1

Chapter 21 continued…

Why is she having a bad hair day???

2

Charging by FrictionSimply rubbing one object against another can transfer electrons. This makes one positive and one negatively charged object.

+

+

+

_

_

_+

+

+___

Before: both neutral

++

+

__

+

+

+

_

_

_

_

After: Silk negative, glass positive

friction

+

+

+___ _

3

Charging by InductionEven without direct contact, you can induce the electrons to move due to the electric force acting on them. Consider the metal spheres on insulating stands shown below.

3. Separate the spheres

4. Remove rod, spheres are now

charged.

- --

2. Negative rod separates charges.

+

1. Both neutral at

first

- ---+ ++

++--+

- --

-+-+ ++

----++

- ---+ -+

+ --

5

Lightning:

When one mass of air moves over another, electrons are transferred. This is just like moving your feet on carpet to build a charge.

A large negative charge on the bottom of a cloud induces a positive charge on the ground below. ( they are attracted)

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If the charge imbalance becomes large enough, lightning occurs. This discharges the accumulated charge.

-- - - --- - - - -

++

+ + ++

+

+ +

++

7

Lightning Rods:

The pointy rod forces electrons to repel jump off before they can accumulate too much and force a lightning strike.

8

Grounding

When items are grounded, it literally means they are connected to the earth/ground. This creates a place for excess electrons to go.

9

A similar static electric discharge should be avoided at the gas pump! Thus, you should touch your car and discharge before pumping. “grounding” the container also removes any accumulated charge!

10

Charge PolarizationImagine bringing a charged balloon near a neutral wall.Notice how the wall is still neutral, but the charge is separated, or polarized.

Since the opposite charges are closer than the like charges, a net attractive force is present.

-

-- --

-

-

+

++

+

--

-

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Why Does it Cling?

Although the paper is still neutral, since its polarized, the (-) is now closer, and the (+) is farther. This means the attractive force is

greater, and the repulsive force is less. ( It sticks!)

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Electric Field:Imagine there is a cluster of – charge as shown.

-

If a small + charge was placed, what force would it feel?

13

This “map of force lines” shows what a positive test charge will do when exposed to any

particular field. When lines are closer together, that means the force is stronger. Here are some

pictures of various fields:

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Questions??

Homework:Read pages 379-385

page 394 Q # 21, 25, 26, 27, 29, 30, 31, 32