Transcript Document

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 attracts
small 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 positions
- electrons - tiny negatives
- - move quickly around nucleus
- some move between atoms
remove electrons - add electrons
transfer 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 hairelectrons transferred to balloon (friction)
balloon acquires negative charge
- ---
++ -++
+- no
+ -++ forces
--
force from balloon charge
attracts +, 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 second
acquires 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
+
+
e-
+
+
+
+
+
+
strongly held e-
conductor
loosely held eAlso conduct heat well move from atom to atom
path for e- to travel
from motion of eExample: wires - transport charge for use in circuits
Insulators - charge cannot freely move
+
+
+
+
+
+
+
+
no loose eget stuck on surface
poor heat conductors
Some materials have both properties
atmospheric air
nitrogen
Oxygen
Carbon dioxide
water (humidity)
Polar - act like
separated charge
+ GOOD
GOOD
INSULATOR
CONDUCTOR
damp day - charges leak off
water molecules form chains
to drain e- to ground
SEMICONDUCTORS - properties of both
normally insulators
add energy
loosely held states
energy from light, heat, electrical
used as switches - add energy for charge to flow
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
Electroscope - early device used to
measure charge
add charge here
metal leaves (gold)
spread apart when charged
-likes repel
-more charge, spread more
Methods to charge objects:
conduction and induction (and friction)
CONDUCTION – touch two charged objects
together to transfer charge
neutral
electroscope
spark
charge transferred
charge shared
leaves move apart
charge becomes evenly distributed
Charge by INDUCTION – two objects never actually touch
charge by using electric forces (induced charge)
NO DIRECT CONTACT
bring charged rod closepushes e- away
leaves separate
+
+++
-- --
e- try to get as
far away as possible
neutral
electroscope
still neutral
same number of + as -
+
+++
e-
connection to ground
e- can get even further
from charged rod
leaves fall
now positively charged
But still connected to ground
-
-
(Earth) Ground –
- reservoir of electrons
- can accept or donate
any number of ew/ no resistance
+ +
+ +
Remove the charged rod
+ redistribute
leaves separate for good
NET POSITIVE CHARGE
Break connection w/ ground
e- can’t go back
leaves try to get as far
away as possible
Separate because likes repel – like hair in Van de Graaf demo
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 (mC, 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
F
from calib
F=k q1q2 / d 2
stiff
wire
q1
empiricalbrute force
simpler model
q2
q1
d
q2
d
F = force (in N)
q1, q2 – charges (in C)
d - separation between charges (m)
k = 9x109 Nm2/C2
Coulomb constant
Coulomb actually measured!
Force is a vector – direction important
force acts along a line
joining two charges
F=k q1q2 / d 2
+ and +
or
_ and _
}
positive force
charges repel
+
}
negative force
charges attract
and
-
or just remember “opposites attract, likes repel”
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 m
model
qp= +1.6x10-19 C
proton–positive charge
+
equal to magnitude of e-
- q e = - 1.6x10
-19
C
d=5.3x10-11 m
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
Ben Franklin – first to experiment
with lightning
+ + +
+
- - -- - + + ++ +
water evaporates
ionized by high
velocity motion
F=k q1q2 / d 2
Large distance but
huge charge – big F
-- --+ +
+
Induces charge on objects
Puts force on cloud charges
greatest force for highest objects (d smaller)
Gigantic discharge – great amount of charge in cloud
causes destructive damage because
of energy stored
ground to cloud, or cloud to ground (depends on – charge)
lightning rod – sticks above buildings to attract charge
thick wire connects to ground
bypasses building to ground
destructive energy goes directly to ground
Heat lightning – lightning between clouds from a distance
Electric Batteries - galvanic cells
History - Galvani and Volta
observed 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
Led to idea of
(flow of electrons)
galvanic cell - battery
produces
electric current
Zn
C
positive terminal
negative terminal
stores chargeHook up to use
electrons can flow
+
+
+
+
+
+
-
discharge-dead
metals used up
e-
Electrolyte- conducting solution
Chemical work-energy to move e- from + to - terminal
provides energy
for electrical work
- light bulb heats
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
wires - no energy lost by e-
-+
-+
-+
Connected in series
} 3X voltage
of a single cell
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)
test mass
Example: gravity
Field points IN
-attractive force
-mass follows line
mass
Mass feels force
from touching field
ELECTRIC FIELD - positive test charge to measure
long distance force of charges
Positive charge will go:
outward
repulsive
Force along field lines
+
-
inward
attractive
Magnetism - acts between moving charges
- current
ANCIENT GREEKS
lodestone-natural magnet like magnetite
attracts small pieces of iron
Magnetic fields different from other forces
1. Field not in direction of force
force perpendicular to field
2. NO MAGNETIC MONOPOLES
-cannot isolate poles
North and South poles always paired
N
S
Field lines form closed loops!
point from N. Pole to S. Pole
CANNOT SPLIT POLES
N
S
N
Break apart get 2 magnets
both have N & S
S
SIMILARITIES:
Like poles repel, opposite poles attract
EARTH’S Magnetic Field
Motion of molten iron core
N
S
EARTH
N
S
Earth North Pole
N
S
Compass
S. Pole of compass
magnet points to
N. Pole of Earth
for navigation
Deflects solar wind - high energy
particles ejected from Sun
Magnetism from electricity
What causes magnetism?
Oersted A current (electron flow) causes a force on
a compass needle
SI UNIT
Current
I = Q / t (C/s=Ampere = 1 A)
how fast electrons are flowing in a wire
N
S
I (current)
N
S
S
N
S
N
Force perpendicular to
both magnet and current
Compass needle points around in
circle surrounding wire
magnetic field forms
circle around wire
A current exerts a force on a
permanent magnet!
Ampere - two currents exert forces on each other
no permanent magnets involved!
Magnetismhas to do with
moving charges
I 2
two wires are attracted
I 1
If currents opposite
repel
Also invented solenoid – electromagnet (wire coiled on bolt)
loop of wire produces
field through center
Coil intensifies
the magnetic field at the center:
Looks like bar magnet
magnetic domains
domain
Permanent magnets:
boundaries
Electrons in atoms move – electric currents
produce field
Atomic magnets line up in
magnetic materials:
iron, nickel, cobalt, etc.
Electricity from magnetism
Faraday : can magnetism produce electricity?
-built on Oersted’s & Ampere’s results
Coil and galvanometer
magnetic sitting in field - no current
take out - current flows
put in - current flows
Faraday’s Law of Induction
induced voltage and current produced by
changing magnetic field or circuit motion
in field
electromagnetic induction
Dynamo - electric generator
uses mechanical energy
to produce electricity
turbine 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 charge
from induced current (generator) or battery
I = charge passing a certain point
= Q / t = J/s (Ampere)
time
Historically: Ben Franklin(first to experiment with electricity)
Wrongly assumed + charges move
conventional current -still used today
Actually - charges move in typical circuits - + fixed
current is flow of electrons in wire
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) / q
SI: J/C = Volt (V)
provides energy to circuits!
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- make collisions w/ atoms
in wire
-does not accelerate
-lose energy
-move at a very small
E speed (drift velocity)
Electric field moves at speed of light
electrons 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 (W)
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
- temperature
superconducting @ low T - no R!
How current flows determines how circuits work!
Combinations of resistances
most circuits are combinations
R
of resistances
and batteries
V
+ and wiresconnections with no resistance
Two ways to combine resistors:
SERIES COMBINATION - same current
thru each resistor
R1
R2
R3
Req
I
V
equivalent
circuit
Equivalent - Total - Combined Resistance:
V
total bigger
than individual
Req = Rtot = R1 + R2 + R3
looks like a longer resistor
-each will resist current
Can analyze I-V characteristics of circuit
How much I
with Ohm’s Law V = I Req
battery life
Parallel Combination of resistors
Divided circuit in which the current can
travel in multiple paths
same potential difference
R1
across each component
R2
Req
R3
V
equivalent
circuit
V
Combined Resistance:
Total smaller
than individuals
1/Req = 1/R1 + 1/R2 +1/ R3
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 W, 8 W,
and 12 W 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 W.
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 HEATING
JOULE’S LAW - wires heat up as current
V
flows
2R
P=
I
A
Joule’s
Experiment
***remember power=(work energy)
time
more current e- make more collisions
higher resistancemore energy lost to atoms
material impedes flow
2R
P
=
I
Can rewrite with
= V2/ R
Ohm’s Law
=IV
(V=IR)
most 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 examples:
A radio uses 0.5 A through a resistance of 6 W
During operation. How much power is consumed?
A 3 W lightbulb is connected is connected to a 120 V
Source of potential difference. How much power
is used?
Joule heating used in many
electrical applications
-hair dryer
-space-heater
-toaster
-stove
-lightbulb - filament heated to > 2500oC
Heat generated also a problem
Broken cord: loose connection
high resistance
heat
Short circuit: bypasses load
large current
heat
P = I 2R
I=V/R
Power Stations provide current to homes
Called power station because it provides
current and voltage
Don’t pay for power
Pay for energy!
kilowatt-hour meter
E=Pt
Safety device to limit dangerous current
fuse- filament heats up too much
and will melt
I from
I to
plant
house -connection to current
source broken
-circuit breaker similar
Low melting point conductor
Voltage lost as current
travels along power lines
Joule heating
TRANSFORMER
steps up the voltage
But at the expense of the current
Constant power device P=IV
increase V, decrease I
Changes voltage by
primary coil
secondary coil