256/449

Experiment 21FExperiment 21F

Transmission of electrical power

Expt. VCD

21.4 Transmission of electrical energy (SB p. 159)
http://../Video/21f.avi


257/449


transmission line

step-downtransformer

step -uptransformer

Transmission of electricity



258/449


Power loss of cables

RIP

2=



259/449


RIP

cable

12 V12 V

I

Power loss of cablesIsIp



260/449


Np : Ns

Vp Vs