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Electricity
N Bronks
Basic ideas…Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.
Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta.
Resistance is anything that resists an electric current. It is measured in _____.
Words: volts, amps, ohms, voltage, ammeter, voltmeter
• Flow of electrons
timepoint a Passed ChargeCurrent
tQI
Current
Electrons are flowing from the negative to positive side of the
battery through the wires
Note current moves from positive to
negative, however electrons are actually
are moving in the opposite direction!
H/W
• 2004 HL Q4
More basic ideas…
Another battery means more current as there is a greater push on the electrons
The extra resistance from the extra bulb means less current
Current in a series circuit
If the current here is 2 amps…
The current here will be…
The current here will be…
And the current here will be…
In other words, the current in a series circuit is THE SAME at any
point
2A
2A
2A
Current in a parallel circuit
A PARALLEL circuit is one where the current has a “choice of routes”
Here comes the current…
And the rest will go down here…
Half of the current will go down here (assuming the bulbs are the same)…
Current in a parallel circuit
If the current here is 6 amps
The current here will be…
The current here will be…
The current here will be…
And the current here will be…6A
2A
2A 2A
Voltage in a series circuit
V
V V
If the voltage across the battery is 6V…
…and these bulbs are all identical…
…what will the voltage across each bulb be? 2V
Voltmeter always in parallel
Voltage in a series circuit
V
V
If the voltage across the battery is 6V…
…what will the voltage across two bulbs be? 4V
Voltage in a parallel circuit
If the voltage across the batteries is 4V…
What is the voltage here?
And here?
V
V4V
4V
Summary
In a SERIES circuit:
Current is THE SAME at any point
Voltage SPLITS UP over each component
In a PARALLEL circuit:
Current SPLITS UP down each “strand”
Voltage is THE SAME across each”strand”
An example question:
V1
V2
6V
3A
A1
A2
V3
A3
Advantages of parallel circuits…
There are two main reasons why parallel circuits are used more commonly than series circuits:
1) Extra appliances (like bulbs) can be added without affecting the output of the others
2) When one breaks they don’t all fail
Georg Simon Ohm 1789-1854
Resistance
Resistance is anything that will RESIST a current. It is
measured in Ohms, a unit named after me.
That makes me so happy
The resistance of a component can be calculated using Ohm’s Law:
Resistance = Voltage (in V)
(in ) Current (in A)
V
RI
An example question:
V
A
What is the resistance across this bulb?
As R = volts / current = 10/2 = 5
Assuming all the bulbs are the same what is the total resistance in this circuit?
Total R = 5 + 5 + 5 = 15 Voltmeter reads 10V
Ammeter reads 2A
More examples…
12V
3A
3A
6V
4V
2A
1A
2V
What is the resistance of these bulbs?
Practice with Ohm’s Law
Ohms Volts Amps4 100 25
15 150 102 30 159 45 56 48 8
H/W
• 2005 HL Q9
VARIATION OF CURRENT (I) WITH P.D. (V)
A
V+
6 V-
Nichrome wire
Method
1. Set up the circuit as shown and set the voltage supply at 6 V d.c.
2.Adjust the potential divider to obtain different values for the voltage V and hence for the current I.
3.Obtain at least six values for V and I using the voltmeter and the ammeter.
4.Plot a graph of V against I
Variations
(a) A METALLIC CONDUCTORWith a wire
(b) A FILAMENT BULB (c) COPPER SULFATE SOLUTION
WITH COPPER ELECTRODES(d) SEMICONDUCTOR DIODE
Done both ways with a milli-Ammeter and the a micro Ammeter
Current-voltage graphsI
VI
V
I
V
1. Resistor 3. Diode
2. BulbCurrent increases in proportion to voltage
As voltage increases the bulb gets hotter and resistance increases
A diode only lets current go in one direction
Factors affecting Resistance of a conductor
• Resistance depends on– Temperature– Material of conductor– Length – Cross-sectional area Temperature
The resistance of a metallic conductor increases as the temperature increases e.g. copperThe resistance of a semiconductor/insulator decreases as the temperature increases e.g. thermistor.
VARIATION OF THE RESISTANCE OF A METALLIC CONDUCTOR WITH
TEMPERATURE
Water Wire wound on frame
Glycerol
Heat source
10ºC
Digitalthermometer
Ω
10º C
Method1. Set up as shown.
2. Use the thermometer to note the temperature of the glycerol, which is also the temperature of the coil.
3. Record the resistance of the coil of wire using the ohmmeter.
4. Heat the beaker.5. For each 10 C rise in temperature record
the resistance and temperature using the ohmmeter and the thermometer.
6. Plot a graph of resistance against temperature.
Graph and Precautions
Precautions - Heat the water slowly so temperature does
not rise at end of experiment-Wait until glycerol is the same temperature as
water before taking a reading.
R
LengthResistance of a uniform conductor is directly proportional to its length.
i.e. R L
Factors affecting Resistance of a conductor
Cross-sectional areaResistance of a uniform conductor is inversely proportional to its cross-sectional area.
i.e. R 1 A
Factors affecting Resistance of a conductor
• MaterialThe material also affects the resistance of a
conductor by a fixed amount for different materials. This is known as resistivity ().
R = L = Resistivity A Unit: ohm meter m
RESISTIVITY OF THE MATERIAL OF A WIRE
Micrometer
Metre stick
l
Bench clamp
Stand
Nichrome wire Crocodile clips
Method1. Note the resistance of the leads when the crocodile
clips are connected together. Could also be precaution.2. Stretch the wire enough to remove any kinks or ‘slack’ in
the wire.3.Read the resistance of the leads plus the resistance of wire
between the crocodile clips from the ohmmeter. Subtract the resistance of the leads to get R.
4.Measure the length l of the wire between the crocodile clips, with the metre stick.
5.Increase the distance between the crocodile clips. Measure the new values of R and l and tabulate the results.
6.Make a note of the zero error on the micrometer. Find the average value of the diameter d.
1. Calculate the resistivity where A =
2. Calculate the average value.
Precautions Ensure wire is straight and has no kinks like ....Take the diameter of the wire at different angles
,Al
Rñ
4
2d
ρ
H/W
• 2004 HL Q4
Resistors in series and Parallel
321 IIIIT
V1
I
I1
V
I2
IT
R1R1
R2 R3
R2
321 VVVVT
Resistors in series and Parallel
321 IRIRIRIRT
V1
I
I1
V
I2
IT
R1R1
R2 R3
R2
321 RRRRT
321 VVVVT
Resistors in series and Parallel
321 R
V
R
V
R
V
R
V
T
V1
I
I1
V
I2
IT
R1R1
R2 R3
R2
321
1111
RRRRT
321 IIIIT
Wheatstone BridgeUses
– Temperature control– Fail-Safe Device (switch
circuit off)– Measure an unknown
resistance
– R1 = R3 (When it’s balanced)
R2 R4
Metre Bridge R1 = R2 (|AB|)
|BC|
I
r 1
r2
r 4
r3
A C
B
D
Effects of an Electric Current
•Heat•Chemical•Magnetic
Chemical Effects of an Electric Current
• Electrolysis is the chemical effect of an electric current
• Voltameter consists of electrodes, an electrolyte and a container
• Inactive electrodes are electrodes that don’t take part in the chemical reaction e.g. platinum in H2SO4
• Active electrodes are electrodes that take part in the chemical reaction e.g. copper in CuSO4
Chemical Effects• Ion is an atom or molecule
that has lost or gained 1 or more electrons
• Charge Carriers in an electrolyte are + and – ions
Uses Electroplating to make metal look better, prevent corrosion Purifying metals Making electrolytic capacitors
Current-voltage graphs
I
V
I
V
1. Active Electrodes
2. Inert Electrodes
e.g. Copper in Copper Sulphate
e.g. Platinum in Water
Current Carriers
Medium Carrier
Solid (Metal) Electrons
Liquid (Electrolyte) Ions
Gas Electrons and Ions
Resistance in Semiconductors
2) Thermistor – resistance DECREASES when temperature INCREASES – Due to more charge carriers being liberated by heat
1) Normal conductor like metal resistance increases as vibrating atoms slow the flow of electrons
Resistance
Temperature
Resistance
Temperature
Fuse – Safety device
Fuses are designed to melt when too large a current tries to pass through them to protect devices.
Prevent Fires
Modern fuse boxes contain MCB (Miniature circuit breakers) that trip when too much current flows to protect the circuit
2A5A
Which Fuse
• A i-pod charge uses 200W and is plugged into the mains at 230v. What fuse is in the plug?
• P=I.V• 200=I.230• I = 200/230 = 0.87A is current used• So the most the fuse should be is a
1A
Other safety devices…1) Insulation and double insulation
2) Residual Current Circuit Breaker
In some parts of Europe they have no earth wire just two layer of insulating material the sign is
An RCCB (RCB) detects any difference in current between the live and neutral connectors and the earth it switches off the current when needed. They can also be easily reset.
Electrical Safety• A combination of fuse and Earth
A.C. Supply
That Hurts!
The fuse will melt to prevent electrocution and the electricity is carried to earth
The casing touches the bare wire and it becomes live
Wiring a plug
Earth wire
Neutral wire
Insulation
Live wire
Fuse
1.
2.
3.
4.
5.
6. Cable grip
Capacitors• A device for storing
charge.• A pair of metal plates are
separated by a narrow gap -
-----
-
+
+++++
- - -
electrons
capacitor charge
charged capacitor
capacitor discharge
electrons
Charge & Discharge
Capacitor Construction• Two metal plates• Separated by
insulating material• ‘Sandwich’
construction• ‘Swiss roll’ structure• Capacitance set by...
d
AC
Uses of Capacitors
• Storing charge for quick release – Camera Flash
• Charging and discharging at fixed intervals – Hazard Lights
• Smoothing rectified current – See Semiconductors
variable capacitor
smoothing capacitors
Parallel Plate Capacitors• The size of the capacitor depends on1. The Distance the plates are apart d
-
-
-
+
+
+
d
Parallel Plate Capacitors
2 /.The area of overlap A
-
-
-
+
+
+
A
Parallel Plate Capacitors• 3/.The material between ()
-
-
-
+
+
+
High material
Called a
DIELECTRIC
--
--
++
++
Finding Capacitance
A
s
V
Vecapacitanc
Equations
C
d
A=
For the parallel plate capacitor
Distance in meters
Area In m2
Permitivity inFm-1
CapacitanceIn Farads
Example 1
0
C0.01m
0.04m2
=
The common area of the plates of an air capacitor is 400cm2 if the distance between the plates is 1cm and ε0=8.5x10-12Fm-1.
C
d
A=
8.5x10-12Fm-
1x=3.4x10-11F.
Capacitance experiment on the internet
Equations
C
V
Q=
Capacitance on any conductor
Potential Difference in volts
Charge in Coulombs
CapacitanceIn Farads
Placing a charge of 35μC on a conductor raises it's potential by 100 V. Calculate the capacitance of the conductor.
Info Q = 35μC and V = 100V find C=?
Using Q=VC or C = Q/V
= 35 x 10-6/100
= 35 x 10-8 Farads
Equations
C½Work Done
(V)2=
Energy stored on a capacitor
Voltage Squared
CapacitanceIn Farads
Energy Stored
Example 3Find the capacitance and energy stored of a
parallel plate capacitor with 2mm between the plates and 150cm2 overlap area and a dielectric of relative Permittivity of 3. The potential across the plates is 150V.
A = 150cm2=0.015m2, d = 2x10-3m,
ε = 3xε0 = 27x10-12Fm-1
As C = ε0A/d = 27x10-12 x 0.015/0.002 = 2.025x10-9 F
Energy stored = ½ C V2 = ½ x 2.025x10-9x (150)2
= 2.28x10-5 Joules
H/W
• 2006 HL Q9
Types of BatteriesType of Battery
Contains Uses
Wet cell rechargeable
Lead and acid Cars, industry
Dry cell rechargeable
Nickel, cadmium, lithium
Mobile phones, power tools
Dry cell non-rechargeable
Zinc, carbon, manganese, lithium
Torches, clocks, hearing aids
Why use rechargeable batteries?
• Long long-term expense
• Can be used many times
• Less energy to produce
Why use standard batteries?
• No need for charger
• Less expensive
• Rechargeables contain carcinogens
There are 2 types of currents:
• Direct Current (DC) – Where electrons flow in the same direction in a wire.
There are 2 types of currents:
• Alternating Current (AC) – electrons flow in different directions in a wire
DC and AC
DC stands for “Direct Current” – the current only flows in one direction:
AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)
Find Root Mean Square of voltage by
Vrms= Vpeak/ √2
1/50th s
240V
V
V
Time
T
The National Grid
If electricity companies transmitted electricity at 240 volts through overhead power lines there would be too much energy lost by the time electricity reached our homes.
This is explained by JOULES LAW
Power stationStep up
transformerStep down
transformerHomes
The National Grid
Power Transmitted is = P = V.I
JOULES LAW gives us the power turned into heat
Power Lost = I2R
So if we have a high voltage we only need a small current. We loss much less energy
Power stationStep up
transformerStep down
transformerHomes
Power loss in Transmission lines
A power company wants to send 100000w of power by a line with a resistance of 12 ohms. If it uses 10A as the current
Power transmitted = V . I 100000 = V . 10 So V=10000Volts
But the loss is from Joules law = I2R= (10)2.12 = 1200watts
Power loss in Transmission lines
If we want the same power but use only 1A as the current
Power transmitted = V . I 100000 = V . 1
So V=100000Volts (Another Zero)But the loss is from Joules law = I2R
= (1)2.12 = 12watts
100 times less!
Joules law
Heating coil Lagging
Calorimeter Water
A
LidDigitalthermometer
10°C
Method1. Put sufficient water in a calorimeter
to cover the heating coil. Set up the circuit as shown.
2. Note the temperature. 3. Switch on the power and simultaneously
start the stopwatch. Allow a current of 0.5 A to flow for five minutes. Make sure the current stays constant throughout; adjust the rheostat if necessary.
4. Note the current, using the ammeter. 5. Note the time for which the current
flowed. 6. Stir and note the highest temperature.
Calculate the change in temperature ∆.
Calculation and GraphRepeat the above procedure for increasing values of current I, taking care not to exceed the current rating marked on the rheostat or the power supply. Take at least six readings. Plot a graph of ∆(Y-axis) against I 2 (X-axis).
A straight-line graph through the origin verifies that ∆ I 2 i.e. Joule’s law.
Electrical Power lost as Heat P I2 is Joules lawThe power lost (Rate at which heat is produced) is
proportional to the square of the current.
∆
I2
H/W
• 2006 HL Q 4
Experiment to Show shape of Electric Field
• The electrode is placed in the shallow glass dish containing a mixture of semolina and castor oil. The semolina aligns itself along the lines of the electric field.
The Electroscope
+
+
++
- - - -
The electroscope detects charge The Gold leaf and post repel each other
Coulomb's Law
• Force between two charged bodies
Force = f Q1.Q2
d2
Q1 Q2d
Put this as a sentence to get a law!
Coulomb Calculations
• We replace the proportional with a equals and a constant to get an equation
Force =f Q1.Q2
d2
Force = f = Q1.Q2
4d2
= permitivity as in capacitors
Coulomb's Law Calculations
• Force between these bodies
Force = f = Q1.Q2
4d2
2C 4mCd=2m
= 3.4 x 10-11
Coulomb's Law Calculations
• Force between these bodies
Force = f = 2 x 0.004
4 x3.4 x 10-11x 22
2C 4mCd=2m
Coulomb's Law Calculations
• Force between these bodies
Force = f = 7.49 x 10-15 N
2C 4mCd=2m
Coulomb's Law Calculations
• Force between these bodies
2C 4mCd=2m
Electric Field Strength = E = F/q
Electric Field Strength =
E = 7.49 x 10-15 N /2C
= 3.75 x 10-15 N /C
Precipitator
• Carbon and ash - can be removed from waste gases with the use of electrostatic precipitators
Precipitator
• Dirt particles are charged then made to stick to oppositely charged plates
Photocopier
• Charging:• Exposure: • Developing:• Transfer: • Fusing:• Cleaning:
Potential Difference (V)
Potential difference is the work done per unit charge to transfer a charge from one point to another (also Voltage)
i.e V = W Q
Potential Difference (V)
V = W Q
Unit Volt V or J C-1
Volt is the p.d. between two points if one joule of work is done bringing one coulomb from one point to the other
Potential at a point is the p.d. between a point and the Earth, where the Earth is at zero potential
Current in a Magnetic Field
N S N S
Current in a Magnetic Field
N S
Force
CurrentMagnetic Field
A conductor carrying a current in a magnetic field will always feel a force
The force is perpendicular to the current and the field. – This is THE MOTOR EFFECT
Fleming’s Left Hand Rule
I used my left hand to show the direction the wire would move
The Size of the Force
Force = F = B.I.lWhere B = Magnetic Field Density in Tesla (T)
I= Current in Amps (A)…………………………… L = length if the conductor in metres…
Example What is the force acting on a conductor of length 80cm carrying a current of 3A in a 4.5T magnetic field?
Using Force = F = B.I.l = 4.5x3x0.8
= 10.8N
Two Parallel Wires• Wires also produce magnetic fields
when a current flows
Attraction
Two Parallel Wires• The fields act like magnets when the
current flows
Repulsion
The Ampere• Basic unit of electricity
F=2x10-
7N/m
1m
The current flowing is 1A when the force between two infinitely long conductors 1m apart in a vacuum is 2x10-7N Per metre of length.
Demo
• OHP and coils and compass
Moving Charge• When any charged particle moves it is like a
small current of electricity• It feels the same force• The crosses show a magnetic field into the
screen
e-Velocity
Force
e -
VelocityForce
e -
Velocity
Forcee-
e-
Moving Charge• A positive will move the other way
e-Velocity
Force
+
All charged particles moving in magnetic fields always have a force at right angles to their velocity so follow a circular path due to FLH Rule
See particles motion
Force 0n a Particle
Force = F = B.q.vWhere B = Magnetic Field Density in Tesla (T)
q=charge on the particle (C)
v=velocity of the particle…
Example What is the force acting on a particle travelling at 80m/s carrying a charge of 0.1C in a 10T magnetic field?
Using Force = F = B.q.v= 10x.1x80
= 80N
Demo
• CRT and magnet
• ..\..\..\My Documents\fnfig-12.jpg
Inductionis where changes in the current flow in a circuit are caused by changes in an external field.
N
Moving Magnet
Circuit turning off and on
Electromagnetic induction
The direction of the induced current is reversed if…
1) The magnet is moved in the opposite direction
2) The other pole is inserted first
The size of the induced current can be increased by:
1) Increasing the speed of movement
2) Increasing the magnet strength
3) Increasing the number of turns on the coil
Demo
• Coils and spot galvo• Internet
http://phet.colorado.edu/en/simulation/faraday
Generators (dynamos)
Induced current can be increased in 4 ways:
1) Increasing the speed of movement
2) Increasing the magnetic field strength
3) Increasing the number of turns on the coil
4) Increasing the area of the coil
Faraday’s LawBasically
1. More turns (N) more EMF
2. Faster movement more EMF
Rate of change of FLUX DENSITY is proportional to induced EMF
Induced EMF = E = - Nd ( =B.A)
dt
Lenz’s LawThe induced EMF always opposes the current/Motion
You get ought for nought
A version of Newton III and of energy conversion
The induction always tries to stop the motion or change in the field.
Aluminum Ring
The ring moves away as the induced current is preventing more induction
Mutual induction
• Induction in a second circuit caused by changes in a first circuit
• Main use in a transformer• As the current changes the
field changes giving a EMF in the second circuit.
TransformersThis how A.C. changes voltage up or down
V In
V Out
Turns 2
Turns 1=
Self Induction
• property whereby an electromotive force (EMF) is induced in a circuit by a variation of current in the circuit its self
D.C. SourceCurrent Back EMF
Another example on LENZ’S LAW
Flux Density• Magnetic flux, represented by the
Greek letter Φ (phi), total magnetism produced by an object. The SI unit of magnetic flux is the Weber
• Magnetic field (B) is the flux through a square meter (the unit of magnetic field is the Weber per square meter, or Tesla.)
As the flux expands the density through any square meter decreases