Electrical Power
Electrical Power
- Power is the rate of energy transfer. In a circuit, p.d. gives energy per coulomb and current gives coulombs per second.
- Multiplying these quantities gives joules per second, which is watts.
Choose the relation from the data
Symbols
- = electrical power transferred (W)
- = p.d. across the component (V)
- = current in the component (A)
For a resistor, substitute only after identifying which quantities the question gives.
Symbols
- = resistance (\Omega)
Symbols
- = resistance (\Omega)
- At fixed resistance, doubling voltage or current makes power four times larger.
- At fixed voltage, decreasing resistance increases power.
- At fixed current, decreasing resistance decreases power.
- If voltage and resistance both change, compare both factors before deciding what happens to power.
Common mistake
State what is fixed. Decreasing resistance increases power at fixed voltage but decreases power at fixed current.
From power to total energy
Symbols
- = energy transferred (J)
- = time (s)
Symbols
- = energy transferred (J)
- = time (s)
Worked example
Power and energy in a heater
A 12 V heater carries 3.0 A for 5.0 minutes. Find its power and the energy transferred.
- .
- Convert 5.0 minutes to 300 s.
- .
Answer
Your turn9702/11/O/N/25 Q31
A resistor is connected to a constant p.d. Express its power in terms of p.d. and resistance. [1] (9702/11/O/N/25 Q31)
Show worked answer
Substitute into to obtain.