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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.

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electricity and magnetism
Electricity and Magnetism

Physics 102

Professor Lee Carkner

Lecture 8

electricity and magnetism2
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
  • 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
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
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
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
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
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
Making Electricity
  • Three ways to do this
  • Chemically
    • e.g.
  • Magnetically
    • e.g.
  • Physically
  • Four basic methods: friction, conduction, induction, polarization
  • 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
Triboelectric Charging




Rubbing a piece of rubber with fur transfers the electrons from the fur to the rubber due to friction.

how does charge move
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
  • Both end up with the same sign charge
    • Example: shocking your friend
conductive charging
Conductive Charging

Neutral Metal

Charged Metal


Both Metal Rods Now Charged

Charge will move from one conductor to another

    • 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
Inductive Charging

Induced Charge on Metal

Charged Metal

A charged conductor will split the charge on a near-by conductor

induction via grounding
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
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

    • 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
origins of electricity
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’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
Next Time
  • Read: 16.5-16.7
  • Homework: Ch 16: P 8, 12, 23, 24