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 X C   Charge is represented by the variable q (or sometimes Q)

Electric Force  F = k q 1 q 2 /r 2  Where:  k is the Coulomb constant (8.99 X 10 9 N m 2 /C 2 )  q 1 and q 2 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-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-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 positiveCharged 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:  Homework: Ch 16: P 8, 12, 23, 24