Electricity & Magnetism
Chapter 8
Student Learning Objectives
• Characterize charge & static electricity
• Differentiate between series & parallel circuits
• Describe properties of magnetic materials
• Analyze electromagnetic systems
What is charge?
 All matter is composed of atoms which contain charged
particles.
Charge
Charge
Mass
electron
proton
neutron
−1
+1
0
–1.6 x 10−19 C +1.6 x 10−19 C
0
9.1 x 10-31 kg 1.67 x 10–27 kg 1.67 x 10-27 kg
http://www.pond5.com/stock-footage/68580/atom5.html
Practice
1) Why do the electrons stay in orbit around the
nucleus?
2) Does an object that has lost electrons have a positive
or negative charge? Does it have more or less mass?
 Law of conservation of charge:
The total amount of electric charge in the universe
remains constant. Charge is never created or
destroyed.
 Neutral atoms have the same number of electrons and
protons.
2
He
4.00260
Atomic Number
Coulomb’s Law
Objects become charged when they lose or
gain electrons. (ionized)
F = k(q1)(q2)
d2
Opposite charges  Attract
Like charges  Repel
 The electric force and the gravitational force are both
inverse square laws, and both apply mutual forces
between two quantities.
Electric Force
Gravitational Force
Attract & Repel
Attract
Strong
Weak
Practice
1) If an object has lost 100 electrons, what is the
object’s charge in Coulombs?
2) What is the electric force between the electron
and the proton in a hydrogen atom? The
average separation between the electron and
proton is 5.29 x 10−11 m.
More Practice
3) Rubbing a balloon on your hair results in the
transfer of 2000 electrons from your hair to
the balloon. What is the force between your
hair and the balloon if the balloon is 2 cm
from your hair?
Hint: determine the total charge on the hair
and the total charge on the balloon.
How do objects become charged?
Electrons are lost, gained, or redistributed
within a charged object.
Friction: electrons are “knocked loose”
Contact: electrons are transferred
Induction: charge is rearranged
Image Credit: sci-culture.com
Static Electricity
A net electric charge on an object results in
static electricity. (Positive or Negative)
Cars & Clothes
Practice
1) Is an object that has been charged by induction
ionized?
2) Static electricity on clothes and us is less common on
humid days. Why?
3) Why does a plastic slide retain static charge while a
metal slide does not?
What is an electric field?
An electric field surrounds charge, modifying
the space near the charge.
Lines of Force
Electric Fields Interact
Objects are affected by the field
established in that space.
Practice
1) What would the electric field lines look like between
two equal positive charges?
2) How would the electric field lines between two
equal negative charges compare to the equal positive
charges?
What are the properties of electric circuits?
Electric current is the net movement of electrons.
Electrons are present in all electrical systems.
When electrons begin to move, there is current.
Two different electric
potentials set-up an
electric field, which
causes existing electrons
to flow from the high
potential to the low
potential.
Potential Difference  E Field  Current
No difference in electric
potential = no charge
will flow.
EPE → KE as charge is
accelerated through the
system.
"Shock War“
Wall Outlets (ac)
Bird on a Wire
Batteries (dc)
Lightening
Electrical Systems
Voltage is created by a difference in electric potential.
Electric resistance is a measure how difficult it is for
electrons to move within the system.
Conductivity
Length
Diameter
Temperature
Voltage & Power
Ohms Law
V = IR
Power is the rate at
which electric energy is
used by a system.
P = IV
Practice
1) A flashlight uses a 1.5 volt battery and has a
resistance of 2 ohms. What is the current within the
flashlight circuit? How many Watts will this
flashlight supply?
2) A 2000 Watt hair dryer, a 100 Watt radio, and five
60 Watt light bulbs are being used simultaneously.
Will this cause a 12 A safety switch to open?
Assume the voltage supplied is 110 volts.
More Practice
3) An Xbox 360 console is attached to a 55 inch plasma
TV. Each of these devices uses about 135 Watts. If
you play the Xbox 360 on this TV for 4 hours, what is
the cost?
The current average utility rate is 24¢/kWh.
Why you should not stick any metal
object into the plug!
4) A person with dry skin has a maximum resistance of
500,000 ; moist skin has a resistance of 1000 . If
the metal prongs of a plug are touched as the plug is
placed into a 110 volt wall outlet, how much current
could pass through the person’s body? Is this
dangerous? Don't try it!
0.001 A
0.005 A
0.010 A
0.015 A
0.070
Noticeable
Painful
Muscle spasms
Loss of muscle control
Disrupts heartbeat
How are circuits wired?
Series Circuits
Single continuous loop
 Voltage is divided. (energy shared)
 Resistance is additive. (increases)
R = R1 + R2 + …
Parallel Circuits
Multiple parallel loops
 Voltage is supplied to each parallel loop. (energy not
shared)
 Resistance will drop. (decreases)
1 = 1 + 1 +…
R R1 R2
Questions
What do you think an
electrical switch does
within the circuit?
Why is too much current
in a wire dangerous?
What are the properties of the magnetic field?
All magnets have two poles;
this is where the magnetic field
is strongest.
North Seeking & South Seeking
The poles are not charged!
Opposite magnetic poles  Attract
Like magnetic poles  Repel
A magnetic field surrounds a magnet, modifying the
space around the magnet.
Practice
1) How do electric charges and magnetic poles
compare and contrast?
2) How do electric fields and magnetic fields compare
and contrast?
What produces magnetism?
The motion of electric charge is what makes
a material magnetic.
Magnetic Moments
Electric Current
Motion of Ionized Fluid
Magnetic Moments
The intrinsic magnetic moments of electrons align
creating magnetic regions called domains.
Non-Magnetic
Material
no domains
Magnetic
Material
domains not
aligned
Magnet
domains
aligned
Electric Current
 Electric current forces magnetic moments to align.
I
B-Field
Motion of Ionized Fluid
 Ionized fluid motion, in Earth’s outer core,
produces a magnetic field that surrounds Earth.
• Protects us from solar wind
• Produces aurora
• And the poles switch!
NASA
http://www.scientificamerican.com/article/earth-s-magnetic-field-flip-could-happen-sooner-than-expected/
How are electric and magnetic effects related?
Electric charge moving induces magnetism.
Magnetic field changing induces current.
E-Fields & B-Fields
Induce Each Other
Flashlight
(no batteries required)
Radio
(no batteries required)
Faraday’s Law
Faraday’s Law:
Induced voltage in a coil is proportional to the
number of loops, multiplied by the rate at which
the magnetic field changes.
More Loops + Turning Faster = More Voltage
Electric Motor vs Electric Generator
Practice
1) Do electric generators create energy or convert
energy?
2) Apply Faraday’s law to the electric generator.
Maxwell’s Law
Maxwell’s Law:
A magnetic field is induced in any region of
space in which an electric field is changing
with time.
DCurrent = Magnetism
Electromagnet
(magnetism from electricity)
Electric Transformer
The relative number
of turns determines
the change in voltage.
V2 = T2
V1 T1
Practice
1) If a laptop needs only 12 volts and the wall outlet
supplies 110 volts, what is the relative number of
turns needed in each coil? Is the laptop transformer
a step-up or step-down transformer?
2) Apply both Faraday’s law and Maxwell’s law to the
electric transformer?