ELECTRICITY!!

• Electricity – From the word “Elektron” – Greek for “amber”– Electricity is simply the flow of electrons.

“Laws of Attraction”

• Opposite Charges Attract

• Like Charges Repel

+ -

+ +

Question:

If you remove the 2 batteries from a working flashlight and reinstall them backward so that they make good contact inside, will the flashlight still work?

Question on batteries

Observations About Flashlights(and electrical circuits)

• They turn on and off with a switch

• More batteries usually means brighter

• The orientation of multiple batteries matters

• Flashlights dim as batteries age

Observations about flashlights

• battery,

• switch,

• light bulb,

• wire.

For a functioning battery we need:

• An electrical current (electrons) runs through all the parts of the circuit (close circuit).

• No current flows when switch is open (open circuit).

• Electrons carry energy from batteries to the bulb.

• Short circuit: A path (short cut) in which the light bulb is cut out.

In a flashlight we are creating an:

Electrical circuit

A Battery

• Battery “pumps” charge from + end to – end– Chemical potential energy is consumed– Electrostatic potential energy is produced

• Current undergoes a rise in voltage– Alkaline cell: 1.5 volt rise– Lead-acid cell: 2.0 volt rise– Lithium cell: 3.0 volt rise

• Chain of cells produces larger voltage rise

Battery

A Light Bulb

• Structure– Contains a protected tungsten filament– Filament conducts electricity, but poorly

• Filament barely lets charge flow through it– Electrostatic potential energy (voltage) is consumed– Thermal energy is produced

• Current undergoes a drop in voltage– Two-cell alkaline flashlight: 3.0 volt drop

A Light Bulb

Electric Current Water Analogy

h V

water flow I

Current: number of electrons passing through per second

Water analogy: number of water molecules passing through per second

Electric Current Water Analogy

I

Electric current --> water

What determines the current through the circuit (Load)?

Ohm’s LawV = I·R or

I = V/R

So, 10V across a 100ohm load = 0.1 AmpWhere 1 Amp = 1 coulomb/sec = 6.25 x 1018 e/sec1Amp=62,500,000,000,000,000,000 electrons/sec

IV

R

What determines the current through the circuit (Load)?

Examples

2. A bulb in a lamp that is connected to a household outlet has a resistance of 100 ohms. What current flows through it?

3. Your skin has a resistance of about 106 to 104 ohms (dry) and 103 ohms (wet) . What current runs through you when you stick your finger in an outlet (conduction to ground)?

1. A battery can produce 1.5 V. When connected to a light bulb a current of 2 A (Ampere) runs through the bulb. What is the resistance of the bulb?

examples

• The severity of an electric shock depends on the magnitude of the current, how long it acts and through what part of the body it passes.

• Can feel ~ 1 mA; pain at a few mA; severe contractions above 10 mA; heart muscle irregularities above 70 mA.

• Resistance of dry skin ~ 104 to 106 W; wet skin 103 W or less.

• A person in good contact with ground who touches a 120 V

line with wet hands can suffer a current mAV

I 1201000

120=

Ω=

Electric shockElectric shock

Positive Charge

• Current points in the direction of positive flow

• Flow is really negative charges (electrons)

• It’s hard to distinguish between:– negative charge flowing to the right

– positive charge flowing to the left

• We pretend that current is flow of + charges

• It’s really – charges flowing the other way

A word about the sign convention….

Positive Charge

Electrical Resistance

• So, we hook up a wire between the terminals on a battery

• The question is, how much current flows?

• We have lots of free electrons able to move in the metal composing the wire

• So, how much charge moves past any point in the wire per second?

Electrical Resistance

• The amount of current that flows is determined by how much resistance there is to the flow of the charges

• What makes this resistance?

• Collisions of the electrons with each other and with the “stationary” atoms of the metal

• Atoms are actually vibrating in the lattice

Electrical Resistance

• Resistance depends on the structure of the material and the temperature

• The higher the temperature, the more the atoms vibrate and the more electrons make collisions with the atoms

• How much current flows was discovered by George Ohm, and we honor him by naming the unit of resistance an ohm

Ohm’s Law

• Ohm’s discovery was that current is proportional to voltage and inversely proportional to resistance

current =voltage

resistance

amperes=voltsohms

Ohm’s Law

• Three forms are convenient

I =VR

V =IR

R=VI

Electric Shock

• We all know not to stick our finger into a light bulb socket or to drop an electrical appliance into our bath tub

• What causes the damage to our bodies?• It is the amount of current that flows

through the body that can cause problems• Human skin has resistance ranging from

100 ohms to 500,000 ohms (wet to dry)

Electric Shock

• We can use Ohm’s Law to calculate current based on the size of the applied voltage

• You can feel 0.001 ampere (1 milliampere)

• 0.005 amperes hurts

• 0.010 amperes causes muscle spasms

• 0.015 amperes loss of muscle control

• 0.070 amperes disrupts heart rythyms (fatal)

Electric Shock

• There must be a potential difference between one part of your body and another

• You must become a conductor of electricity

• OK for birds to sit on a 5000 V transmission line as long as no part of the bird touches something else

• Squirrels get across the transformers

Power

• Power is energy per unit of time

• Power is measured in joules/second or watts

• Batteries are power sources

• Loads are power consumers

Battery Powerpower produced by the battery

• Current: units of charge pumped per second

• Voltage rise: energy given per unit of charge

P = Vrise·I

current · voltage rise = power produced

IVrise

Load Power

• Current is units of charge passed per second

• Voltage drop: energy taken per unit of charge

current · voltage drop = power received

P = Vdrop·II

Vdrop

ElectrostaticsElectrostaticsthe study of electrical

charges at rest

ElectrodynamicsElectrodynamicsthe study of electrical

charges in motion

Two oppositeopposite types of charge exist, namedpositivepositive and negativenegative by Benjamin Franklin.

Charge is aproperty of

matter.

charges

Charged particlesCharged particles exist in atoms. exist in atoms.

ElectronsElectrons are responsible are responsiblefor for negativenegative charge; charge;

protonsprotons for for positivepositive charge; charge;neutronsneutrons have have nono charge. charge.

ONLY ELECTRONS MOVE!!!!ONLY ELECTRONS MOVE!!!!

Small amountsSmall amounts of ordinary matter contain of ordinary matter containincredible amountsincredible amounts of subatomic particles! of subatomic particles!

When charged particles near one another,They give rise to two different forces.

A force is a push or a pull on an object.

Basic Law of ElectrostaticsBasic Law of ElectrostaticsLike charges repel; unlike charges attractLike charges repel; unlike charges attract

Link

Electrostatic force

An electric field extends outward through space from An electric field extends outward through space from every charged particle. When a charged particle moves every charged particle. When a charged particle moves

into the electric field of another, it is either pushedinto the electric field of another, it is either pushedor pulled.or pulled.

Field lines Field lines point away from positivepoint away from positive and and toward negative chargestoward negative charges..

field

Static Electricity: The buildup of electric charges on an object.

A Neutral object gains an electric charge when it gains or loses electrons.

Three Methods of Charging an Object1. Friction - Rubbing two objects together2. Conduction-touching two objects together.3. Induction-rearrangement of charges on the object.

static

Charging objects by friction.Charging objects by friction.

Example: Charging a balloon by rubbing it on your hair and Example: Charging a balloon by rubbing it on your hair and then sticking it to a neutral wall.then sticking it to a neutral wall.

Conduction-Touching of ObjectsCertain Materials permit electrical charges to flow freely through them.

Conductors: Metals, Earth

Semi-Conductors: Silicon, Germanium

Non-Conductors: rubber, glass, wood, plastic

INDUCTIONcharging without touching of objects

charging a rodrod and electroscopeelectroscopepositivelypositively and negativelynegatively

by conductionconduction and inductioninduction

When charging by conductionconduction,the rod touchestouches the electroscope.

The electroscope gets the same chargesame charge as the rod.

When charging by inductioninduction, the rod does notdoes nottouchtouch the electroscope. The electroscope gets

the opposite chargeopposite charge of the rod.

electroscope

Go Go here , here, here, and here to view simulations of charging to view simulations of charging an electroscope. Read more an electroscope. Read more here.

links

COULOMB’S LAWCOULOMB’S LAWThe force between two charged objects isThe force between two charged objects is

directly proportional to the product of theirdirectly proportional to the product of theircharges and inversely proportional to theircharges and inversely proportional to their

separation distance squared.separation distance squared.

link1, link2, link3, link4, link4

In equation form:In equation form:

F =

FF is the is the force of attractionforce of attraction, measured in , measured in NEWTONSNEWTONS,,between charges qbetween charges q11 and q and q22

k

kk is the is the Universal Electrostatic ConstantUniversal Electrostatic Constant, equal to , equal to

9.00 x 109.00 x 1099 N N mm22/coul/coul22

q1 q2

qq11 and and qq22 are the are the attracting chargesattracting charges, measured in, measured in

CoulombsCoulombs

d2

dd is the is the distance between the chargesdistance between the charges, , and is measured in and is measured in METERSMETERS

The SI unit of chargeThe SI unit of chargeis the is the CoulombCoulomb,,

named in honor ofnamed in honor ofCharles Augustin Coulomb..

1 C = charge on 1 C = charge on 6.25 x 106.25 x 101818 electrons (or protons) electrons (or protons)

1 e1 e-- = = 1.60 x 101.60 x 10-19-19 CoulCoul = elementary charge = elementary charge

Electric force is a Electric force is a vectorvector and must be treated as such. and must be treated as such.

Electric FieldsElectric FieldsAn electric field exists in a regionAn electric field exists in a regionif space if a charge placed in thatif space if a charge placed in that

region experiences an electric force.region experiences an electric force.

The magnitude of an electric field at any The magnitude of an electric field at any given point is defined to be the ratio ofgiven point is defined to be the ratio ofthe force on athe force on acharge at thatcharge at thatpoint to the point to the amount of charge.amount of charge.

E = F/QElectric field strength has unitsElectric field strength has units

of Newtons/Coulomb (N/C).of Newtons/Coulomb (N/C).

Click Click here to view a simulationshowing the magnitude and direction of theshowing the magnitude and direction of the

electric force on a test charge whenelectric force on a test charge whenplaced near other charges.placed near other charges.

Click Click here to view a simulation of acharged particle moving through a regioncharged particle moving through a region

occupied by other charges.occupied by other charges.

Voltage Sources

• If we want to move charge from one place to another, we must apply a force to make it move

• Another way of thinking about this is to say that we must give the charges some potential energy

Voltage Sources

• We can give an object gravitational potential energy by lifting it into the air

• We have to do work on the object to lift it into the air

• Doing work uses energy

• Conservation of Energy

Voltage Sources

Voltage Sources

• We need an pump that pumps charges!• The simplest charge pump is a battery• It uses chemical reactions to separate

charges and thus create electrical potential energy

• More convenient to talk about PE/charge or electric potential measured in volts

• So, a battery is a kind of electric pump

Voltage Sources

• Another kind of source is a generator

• A generator converts mechanical energy into electrical energy

• In any case, what we need is a device to separate charges!!!

Electric Current

• The movement of charge is called a current

• Metals have some electrons that are not needed to bond the atoms together in the solid

• They are pretty free to just roam about the material and are not associated with any individual metal atom

Electric Current

• Compare to the flow of water in a hose

Electric Current

• We measure the flow of water in gallons/minute

• We measure the flow of charges in coulombs/sec

• 1 coulomb/sec = 1 ampere

• Remember this is 6.25 x 1018 electrons moving past a point in a wire per second

Electric Current

• In a wire, the electrons actually move quite slowly, less than 0.01 meters/sec

• However, their electric field moves at the speed of light!!!

• So, we can send signals down a wire very quickly, because the information moves at the speed of the changing electric field which is at the speed of light

Electric Current

+ -

Electron flow

Electric Current

• In the 1700’s people figured out that charges could move

• They had two choices– Positive charge moves– Negative charge moves

• They guessed WRONG!!!!!

• We know electrons move

Electric Current

• Hook up a battery and electrons flow from minus terminal to plus terminal

• Ancients defined current as positive charge flow

• Direction of “current” is from plus to minus

Electric Potential DifferenceElectric Potential Difference

the change in the change in electric potential energyelectric potential energyper unit chargeper unit charge V = W/Q

The SI unit of electric potentialdifference is the VOLTVOLT, named in

honor of Alessandro Volta.

One One VOLTVOLTis the is the electric potential differenceelectric potential difference

between two points when between two points when one Jouleone Joule of work of workis done in moving is done in moving one Coulombone Coulomb of charge of charge

between the points.between the points.

the flow ofthe flow of charged particlescharged particles;

can be positive or negative,but usually negative (electrons)

through a conducting metal

current

electric cell - a device thatconverts one form of energy

to electrical energy

Chemical cells convert chemicalenergy into electrical energy.

Chemical cells can be“wet” or “dry”.

SolarSolar cellscells convert light energyinto electrical energy.

A generatorgenerator converts mechanicalenergy into electrical energy.

batterybattery - two or more cellsconnected in series or in parallel

Electric current is measuredin Amperes, in honor of

Andre Marie Ampere.

One Ampere is the flow ofOne Ampere is the flow ofone Coulomb of charge per second.one Coulomb of charge per second.

1 Amp = 1 Coulomb per second = 1 C/s1 Amp = 1 Coulomb per second = 1 C/s

II = QQ/tt

AmmeterAmmetera device that measures current

VoltmeterVoltmetera device that measures

electric potential difference

powerpower = = workwork//timetime

= (= (workwork//chargecharge))..((chargecharge//timetime))

== electric potential differenceelectric potential difference ..

currentcurrent

PP ( (WattsWatts) = ) = VV ( (VoltsVolts))..II ( (AmpsAmps))

Analogies of simple circuits are these links:Analogies of simple circuits are these links:

Water circuit analogy Water circuit analogy linkAir flow Air flow linkVarious Various link

Teaching with Analogies Teaching with Analogies link1, link2

ResistanceResistance

determines the amount of current flowdetermines the amount of current flow

= the ratio of potential difference to current= the ratio of potential difference to current

RR =VVII

The SI unit of resistance is the OhmOhm, WW, namedin honor of Georg Simon Ohm.

One OhmOne Ohm of resistance is the resistancesuch that one Voltone Volt of potential differenceis needed to obtain a current of one Ampone Amp.

The resistance of a circuit element depends on:

1. the length of the conductorthe length of the conductoras length increases, resistance increases proportionally

2. the cross-sectional area of the conductorthe cross-sectional area of the conductoras area increases, resistance decreases proportionally

3. the resistivity of the conductorthe resistivity of the conductoras resistivity increases, resistance increases proportionally

ResistivityResistivityThe resistivity, r, of a conductor is equal to the

resistance of a wire 1 cm long and having a cross-sectional area of 1 cm2.

RR

R = resistance, measured in OhmsR = resistance, measured in Ohms

= rr

r = resistivity, usually in units of Wr = resistivity, usually in units of W ..cmcm

ll

l = length, measured in cml = length, measured in cm

AA

A = cross-sectional area, measured in cmA = cross-sectional area, measured in cm22

Ohm’s LawOhm’s LawThe ratio of potential difference to currentThe ratio of potential difference to current

is constant.is constant.

If R = V/I is a constant valueIf R = V/I is a constant valuefor a given resistor, then thatfor a given resistor, then that

resistor is said to obey Ohm’s Law.resistor is said to obey Ohm’s Law.

Click Click here and here to link to pages describing resistorcolor codes.color codes.

Click Click here and here to runsimulations of Ohm’s Law.simulations of Ohm’s Law.

Many circuit elements do notMany circuit elements do notobey Ohm’s Law. Resistorsobey Ohm’s Law. Resistorsthat get hot, like light bulbsthat get hot, like light bulbsand heating elements, do notand heating elements, do notkeep a constant resistance. keep a constant resistance. Resistance generally increases as Resistance generally increases as objects become hotter.objects become hotter.

Resistor CircuitsResistor CircuitsSeries

1. 1. total resistancetotal resistance is the sum of the is the sum of the separate resistorsseparate resistors

RRTT = R = R11 + R + R22 + R + R33 + ... + ...

2. 2. currentcurrent is the same through each resistor is the same through each resistor

IITT = I = I11 = I = I22 = I = I33 = ... = ...

3. 3. total potential differencetotal potential difference is the sum of each is the sum of each

VVTT = V = V11 + V + V22 + V + V33 + ... + ...

In other words, in a series circuit,In other words, in a series circuit,resistance and voltage add,resistance and voltage add,but current stays the same.but current stays the same.

Resistor circuits

RR11

RR22

RR33

E = 12 VE = 12 V

RRTT = =

VVTT = =

IITT = =

RR11

RR22

RR33

V,V,VV

PPTT = =

R,R,WW

I,I,AA

P,P,WW

8.0

5.0

2.0

RR11

RR22

RR33

E = 12 VE = 12 V

RRTT = 15 = 15 ΩΩ

VVTT = 12 V = 12 V

IITT = 0.80 A = 0.80 A

RR11

RR22

RR33

V,V,VV

PPTT = 9.6 W = 9.6 W

R,R,WW

I,I,AA

P,P,WW

8.0

5.0

2.0

0.80

0.80

0.80

6.4

1.6

4.0

5.1

1.3

3.2

Parallel 1.1. reciprocal of the total resistance reciprocal of the total resistance is the is the sum of the reciprocals of the separate sum of the reciprocals of the separate resistorsresistors

1/R1/RTT = 1/R = 1/R11 + 1/R + 1/R22 +1/R +1/R33 + ... + ...2. 2. total currenttotal current is the sum of the current is the sum of the current through each resistorthrough each resistor

IITT = I = I11 + I + I22 + I + I33 + ... + ...3. 3. potential differencepotential difference is the same across is the same across each resistoreach resistor

VVTT = V = V11 = V = V22 = V = V33 = ... = ...

In other words, in a parallel circuit, resistance adds as reciprocals,

voltage stays the same, and current splits.

In other words, in a parallel circuit, resistance adds as reciprocals,

voltage stays the same, and current splits.

E = 12 VE = 12 V

RRTT = =

VVTT = =

IITT = =

RR11

RR22

RR33

R,R,WW

V,V,VV

I,I,AA

PPTT = =

RR33

RR11

RR22

P,P,WW

12

8.0

12

E = 12 VE = 12 V

RRTT = 3.42 = 3.42 ΩΩ

VVTT = 12 V = 12 V

IITT = 3.50 A = 3.50 A

RR11

RR22

RR33

R,R,WW

V,V,VV

I,I,AA

PPTT = 42 W = 42 W

RR33

RR11

RR22

P,P,WW

12

8.0

12

12

12

12

1.0

1.5

1.0

12

12

18

Go to Go to link1, , link2, , link3, , link4, , link5, and link6 to view, and link6 to viewpages and simulations examining pages and simulations examining

Kirchhoff’s Loop and Junction Rules.Kirchhoff’s Loop and Junction Rules.

Kirchhoff’s RulesKirchhoff’s Rules

Loop Rule: Loop Rule: The sum of the potential differencesThe sum of the potential differencesaround any closed circuit loop is zero.around any closed circuit loop is zero.

Junction Rule: Junction Rule: The sum of the currentsThe sum of the currentsinto any circuit junction is zero.into any circuit junction is zero.

The site linkedThe site linkedhere allows youhere allows youto build and test to build and test your own series,your own series,parallel, and/orparallel, and/orcombination circuits.combination circuits.

For a complete interactive tutorial onFor a complete interactive tutorial onelectricity and magnetism, go here.electricity and magnetism, go here.

POWER

THE RATE OF DOING WORK OR CONVERTING ENERGY.

Power = Work / Time

Power = Energy / Time

UNIT- WATT = JOULE PER SECOND

POWER

Watt Demonstration:Lift a 1-N mass a distance of 1 meter, and you have done 1 Joule of Work.Do this once each second, and your power output is 1 Watt.

POWER

Light bulbs are rated by the power used.

POWER

Power = Current × Voltage

What are the units for Current, Voltage, and Power?

Ampere = coulomb / second

Volt = joule / coulomb

POWER

What is a Volt X Ampere ?

Volt X Ampere = joule/coulomb X coulomb/second

Volt X Ampere = joule/second = watt

POWER

A PAIR OF LEMON BATTERIES GENERATED ABOUT 0.5

MILLIWATTS

AN XMAS BULB REQUIRES ABOUT 0.5 TO 1 WATT

POWER

ACTIVITY

HAND HELD GENERATOR

Electromagnetism

21-1 Magnetism from Electricity

• Electromagnetism – relationship between magnetism and electricity

• Oersted – (1820) Danish physicist who first discovered that an electric current produces a magnetic field.

Example of Oersted’s Discovery

• Solenoid – long coil of wire with a current passing through wrapped into many loops creating a magnetic field. Acts like a magnet when a current is passing through. Strongest at the ends – like a magnet.

• Increase the strength of the magnetic field:

1) increasing the amount of coils and/or

2) increasing the amount of current.

OR by adding magnetic material between the wires.

• An electro-magnet – solenoid with magnetic material (iron nail) inside the coils. Greatly increases the strength.

Strong temporary magnets

• Just as an electric current exerts a force on a magnet. A magnetic field exerts a force on an electric current (charged particles)

• Practical devices: Electric motors –

•converts electrical energy into mechanical energy. Contains a coil of wire connected to a cylinder (armature). Armature free to spin between the poles of a magnet.

Galvanometers – instruments used to detect small currents (coil of wire connected to an electric circuit and a needle)

Examples: ammeters and voltmeters

21-2 Electricity from Magnetism

•Faraday – 1831 Englishman – and Henry an American – first to prove that electricity can be produced from magnetism.•Electromagnetic Induction - Faraday noticed that changing the magnetic field produced an electrical current

– induced current.

Faraday’s experiment

Another experiment illustrating electromagnetic induction

• An electric current will be produced (induced) in a circuit exposed to a changing magnetic field.

• The direction of the current depends on the direction of the moving magnet.

•Generators – converts mechanical energy into electrical energy – opposite of a motor (electrical energy into mechanical energy)

•Consist of loops of wire connected to a rotating rod.

AC generator

DC generator

• In large generators turbines provide the mechanical energy to turn the axles. Either moving wind, water or steam turn the turbines.

• Wind power, hydroelectric power, nuclear power or electricity from burning of fossil fuels - coal and oil.

• Transformers – devices that increase or decrease the voltage of alternating current. In US 60 hertz

• Consists of 2 coils wrapped around an iron core. AC current passes through first coil, inducing a current in second wire.

• Step-up transformers – when primary coil has less loops than second coil. Voltage in second coil greater than first coil – step-up transformer (increases voltage)

• Step-down transformers – more loops in first coil, less in second – voltage in secondary coil less.

•Power can be conserved if voltage is stepped up before transmission and stepped down before being used by consumer

DC and AC

• Hook up a battery and we have direct current, i.e. current flows ALWAYS in one direction

• Modern generators in power plants reverse the polarity of the output terminals 60 times per second

• This is called alternating current• It is what comes out of the wall plug

DC and AC