Chapter 6 - Electricity (& Magnetism)Electricity - deals with interactions

between electric charges

* causes forces motion

* two types of charges:

+ positive proton

- negative electron

Ancient Greeks - rub amber and it attractssmall objects

electron - from Greek for “amber”Law of Electric charges - basic law of interaction

“opposites attract, likes repel”Where do charges come from? Atomic Theory - smallest particles of nature

Neutral atom-

-

-

-

-

-++++++ -

+nucleus - made of protons - fixed positionselectrons - tiny negatives - move quickly around nucleus - some move between atoms

remove electrons - add electronstransfer charges between objects

Charges are transferred between objects

ions - charged atoms atom acquires extra electron - negative ion loses an electron (to another atom) - positive ion

Rub balloon on your hair-electrons transferred to balloon (friction)balloon acquires negative charge

+ -+++

++++-

--

---

- ------

no forces

force from balloon chargeattracts +, repels - attracted to balloon

Induced charge - uses law of electric charges to separate charge

LAW OF CHARGE CONSERVATION - when one body acquires a charge from another, the secondacquires an equal and opposite charge from the first

-net charge in universe constant-charge neither created nor destroyed

charges don’t just appear out of nowhere!

Electrical properties of materials

Two general behaviors of matter regarding electricity:how they act in the presence of charge

Conductors - transmit charge readily

+ + + +

++++

+ fixed nucleus

loosely held e-move from atom to atompath for e- to travel

strongly held e-

conductor

Also conduct heat well from motion of e-

Example: wires - transport charge for use in circuits

Insulators - charge cannot freely move

+ + + +

++++

no loose e-

get stuck on surface

poor heat conductors

e-

Electrostatics - charge is confined to an object

- charge assumed not moving

- static electricity - accumulated charge at rest

like charge on balloon

or charge on your body from walking

Some materials have both properties

atmospheric airnitrogen water (humidity)OxygenCarbon dioxide

GOOD INSULATOR

GOOD CONDUCTOR

Polar - act like separated charge

+ -

damp day - charges leak offwater molecules form chains to drain e- to ground

SEMICONDUCTORS - properties of bothnormally insulatorsadd energy loosely held statesenergy from light, heat, electricalused as switches - add energy for charge to flow

Electroscope - early device used to measure charge

metal leaves (gold)spread apart when charged

-likes repel-more charge, spread more

add charge here

Methods to charge objects:conduction and induction (and friction)

CONDUCTION – touch two charged objects together to transfer charge

--

--- -

--

neutral electroscope

sparkcharge transferred

--

-- -

- --

charge sharedleaves move apart

charge becomes evenly distributed

leaves try to get as far away as possible

Separate because likes repel – like hair in Van de Graaf demo

Charge by INDUCTION – two objects never actually touchcharge by using electric forces (induced charge)

NO DIRECT CONTACT

--

--- -

--

neutral electroscope

+++ +

----

bring charged rod close- pushes e- away leaves separate

e- try to get as far away as possible

still neutral same number of + as -

--

--- -

--

+++ +

--

- -

connection to ground e- can get even further from charged rod

leaves fall(Earth) Ground – reservoir of electrons can accept or donate any number of e- w/ no resistance

now positively chargedBut still connected to ground

+

+

+

+Break connection w/ ground

e- can’t go back

--

--- -

--

Remove the charged rod + redistribute leaves separate for good

NET POSITIVE CHARGE

e-

ELECTRIC forces between charges

CHARGE – physical quantity; described by the Coulomb

SI UNIT : for charge (Q,q) Coulomb (C)

actually very large charge, 10-6 C on a balloon (C, nC) FUNDAMENTAL CHARGE

electron (e-) charge = 1.6 x 10-19 C cannot transfer less than 1 e- to charge objects all charge in multiples of an electron – fundamental

charge not continuous

Coulomb’s Law - forces on charges

stiffwire

dq1

q2

Ffrom calib

F=k q1q2 / d 2

q1 q2

d

simpler model

F = force (in N)

q1, q2 – charges (in C)

d - separation between charges (m)

k = 9x109 Nm2/C2

Coulomb constantCoulomb actually measured!

empirical-brute force

Force is a vector – direction important

F=k q1q2 / d 2

+ and +or

_ and _ } positive forcecharges repel

+ and - } negative forcecharges attract

or just remember “opposites attract, likes repel”

force acts along a line joining two charges

Example: What is the electric force between an

electron and proton in a hydrogen atom,

spaced about 0.53 A apart?

1 A = 10-10 mmodel

+ -

d=5.3x10-11 m

proton–positive charge equal to magnitude of e-

qp= +1.6x10-19 Cq e = - 1.6x10-19 C

Another example: A balloon charged to 3.4x10-5 C is

located 2.6 m from a can charged at -5.6x10-5 C. What

is the direction and magnitude of the force between them?

Application: Lightning – electric discharge from clouds

water evaporatesionized by high velocity motion

+

-

+

+

+

++ ++

+

------

- - --

F=k q1q2 / d 2

Induces charge on objectsPuts force on cloud charges

greatest force for highest objects (d smaller)

+++

Ben Franklin – first to experiment with lightning

Large distance but huge charge – big F

Gigantic discharge – great amount of charge in cloudcauses destructive damage because of energy storedground to cloud, or cloud to ground (depends on – charge)

lightning rod – sticks above buildings to attract chargethick wire connects to groundbypasses building to grounddestructive energy goes directly to ground

Heat lightning – lightning between clouds from a distance

Electric Batteries - galvanic cells

History - Galvani and Voltaobserved frog leg twitch in presence of dissimilar metals

Galvani: “animal electricity”stored electricity released when tissue touches metal

Volta: dissimilar metals in contact through a solution produce a current

(flow of electrons)

ZnC

Led to idea of galvanic cell - battery

Electrolyte- conducting solution

Zn+2

Zn+2

Zn+2

++

+++

+

--

-

--

-

produces electric current

Zn+2

e-

positive terminal negative terminal

stores charge-Hook up to useelectrons can flow

discharge-deadmetals used up

Chemical work-energy to move e- from + to - terminal

+-

e-

e- uses energy as it goes from - terminal to + terminal

battery used up when metal used up

RECHARGABLE - able to reverse chemical process

lithium ion, NiCad, wet/dry cell, fuel cells, solar

+- +- +-

POTENTIAL DIFFERENCE - “voltage”Describes amount of chemical energy available to charge

V = Work/q work per charge J/C SI Volt (V) how much work a charge is able to do

related to chemical work (potential) PE or Work W=qV

Increase battery :voltage (potential)add more galvanic cells

} 3X voltageof a single cell

Connected in series

wires - no energy lost by e-

provides energy for electrical work - light bulb heats

FORCE FIELDS - visual representation of invisible“action-at-a-distance”interactions

-shows lines of force - extends all thru space

- force on object in direction of lines

- measure with test particle (field map)

Example: gravitytest mass

massField points IN -attractive force -mass follows line

ELECTRIC FIELD - positive test charge to measurelong distance force of charges

Mass feels force from touching field

-+

inwardattractive

outwardrepulsive

Positive charge will go:

Force along field lines

Magnetism - acts between moving charges - current

ANCIENT GREEKSlodestone-natural magnet like magnetiteattracts small pieces of iron

Magnetic fields different from other forces1. Field not in direction of force force perpendicular to field2. NO MAGNETIC MONOPOLES -cannot isolate poles

North and South poles always paired

S

N Field lines form closed loops! point from N. Pole to S. Pole

CANNOT SPLIT POLES

S

NS

N

Break apart - get 2 magnets both have N & S

SIMILARITIES: Like poles repel, opposite poles attract

EARTH’S Magnetic Field

EARTH

N

S N

S

SN

Compass S. Pole of compass magnet points to N. Pole of Earth

Motion of molten iron core

Deflects solar wind - high energy particles ejected from Sun

for navigationEarth North Pole

NSN

S

Magnetism from electricity

What causes magnetism?

Oersted A current (electron flow) causes a force ona compass needle

SI UNITCurrent I = Q / t (C/s=Ampere = 1 A)

how fast electrons are flowing in a wire

Compass needle points around in circle surrounding wire

magnetic field forms circle around wire

NS

NS

I (current)

A current exerts a force on a permanent magnet!

Force perpendicular to both magnet and current

Ampere - two currents exert forces on each other

I 1

I 2

two wires are attracted

If currents opposite repel

Also invented solenoid – electromagnet (wire coiled on bolt)

Magnetism-has to do with moving charges

no permanent magnets involved!

loop of wire produces field through center

Coil intensifies the magnetic field at the center:Looks like bar magnet

Permanent magnets:Electrons in atoms move – electric currents

produce field

Atomic magnets line up in

magnetic materials:

iron, nickel, cobalt, etc.

magnetic domainsdomain

boundaries

Electricity from magnetismFaraday : can magnetism produce electricity?

-built on Oersted’s & Ampere’s resultsCoil and galvanometer

magnetic sitting in field - no current

take out - current flows

put in - current flows

Faraday’s Law of Inductioninduced voltage and current produced by

changing magnetic field or circuit motion

in field electromagnetic inductionDynamo - electric generatoruses mechanical energy to produce electricityturbine turns circuit in magnet water wheel, steam. Nuclear

Produces current- electricity force electrons through a circuit

Applications of Electromagnetism

Electric meter - detects flowing currents “galvanometer” -coil wound on on pointer needle -force when current flows in magnet -force bigger when current larger

use to measure I, V, and R

Electromagnetic Switch (Relay) -small switch closes to produce small current in

solenoid

-solenoid produces magnetic field to pull in metal contact so larger current can flow

Telephone -receiver - carbon granules compress with diaphragm changing resistance

-changes current which is transmitted

Speaker -current changes in magnetic field -force on coil moves cone

Electric Motor

-converts electrical energy to mechanical energy

-rotating electromagnet spins in stationary magnetic field

-electromagnet current changes direction to maintain rotation (always repels in magnet)

-armature and commutator change current

-generator in reverse

+-

Electric currents provide electrical work

Electric current - flow of chargefrom induced current (generator) or battery

I = charge passing a certain pointtime

= Q / t = J/s (Ampere)

Electric field in wires forces e- to go from - to + .Does work on electrons - gives them energy

POTENTIAL DIFFERENCE - energy/charge available to electrons - “voltage”

V=work/charge = (Work Energy) / qSI: J/C = Volt (V)

provides energy to circuits!

Historically: Ben Franklin(first to experiment with electricity)

Wrongly assumed + charges move

conventional current -still used today

Actually - charges move in typical circuits - + fixedcurrent is flow of electrons in wire

Example : Car battery

A 12 V car battery is used to start a car. If 1x109

electrons go from the negative terminal to the

positive terminal, then how much work is done?

charge equivalent: 1 e- = 1.6x10-19 C

V = W/q W = qV

current flow in wires

E

e-

e- make collisions w/ atomsin wire

-does not accelerate

-lose energy

-move at a very small

speed (drift velocity)

Electric field moves at speed of lightelectrons move very slowly (hours to

from switch to light socket)

Large number of charges (1015) produce current - drip out like full water hose

George Simon Ohm - how current flows in conductors

V+-

A

Current depends on potential difference (V)

OHM’S LAW

I=V/ R

R - resistance to a flow of current

how difficult it is to pass a current

Resistance (R) SI: Volt/Amp = ohms ()how energy is lost - flow of electrons impeded

depends on:

- type of material (copper, gold, graphite)

- length of wire - longer, more resistance

- cross-sectional area thinner wire, more resistive

less charge can flow

- temperaturesuperconducting @ low T - no R!

How current flows determines how circuits work!

Combinations of resistancesmost circuits are combinations of resistances

and batteries

and wires-connections with no resistance

R

V

Two ways to combine resistors:

SERIES COMBINATION - same current thru each resistor

V

I

Equivalent - Total - Combined

Resistance:

Req = Rtot = R1 + R2 + R3

V

R1 R2 R3

equivalent circuit

Req

looks like a longer resistor -each will resist current

Can analyze I-V characteristics of

circuit with Ohm’s Law V = I Req

How much I battery life

total bigger than individual

+ -

Parallel Combination of resistorsDivided circuit in which the current can travel in multiple paths

same potential difference across each component

R1

R3

R2

V

Vequivalent circuit

Req

Combined Resistance:

1/Req = 1/R1 + 1/R2 +1/ R3Total smaller than individuals

must take reciprocal for Req

“path of least resistance” - most of the divided current will go through resistor with the smallest resistance

For parallel, current can bypass broken circuit (burned out) elements

Christmas lights - will stay lit even if one light burns out

Home outlets wired in parallel

Example : light bulbs

1. Three light bulbs with resistances of 5 , 8 ,

and 12 are connected in parallel across a

5 V battery.

a) What is the total (combined, equivalent)

resistance of the combination?

b) How much current is drawn from the battery?

REMEMBER for parallel : flip for resistance

2. Three light bulbs are connected in series across

a 20 V battery. The resistance values of the light

bulbs are all 5 a)What is the equivalent resistance of the combination?

b) What is the current flowing thru the circuit?

Heat Power of Currents

Collision of electrons with atoms

- hit atoms

- atoms vibrate (gain energy)

-heats wire- JOULE HEATINGJOULE’S LAW - wires heat up as current

flows

A

V

P= I2 R ***remember power=(work energy)

time

Joule’s Experiment

Can rewrite with Ohm’s Law

(V=IR)

P = I2R= V2/ R

= I Vmost general

Example: car revisited

How much energy is used to start a car?

The car uses 10 A for 4 second with a 12 V car battery.

more current - e- make more collisionshigher resistance-more energy lost to atomsmaterial impedes flow

Joule heating used in many electrical applications

-hair dryer-space-heater-toaster-stove

-lightbulb - filament heated to > 2500oC

More examples:

A radio uses 0.5 A through a resistance of 6 During operation. How much power is consumed?

A 3 lightbulb is connected is connected to a 120 VSource of potential difference. How much power is used?

Heat generated also a problem

Broken cord: loose connectionhigh resistance heat

Short circuit: bypasses loadlarge current heat

P = I2R

I=V/R

Power Stations provide current to homesCalled power station because it providescurrent and voltage

Don’t pay for powerPay for energy!kilowatt-hour meterE=Pt

Safety device to limit dangerous current

fuse- filament heats up too muchand will melt-connection to current source broken-circuit breaker similar

Low melting point conductor

I from plant

I tohouse

Voltage lost as current travels along power lines

Joule heatingTRANSFORMER

steps up the voltageBut at the expense of the current

Constant power device P=IV increase V, decrease I

Changes voltage by primary coil

secondary coil