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
Example — a current of 0.50 A flows through a 9.0 V circuit for 5.0 minutes. Calculate the total energy transferred.
- Convert the time to SI units: t = 5.0 min × 60 = 300 s
- E = I × V × t = 0.50 × 9.0 × 300 = 1350 J