Georg Simon OhmGeorg Simon Ohm

ResistancePotential Difference

Current

RVI

SI Unit: Ohms (Ω)

For many materials the Resistance is constant over a wide range of applied potential differences

VI

Constant

ΔV=IR

m=?

Do not follow Ohm’s Law (resistance does vary with the applied voltage)

Potential Difference (V)

Have NO resistance below a critical temperatureHave NO resistance below a critical temperature

Good Superconductors:

*Zn, Al, Sn, Hg, Nb, Nb3Ge, YBa2Cu3O7, TI-Ba-Ca-Cu-O

*Cu, Ag, and Au are not Superconductors

• Resistors in a circuit can change the current.– Variable resistors (potentiometers) are used in

dimmer switches and volume controls.– Resistors on circuit boards control the current to

components.

• The human body’s resistance ranges from 500 000 (dry) to 100 (soaked with salt water).– Currents under 0.01 A cause tingling.– Currents greater than 0.15 A disrupt the heart’s

electrical activity

P=W/t… (rate at which work is done)

Electric power: rate at which charge carriers convert electrical potential to nonelectrical forms of energy (ΔPE/t…conversion in book)

P = IΔV = I(IR) = I2R P = IΔV = (ΔV/R)ΔV = (ΔV)2/R

SI Unit for Power: Watt 1W= 1J/s (1 joule of energy being converted into another

form per second)

Electric Companies charge for energy, not power. Electricity is measured in terms of kWh (energy delivered in 1 h at constant rate of 1 kW)

1KW h103W

1kW60min

hour60s

min3.6106W s3.6106J