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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