Post on 24-Dec-2015
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.