Physical Science
Electricity and
Magnetism
Slides subject to change
Electric Charge
Charge is a fundamental quantity (just as
mass is a fundamental quantity).
 Positive and negative charges.
 Charge is measured in coulombs (C).
 Elementary charge e = 1.6x10–19 coulombs.
 At atomic level
 protons +e
 electrons −e
 neutron no charge

Atomic Level
Most atoms have equal number of
electrons and protons.
 Hydrogen 1 proton, 1 electron
 Uranium 92 protons, 92 electrons
 If unbalanced number of charges, atom is
called an “ion.”

Sodium ion, one less
electron than usual
Na+
Chlorine ion, one
more electron than
usual
Cl–
Forms sodium
chloride, “salt”
Exercise
If an object has 1 million more electrons
than protons. What is the net charge?
Given
Formula
N = 1x106 excess electrons
q = Ne
e = charge per electron
= –1.6x10–19 C

q = Ne = (1x106)(–1.6x10–19)
= –1.6x10–13 C
Static Electricity
Excess charge.
 zzzzzt; crackle!

Benjamin Franklin

In 1750 Franklin proposed an
experiment to prove that lightning is
electricity—fly a kite in a storm that
appeared capable of becoming a
lightning storm.

May 10, 1752 Thomas-François Dalibard of
France conducted Franklin's experiment using a
40-foot-tall iron rod instead of a kite, and he
extracted electrical sparks from a cloud.
Physics Backfires


Prof. Georg Wilhelm
Richmann electrocuted
July 26, 1753, following
Franklin's experiment.
Electrocuted in St.
Petersburg, Russia,
while "trying to quantify
the response of an
insulated rod to a
nearby storm."
Current

When charge “q” flows, we have current “I”
measured in amperes (A):
I = q/t
Current is the rate of flow of electric
charge (units are coulombs/second or
amperes).
 Metal is good electrical conductor, other
materials such as glass are poor
conductors (called “insulators”).

Exercise

If 0.16 coulombs of charge are released
across a wire in 0.10 s, what is the
current?
Given
Formula
 q = 0.16 C
I = q/t
 t = 0.1 s
I = q/t = (0.16) / (0.10)
= 1.6 ampere
Coulomb’s Law

Force between charges:
F = k q1q2
r2
k = 9.0x109 N-m2/C2
 Electric force is proportional to the charges
on the objects and inversely proportional
to the square of the distance between
them.

Voltage
Amount of work it takes to move a charge
of 1 coulomb between two points.
 Measured in volts (V).
 A battery supplies this work by electrochemical action.

1.5 V
9V
12 V
Schematic
Resistance
Opposes current flow.
 Metals offer little resistance, insulators
great resistance, semi-conductors
moderate resistance.
 Units are called ohms (Ω).

Schematic
Ohm’s Law

Voltage V equals the current times the
resistance.
V = IR
I
V
R
Ohm’s Law

Voltage V equals current I times
resistance R:
Given
R = 3.0 ohms.
V = 12.6 volts
V = IR = 12.6 = I (3.0)
I = 4.2 amps
Formula
V = IR
I
R=3.0 Ω
V
Add a Switch


I=0
“Open circuit” if switch
is open – no current
flows.
“Closed circuit” if
switch closed, current
flows.
V
R
Car Headlights
A typical car alternator generates up to 60
amps at 14 volts.
 “Open circuit” if switch is open – no current.
 “Closed circuit” if switch closed, current
flows.

V
Power

Power P equals current I times the voltage V:
P = IV
Joule’s Law
Electrical power units same as with
mechanical energy, watts (W).
 Car headlight (halogen)
 I = 4.2 A, V = 13 V.
 P = IV = (13)(4.2)
 P = 55 W

Light Bulb
Original commercial bulb: carbonized
filament.
 Today: tungsten filament in an inert gas.
 Thomas Edison – in his lab in 1880 tested
over 6,000 various carbon filaments before
he got a carbonized cotton thread to last
15 hours before burning out.
 Edison Electric Light Company → GE in
1892.

Car Battery Power

A typical car battery has voltage of 12.6 V,
maximum “cold cranking amps” of 500 A.
How much power is available to start your
car?

P = IV = 500 x 12.6 = 6,300 W
Recall 1 hp = 746 W
 Therefore, P = 8.4 hp
 “Turns over” the engine.

Connect Elements in Series

Both lamp elements have same current.
Rseries= R1 + R2

What if one light burns out? No current, open
circuit!
R1
R2
Connect Elements in Series






Both lamp elements have same current.
Let
R1 = 30 Ω
R2 = 30 Ω
Find equivalent series resistance –
Rseries = R1 + R2
= 30 + 30
= 60 Ω
R1
R2
Connect Elements in Parallel

Both lamp elements have same voltage.
1/Rparallel= 1/R1 + 1/R2

What if one light burns out? No problem!
R1
R2
Connect Elements in Parallel







Both lamp elements have same voltage.
Let
R1 = 30 Ω
R1
R2 = 30 Ω
R2
Find equivalent parallel resistance –
1/Rparallel = 1/R1 + 1/R2
 Resistors in parallel
= 1/30 + 1/30
have lower equivalent
= 1/15
resistance than
Therefore, Rparallel = 15 Ω
resistors in series.
Magnetism
North pole, South pole.
 Like poles repel, unlike poles attract.
 Always appear as pairs.
Invisible magnetic

field
Source of Magnetism

Source of magnetic field − moving
electrons, and in some cases, “spinning”
electrons.
 Electric current in a wire creates a
magnetic field.
 Magnetic materials. A few elements,
iron, nickel, and cobalt, and their alloys,
have a strong magnetic effect.
Magnetic Materials
Every electron, by its nature of
spinning, is a small magnet.
 Ordinarily, countless electrons in a
material are oriented in different directions,
leaving no effect.


In an iron bar magnet the electrons are
aligned in the same direction, so they act
cooperatively, creating a net magnetic
field.
Earth’s Magnetic Field
Earth's magnetic field can be modeled as
a simple bar magnet.
 The direction in which a compass points is
known as magnetic north.
 The Earth's North Magnetic Pole is a
wandering point on the Earth's surface,
with evidence that it flips about every
300,000 years.
 Drifting at about 35 miles per year.

85.1° N 135.6° W in 2012.
82.7° N 114.4° W in 2005.
81.3° N 110.8° W. in 2001.
Declination
The direction of magnetic north from true
north is called “declination.”
 Needed to make corrections in navigation.
 In Los Angeles, magnetic north is about
12° east of true north.
N Magnetic

north
E
W
S
Electromagnetism
Michael Faraday (1791–1867) studied the
magnetic field around a conductor
carrying a direct current
 Established the basis for the “magnetic
field” concept in physics.


Moving a magnet
through a static loop
of wire creates
electric current in the
wire.
Generator

Moving wire loop in a static magnetic field
also produces an electric current.

Generator converts
mechanical energy
to electrical
energy.
History of Local Power

Pomona College’s first president Cyrus Baldwin
helped found San Antonio Light and Power
Company in 1891.

It was the first generating plant to provide electricity
in the Pomona and San Gabriel Valleys, using a
waterfall in San Antonio Canyon. Transformers built
by George Westinghouse provided for transmission
of 10,000 volts from this plant to Pomona.

The source of water proved to be undependable, so
the project failed to provide the expected results.
Hoover Dam Power
8 of 17 turbines
Mutual Induction

Two insulated coils of wire.

Passing a current
through one coil, a
momentary current is
induced in the other
coil.
Transformer
Transformers are passive devices.
 Transform alternating current from
one circuit into another through
electromagnetic induction.
 Number of windings in secondary
determines “step up” or “step down”
voltage.

Vs = (Ns/Np) Vp
Ratio of windings
Transformer
Motors

Current in a conducting wire experiences a
force from magnetic field.
Motor

Split-ring commutator changes direction of
the current.
 Transforms
electrical energy
to mechanical
energy.
No Motor
Suspend, guide, and propel a train using
electromagnets.
 Magnetic levitation (Maglev) trains.


19-mile Shanghai Maglev train
268 mi/hr.
Utility-Scale Power

Turns a turbine and generator.
Utility-Scale Energy Sources






Fossil energy − burn fossil fuels, heat water,
and create steam, turn turbine.
Nuclear – heat water, create steam, turn
turbine.
Geothermal – steam from deep in Earth
(6000 feet – Hawaii), turn turbine.
Gas turbine – hot gases direct into turbine.
Wind – wind turns turbine.
Hydroelectric – water pressure turns turbine.
U.S. Sources of Energy

US DOE, 2006
Historical View
Energy Information Administration /
Annual Energy Review 2007
http://www.eia.doe.gov/cneaf/electricity/epa/epa_sum.html
Iron and Permanent Magnets

Soft iron



Very strong magnetic field when
an electromagnetic field applied.
Iron’s magnetic field disappears
after field removed.
Hard iron


Retains magnetic field,
“permanent magnet”
Will lose permanent field if
heated or struck.
Toy Train Transformer
Vp = 120 V ac
 Np = 1,000 windings
 Ns = 160 windings.

Primary
coil
Secondary
coil

What is Vs?
 Vs = (Ns/Np) Vp
 Vs = (160/1000) 120
= 19 V

Fewer windings on secondary side creates
a step-down (lower voltage) transformer.
Light Bulb
Typical 100-watt light bulb
 V = 120 volts
 P = 100 watts
 What is the current?
 P = IV
 100 = I (120)
 I = 0.83 amperes

Analyze Simple Flashlight

1.0 W flashlight bulb, 3.0 V battery. What
is current?
Joules’ Law: P = IV = 1 = I (3)
 I = 1.0/3.0 = 0.33 A

V
V
Continue Simple Flashlight
What is the bulb resistance?
 From previous slide, V = 3.0 V, and
I = 0.33 A
 Ohm’s Law: V = IR, 3 = 0.33R, R = 10 Ω

V
V