Power

Energy Transformation

Electric energy can be transformed into other more useful forms

Power is the rate of energy transformation P = ΔPE/t ΔPE = QV P = QV/ t I = Q/ t P = IV units – Watt (W)

Example

What is the resistance of a 40 W car headlight designed for a 12V car battery?

KiloWatt - Hour

Electric companies charge by the amount of energy used

Larger unit than Joule is used – called the kilowatt-hour (kW•hr)

1 kW•hr = 3.6 x 106 J

Example

A heater draws 15.0A on a 120V line. How much power does it use and how much does it cost per month (30 days) to operate if it runs 3.0 hours per day and the electric company charges 10.5 cents per kW•hr?

Example

A lightning bolt transfers 109 J of energy across a potential difference of 5 x 107V in .2 s. Find the charge transferred, the current, and the power of the bolt.

Overload

All wires have resistance and some of the electric energy is changed to heat

Overload is when wire carries more than the safe amount of current

Fuses and circuit breakers protect from overload

Causes of overload: too many devices drawing too much current, “short” which is crossed wiring

Example

A 100W lightbulb, 1800W heater, 350W stereo, and 1200W television are all connected to the same circuit which has a 20A fuse. Can all of these be operating at the same time?