1. Resistance and Ohm’s Law Alessandro VoltaAndre Ampere Georg Ohm

2. 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

3. Factors that Affect Resistance in a Wire 1. Resistance increases as the length of the wire increases

4. 2. Resistance increases as the width decreases - wire gauge describes its width American Wire Gauge (AWG)

6. 3. Resistivity of a material affects resistance. Material Resistivity (ohmmeter) 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 10 5 Polyethylene 10 8 - 10 9 Glass 10 10 - 10 14 Hard Rubber 10 13

7. R =  _L_ A  is resistivity L is length A is cross-sectional area (width) http://phet.colorado.edu/en/s imulation/resistance-in-a-wire

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

9. Ohm’s Law Resistance = Voltage Current R = V I More commonly seen in this form: V = IR

10. Now go to physicsclassroom.com and answer the questions about Resistance and Ohm’s Law. http://www.physicsclassroom.com/Class/circu its/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.