Calculating Electrical Power
- In “Work, Energy and Power”, Power P was defined as the rate of doing work
- Potential difference is the work done per unit charge
- Current is the rate of flow of charge
- So, the power dissipated (produced) by an electrical device is defined as:
Power of a component in an electrical circuit
- Using V = IR to rearrange for either V or I and substituting into the power equation means we also write power in terms of resistance R
Power equation in terms of resistance
- This means for a given resistance for example, 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 blows
C. Only the 41 W lamp lights
D. Both lamps light at less than their normal brightness
ANSWER: 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 equationsWhich equation to use will depend on whether the value of current or voltage has been given in the question
