# How Things Work

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12/18/2009
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```							10.1-10.2 Static Electricity & Xerographic Copiers

Ideas for today:
•Charge and Static Electricity •Electric fields and electric potential •Photocopiers

• Static electricity builds up on non-conductive things • Objects with static charge either cling or repel • Static electricity can lead to shocks • Static electricity can make your hair stand up

Ben Franklin named charge as “positive” and “negative”
protons, positrons,… + + +

Charge comes in fundamental units of 1.6 · 10-19 Coulombs Charge is conserved!

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electrons , muons,…

Forces Between Charges
Charge Dependence
Like Charges Repel

Distance Dependence F 1/r2

Opposite Charges Attract

Net Charge
• Net charge is the sum of an object’s charges • Most objects have zero net charge (neutral) • Neutral objects contain equal + & – charges
+ + -+ -

+ + -+ -

+ + -+

+ + - -+ -

before

after

Polarizing Objects:

Click me

Clicker Question
What would happen if the force between charges did not depend on distance? A. Static cling would get stronger. B. Static cling would get weaker. C. Static cling would disappear. D. Nothing would change in the world.

The Xerographic Process
An terrific use for static electricity

What’s up with that Corona Wire?

What do lightning rods do?

There is a different way to think about forces on charges… •Electric fields •Potential

Electric Fields
Two views of charge forces: • Charge/Charge: – Charge 1 pushes directly on Charge 2 • Charge/Field/Charge: – Charge 1 creates an Electric Field – Electric Field pushes on Charge 2 Electric Fields are Real!

Electric Fields

Electric Fields
• The magnitude of the electric field is proportional to the magnitude of the force on a positive test charge • The direction of the field is the direction of the force on a positive test charge

Voltage
• Charge has electrostatic potential energy (EPS) • Voltage measures the EPS per unit of charge – Raising the voltage of a positive charge takes work – Lowering the voltage of a negative charge takes work • Voltage is measured in joules/coulomb or volts

Voltage

• Objects accelerate to reduce potential energy • A charge – has electric potential energy proportional to voltage – so it accelerates toward lower voltage – but it also accelerates because of an electric field • Voltage gradients are electric fields!

Clicker Question:
If you separate two oppositely charged balloons, A. the voltage of the positively-charged balloon does not change. B. the voltage of the positively-charged balloon increases. C. the voltage of the positively-charged balloon decreases.
+ + + + +

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Charges In and Around Metals
• Charges can move inside metals – They move to minimize potential energies – They give the metal a uniform voltage – There is thus no electric field inside a metal • However, outside metals – Charges can’t move so easily – so voltages can vary with location – and there can thus be electric fields

Getting shocked

Click me

Rapidly changing potential means very large electric fields / forces!

Corona Discharges
• Outside a sharp or narrow charged metal • the electric field is very strong • charges are pushed onto passing air particles: corona discharge • Corona discharges can dissipate static electricity

Clicker question
What is the purpose of a lightning rod?

(A)To attract lightning to itself (B)To create a large electric field and dissipate charge from the building (C)To repel lightning from the building

The Xerographic Process
An terrific use for static electricity

What’s that photoconductor stuff?

Photoconductors
•In the dark, a semiconductor is insulating •In the light, a semiconductor may conduct

Photocopier!

And laser printers…

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