Electric PowerPower is the rate that work is done or
energy is transferred, that is
Power =
Power is measured in Watts, W
)onds(secTime)Joules(Energy
Electric power delivered to a circuit by a power supply is given by
Power = Current x VoltageP = 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.
ExamplesHow much energy does a 75 W light bulb give off in
five minutes?Power = Energy / timeEnergy = Power x timeEnergy = 75 W x (5 x 60) seconds Energy = 22500 JWhat is the power output of a 3 A motor running on
regular house voltage?P = IVP = 3 A x 120 VP = 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
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) = I2R
• 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 W
• Multimeter- can measure current, resistance, and voltage!
Req = R1 + R2 + R3 + …...R1
R1
R1
R1
321eq
Each bulb has the same current running through it! That current is the TOTAL current pushed by the battery, IT = Vbattery / Req
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 = V2 / R
Series CircuitParallel Circuit
Electrical Safety
Open circuit Closed circuit