Electrical Power
- In mechanics, power P is defined as the rate of doing work
- Potential difference is the work done per unit charge
- Current is the rate of flow of charge
- The electrical power dissipated (produced) by an electrical device is defined by the equation:
- The unit of power is Watt (W)
- Using Ohm's Law V = IR to rearrange for either V or I and substituting into the power equation, means power can be written in terms of resistance R
- This means for a given resistor if the current or voltage doubles the power will be four times as great
Worked example
Two lamps are connected in series to a 150 V power supply.
Which statement most accurately describes what happens?
A. Both lamps light normally
B. The 15 V lamp blowsC. Only the 41 W lamp lights
D. Both lamps light at less than their normal brightnessANSWER: A
Examiner Tip
You can use the mnemonic “Twinkle Twinkle Little Star, Power equals I squared R” to remember whether to multiply or divide by resistance in the power equations.When doing calculations involving electrical power, remember the unit is Watts W, therefore, you should always make sure that the time is in seconds
