Electric Power Power is the rate that work is done or energy is transferred, that is Power = Power is measured in Watts, W

Electric power delivered to a circuit by a power supply is given by Power = Current x Voltage P = IV

Electrons do NOT leave the circuit- Energy leaves the circuit through the different “resistors” in the form of light, heat, and any kind of work done by the appliance the current is running through. The rate that the energy leaves the circuit is the power output.

Examples How much energy does a 75 W light bulb give off in five minutes? Power = Energy / time Energy = Power x time Energy = 75 W x (5 x 60) seconds Energy = J What is the power output of a 3 A motor running on regular house voltage? P = IV P = 3 A x 120 V P = 360 W

“Power” lines The higher the current, the more the wires in the circuit heat up, thereby “wasting” energy. This is a big problem when electric companies must provide electricity at great distances away from the power plants. The solution: Electric lines that carry current great distances are at very high voltage, so the current can be relatively small. P = IV

High Voltage / Low Voltage TRANSFORMERS: devices that “step- up” the voltage at the power plant and then “step-down” the voltage at the customers’ location.

Even with very high voltage, there is still some current running through those wires. Therefore, some electrical power (dissipated through heat) is lost. If the entire length of wire has a total resistance R, the power lost along the way is given by Power lost (dissipated) = I 2 R

Circuit applet

Voltmeter- device to measure voltage The voltmeter is placed “in parallel” with the component whose voltage is being measured.

Ammeter- device to measure amps (current) The current must flow THROUGH the meter, therefore an ammeter is placed “in series” with the component whose current is being measured.

Ohmmeter- device to measure resistance Resistance is measured with the power OFF ! 2.3 

Multimeter- can measure current, resistance, and voltage!

R eq = R 1 + R 2 + R 3 + … Each bulb has the same current running through it! That current is the TOTAL current pushed by the battery, I T = V battery / R eq If one of the bulbs goes out, they ALL go out! Each bulb has the same voltage across it! That voltage is the voltage of the battery. However, the current splits up so that each bulb only receives a portion of the total current pushed by the battery. If one of the bulbs goes out, the others remain lit, and will have EXACTLY the same brightness (WATTS) as they did before the bulb went out. P = V 2 / R Series CircuitParallel Circuit

Recap

Open circuit Closed circuit