Energy from power and time
E = P × t
- where:
- E = energy transferred (J)
- P = power of the device (W)
- t = time the device runs for (s)
- Substituting P = I × V gives a second form that uses circuit quantities directly:
E = I × V × t
- The two equations are equivalent, so pick whichever set of quantities the question hands you
Energy and work
- The work done by the charge flowing through a component is exactly the energy transferred to that component
- In a complete circuit, the total energy delivered by the battery equals the total energy transferred to every component on the loop, which is conservation of energy
Calculating energy transferred (E = P × t)
A lamp has a power of 60 W and is left on for 5 minutes. Calculate the energy transferred.
Solution:
- Convert time to seconds: 5 min = 300 s
- Write the equation: E = P × t
- Substitute: E = 60 × 300
- E = 18 000 J