Electricity

+ -

Charging by Rubbing

Charged Objects

-

Add + Charge

Remove - Charge

Add - Charge

Remove + Charge

+

Simulation

Electric Force

-+

+ +

Simulation

Coulomb’s Law

Fe -Fe

r

q1 q2

Fe = k q1 q2/ r2

k = 9 x 109 Nm2/C2

r q1 q2 Fe

1 1 1 k

r q1 q2 Fe

2 1 1 ¼ k

r q1 q2 Fe

1 2 1 2k

1 2 2 4k

r q1 q2 Fe

1 1 1 k

2 1 1 ¼ k

1 2 1 2k

1 2 2 4k

2 2 2 k

Fe = k (1)(1)/ (1)2 Fe = k

Fe = k (1)(1)/ (2)2 Fe = ¼ k

Fe = k (2)(1)/ (1)2 Fe = 2k

Fe = k (2)(2)/ (1)2 Fe = 4k

Fe = k (2)(2)/ (2)2 Fe = k

As r increases Fe increases as the inverse square of r.

As q increases Fe increases in direct proportion to q.

The Unit of Charge

1 Coulomb is a very large amount of charge.

The unit comes from the definition of electric current.

1 ampere = 1 coulomb/second

1 coulomb = 6.25 x 1018 electron charges

6,250,000,000,000,000,000 electrons

6.25 quintillion electrons

Electron charge = 1.6 x 10-19 coulombs

Electron charge = 1/6.25 x 1018 coulombs

Electron charge = .00000000000000000016 coulombs

Induced Charges

- + - + - +

+ + + + + + + + +

+ + + + + + + + + - + - + - +

+

+ + + + + + + + + + + + + + + + + +

+ + + + + + + + +

- + - + - +

Simulation

Grounding50- 50-100-

1-

25- 75-

99-0

100-

0

Earth

Top 8 Conductors of Electricity

1. Silver 1.47 x 10-8

2. Copper 1.72 x 10-

8

3. Gold 2.44 x 10-8

4. Aluminum 2.63 x 10-8

5. Tungsten 5.51 x 10-8

6. Steel 20 x 10-8

7. Lead 22 x 10-8

8. Nichrome 100 x 10-8

Aluminum CopperGoldLead NichromeSilverSteelTungsten

The following metals are listed in alphabetical order. Arrange them in order of conductivity from best to worst.

Ohm’s Law

I = Current measured in amperes. V = Voltage measured in volts.R = Resistance measured in ohms.

I = V/R

1 amp = 1 volt/1 ohm

Current = Voltage/Resistance

Electric Power

I = Current measured in amperes. V = Voltage measured in volts.P = Power measured in Watts.

P = I x V

1 watt = 1 amp x 1 volt

Power = Current x Voltage

Electric Power

E = P t

Energy = (Power)(time)

You pay for electricity in kilowatthours (kwh)

1 kwh = (1000)(1J/s)(3,600s) = 3,600,000 J Cost per kwh = $.10 (approximately)

Electric Power (cont)

# kwh = (P x t)/1000

#kwh = 18

A 600 watt hair dryer is operated for 1 hour per day for 30 days. If the electric company charges $.10 per kilo-watt-hour, what is the cost to operate the dryer?

P = 600 wattst = 30 hoursCost = ? $

#kwh = 600 x 30/1000

18 kwh x $.10 /kwh = $1.80

Electric Shock

What is the resistance of the human body?

500,000 Ώ If dry and insulated

100 Ώ If soaked in salt water

.001 amp can be felt

.005 amp is painful

.010 amp causes involuntary muscle contractions (spasms)

.015 amp causes loss of muscle control)

.070 amp If through the heart, serious disruption; probably fatal if current lasts for more than 1 second.

.10 amp Almost always fatal.

A

B

C D

Electric Shock (cont.)What are the effects of electric shock?

1. Muscle contractions and spasms.

2. Heating and burning.

A wet swimmer standing on a concrete floor has a resistance of 500 Ώ.He comes in contact with a 120 volt “hot” electric wire. What happens?

I = V/R = 120/500 = .24 amp Death!

A dry person standing on a concrete floor has a resistance of 100,000 Ώ.He comes in contact with a 120 volt “hot” electric wire. What happens?

I = V/R = 120/100,000 = .0012 amp Shock felt.

I = V/R = 120/500,000 = .00012 amp Nothing.

A dry person standing on a wood floor has a resistance of 500,000 Ώ.He comes in contact with a 120 volt “hot” electric wire. What happens?

Electric Circuit

V-

+

R

1) Source of Voltage (Battery)

2) Device to convert energy (Resistor)

3) Complete conducting path (wires and switch)

Series Circuits

V = V1 + V2 + V3

I = I1 = I2 = I3

Simulation

Current is the same for all bulbs.

The voltage across each bulb adds up to the total voltage of the battery.

If one bulb goes out, they all go out.

Parallel Circuits

VideoSimulation

V = V1 = V2 = V3

I = I1 + I2 + I3

Voltage is the same for all bulbs.

The current in each bulb adds up to the total current from the Battery.

If one bulb goes out, the others stay on.