Electricity and Magnetism

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# Electricity and Magnetism - PowerPoint PPT Presentation

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

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Presentation Transcript
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

e-

Rubber

Fur

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
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

Induced Charge on Metal

Charged 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
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
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