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Electricity and Magnetism
Physics 102
Professor Lee Carkner
Lecture 8
Electricity and Magnetism

In E&M, we will deal with forces that depend
upon charge
Charged particles generate E&M forces
Stationary charges exert an electric force

E&M forces only affect charged particles
Forces
The electric force is much stronger than the
gravitational force

Why don’t we feel this strong force?

Most things have roughly equal numbers of
positive and negative particles

Like charges repel, opposite attract
Charges want to arrange themselves so there
is no force
e.g. lightning, static electric shock
Plus and Minus
The basic particle of negative charge is the
electron
The basic particle of positive charge is the
proton

They are bound in the nucleus

Adding electrons makes something negative

All E&M forces depend on what the electrons
are doing
Using Electricity
Why is the electrical force important?

Convert electrical energy into work

Convert electrical energy into heat and light

Convert electrical energy into sound

Electrical energy can be very finely controlled
Computer, internet
Units of Charge
The unit of charge is the Coulomb (C)

The electron and the proton have equal
and opposite charges:
1 e = 1.60 X10-19 C

Charge is represented by the variable q
(or sometimes Q)
Electric Force

F = k q1 q2/r2
Where:
k is the Coulomb constant (8.99 X 109 N
m2/C2)
q1 and q2 are the two charges (in Coulombs)

You must assign a sign to F at the end
r is the distance between them (in meters)
Direction of Forces

but,
to find the direction of the force we need
to use the rule:
opposites attract, like repel

n.b., the direction of the force does not
come out of the equation, you have to find
it yourself
Making Electricity

Three ways to do this
Chemically
e.g.
Magnetically
e.g.
Physically

Four basic methods: friction, conduction,
induction, polarization
Triboelectricity
Rubbing will transfer electrons from
one substance to the other by friction

Example: Glass rubbed with silk becomes
positive, rubber rubbed with fur becomes
negative
Triboelectric Charging
eFur
Rubber
Rubbing a piece of rubber with fur
transfers the electrons from the fur to the
rubber due to friction.
How Does Charge Move?

Conductors

When you charge a conductor, the electrons will flow
through it

Other types do not allow electrons to flow (e.g. glass,
rubber)
Insulators
If you charge them, the charge stays put
Conduction

Both end up with the same sign charge

Example: shocking your friend
Conductive Charging
Neutral Metal
Charged Metal
e-
Both Metal Rods Now Charged
Charge will move from one conductor to
another
Induction

attract the opposite sign charges to the
near end

The whole conductor has no net charge,
but each end does
Will always attract the original charged
object
Inductive Charging
Charged Metal
Induced Charge on Metal
A charged conductor will split the charge
on a near-by conductor
Induction via Grounding
If you connect a conductor to the Earth,
an endless amount of charge can flow
from it to the ground

If you place a charged object near a
grounded conductor it will repel the
same sign charges to the ground

Charge is opposite that of the inductor
Inductive Charging with
Grounding
Charged Metal
Metal becomes positive
Electrons
pushed to
ground
A charged conductor will push out the
same charge to the ground, leaving the
other conductor with an opposite charge
Polarization

You have charged the balloon but not the
wall

The negative charge on the balloon
attracts the positive charged parts of the
molecules of the wall, polarizing it
Polarization
Origins of Electricity
Moving charges by
rubbing has been
known since ancient
times

Benjamin Franklin
proposed the terms
positive and
negative for the two
types of charge
Franklin’s Kite
Franklin
demonstrated that
lightning is a form
of electricity
He flew a kite in a
thunderstorm and
saw that charge
flowed down the
string

Next Time
Read: 16.5-16.7
Homework: Ch 16: P 8, 12, 23, 24