Physics APB
Dunbar
E°
1/9/13
Page 1
Derek Lee
 Science of electric charge is like the science of opera
 Opera music has transformed the world of music
 Range of voices is from lowest bass to highest soprano
 Operatic arias are often the basis of popular music, commercials, and cartoons
 Entire lecture was accompanied by the soundtrack of Mozart’s “The Marriage of
Figaro.”
 Electrical conductors
 In a conductor, charges are free to move about freely – usually to repel one another.
 The excess charge flows to the outside surface immediately.
 Example: Mylar or a solid chuck of metal [wire, bar]
 Electrical insulators
 Allow charges to shift a tiny amount
 Charges on an insulator generally stay where it’s put.
 Example: a balloon, Styrofoam, plastic
 Grounding
 Touching the earth (connected to ground)
 In houses, the ground is a copper rod 3-4 meters below surface. The earth is the largest
object that people can touch; earth can accept or donate a large number of charged
particles.
 Objects often become electrically neutral when grounded, but not always.
 How to charge objects electrically [not buying with a credit card]
 By contact (touching things together)

One object has greater affinity for charges -- electronegativity; example, rubbing a
rubber rod on rabbit fur

One substance steals electrons from another substance when in contact – it’s a
chemistry thing
 By induced charge separation

Charge a balloon on your hair and the balloon will stick to a neutrally-charged object,
such as a screen.

The screen is being charged temporarily by an electrical field from an outside source,
which in turn causes some internal charges to shift.

Example, a balloon is becomes negatively charged after being charged by contact.
The negative particles on the screen are repelled by the balloon and the positive
charges on the screen are attracted to the balloon.

When the balloon is taken away, the balance of charges is restored.
Physics APB
Dunbar
E°
1/9/13
Page 2
Derek Lee
 By induction

Steps
o Induce a charge separation
o Ground the object while the charge is separated
o Remove the ground

Example
o Rub a block of Styrofoam to charge it negatively
o Place a metal object on the Styrofoam (an insulator) to induce a charge
separation; the +charges migrate toward the surface of the Styrofoam
o Grounding the metal with (say) a finger draws the negative charges away from the
metal, leaving +charges.
o Remove the ground to leave the +charges on the metal so that the metal is now
electrically charged.
 Summary
 Conductors vs insulators – they behave differently
 Grounding does not automatically make an object electrically neutral
 Three ways to electrically charge an object
 HW – Chapter 18 Day 1 – write brief sentences