Resistance and

Ohm’s LawAlessandro Volta Andre Ampere

Georg Ohm

Resistance

– a measure of how much a material resists the flow of electrons – SI unit is Ohms (Ω) (Greek letter omega)–Good conductors have a low resistance• Ex: copper 0.00000004 Ohms

–Poor conductors have a high resistance • Ex: dry human skin 500,000 Ohms

Factors that Affect Resistance in a Wire

1. Resistance increases as the length of the wire increases

2. Resistance increases as the width decreases

- wire gauge describes its width

American Wire Gauge (AWG)

3. Resistivity of a material affects resistance.

MaterialResistivity(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

Carbon 3.5 x 105

Polyethylene 108 - 109

Glass 1010 - 1014

Hard Rubber 1013

R = r _L_ A

r is resistivityL is lengthA is cross-sectional area

(width)

http://phet.colorado.edu/en/simulation/resistance-in-a-wire

Analogy: Water experiences more resistance as pipes get smaller and longer, so current decreases.

Ohm’s Law

Resistance = Voltage

Current

R = V

I

More commonly seen in this form:

V = IR

Now go to physicsclassroom.com and answer the questions about Resistance and Ohm’s Law.

http://www.physicsclassroom.com/Class/circuits/u9l3b.cfm

Answer the questions on the website above and check your answers then hit next section: Ohm’s Law on the bottom of the site to go to the next page. Answer the questions about Ohm’s Law and check your answers.