Electricity and Magnetism
Physics 102Professor Lee
CarknerLecture 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
Fur Rubber
e-
Rubbing a piece of rubber with fur transfers the electrons from the fur to the rubber due to friction.
Triboelectric Charging
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
Neutral Metal
Charged Metal
e-
Charge will move from one conductor to another
Conductive Charging
Both Metal Rods Now Charged
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
Induced Charge on Metal
Charged Metal
A charged conductor will split the charge on a near-by conductor
Inductive Charging
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
Metal becomes positive
Charged Metal
A charged conductor will push out the same charge to the ground, leaving the other conductor with an opposite charge
Inductive Charging with Grounding
Electrons pushed to ground
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