Unit 4Electric Potential, Current, and Resistance

If charges A & B were allowed to move, in what direction would they move because of the electric field? Ignore the interactions between charge A & B

BA

A

A

B BWhat do

these images have in

common?

Electrical Potential Energy(In a uniform electric field)

Electrical Potential Energy = - quantity of charge being moved * the strength of the

Electric Field * the displacement

Potential Difference

Potential Difference = The change in potential difference at two reference points, divided by the charge = The strength of the electric

field multiplied by the distance moved through it.

Do a unit analysis of this quantity. What everyday object is measured with this unit?

Question?

• What’s the purpose of defining a new term “potential difference”, when in the past, “potential energy” would suffice?

Let’s try a problem…

A charge moves a distance of 2.0 cm in the direction of a uniform electric field whose magnitude is 215 N/C. As the charge moves, its electrical potential energy decreases by 6.9 x 10-19 J.

A. Find the charge on the moving object. B. What is the potential difference between these two points?C. Do a unit analysis on the potential difference.

Potential & Candle Flames

• http://www.youtube.com/watch?v=a7_8Gc_Llr8

Practice A

• Problems 1-3

Current

Definition: The rate at which electrical charges pass through a given area.

Another new unit!

I

Current & Moving Charge

Create a list of as many items as you can that involve current (moving charges).

Practice C

• Problems 1-5

Conceptual Challenges pg. 611

Why it matters…..

Bell Work

A. In your own words, why do electrons want travel from the negative terminal of a battery to the positive terminal when they are connected?

B. Similarly, when lightning hits the ground around you, it would go up one leg and out the other, whereas if you stood on one leg, it wouldn’t travel inside you. How does the lightning know where it can travel?

Resistance

Definition: The opposition presented to electric current by a material or device.

4 Factors:

LengthCross Sectional Area

MaterialTemperature

R = resistance (Ohm) = resistivity of material (Ohm*m)

l = length of material current is traveling through (m)A = cross sectional area of the wire (m2)

Why does this equation have no temperature term?

Mate

rial (ρ

)

Material Resistivity(ohm•meter)

Silver 1.59 x 10-8

Copper 1.7 x 10-8

Gold 2.4 x 10-8

Aluminum 2.8 x 10-8

Tungsten 5.6 x 10-8

Iron 10 x 10-8

Platinum 11 x 10-8

Lead 22 x 10-8

Nichrome 150 x 10-8

Carbon 3.5 x 10-5

Polystyrene 107 - 1011

Polyethylene 108 - 109

Glass 1010 - 1014

Hard Rubber 1013Note: The quantity, “conductivity” is the inverse of a material’s “resistivity”.

Combining what we know so far…

Ohm’s Lawhttp://phet.colorado.edu/sims/ohms-law/ohms-law_en.html

Resistance Simulation

http://phet.colorado.edu/sims/resistance-in-a-wire/resistance-in-a-wire_en.html