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