Calculating Energy Transfers
- Work is done when charge flows through a circuit
- Work done is equal to the energy transferred
- The amount of energy transferred by electrical work in a component (or appliance) depends upon:
- The current, I
- The potential difference, V
- The amount of time the component is used for, t
- When charge flows through a resistor, for example, the energy transferred is what makes the resistor hot
- The energy transferred can be calculated using the equation:
E = P × t
- Where:
- E = energy transferred in joules (J)
- P = power in watts (W)
- t = time in seconds (s)
- Since P = IV, this equation can also be written as:
E = I × V × t
- Where:
- I = current in amperes (A)
- V = potential difference in volts (V)
- The electrical energy transferred also depends on the charge and potential difference:
E = Q × V
- Where:
- Q = charge in coulombs (C)
- V = potential difference in volts (V)
- When charge flows around a circuit for a given time, the energy supplied by the battery is equal to the energy transferred to all the components in the circuit
- These can be rearranged using the following formula triangles:
Energy, charge, potential different formula triangle
Energy, power, time formula triangle
Worked example
Calculate the energy transferred in 1 minute when a current of 0.7 A passes through a potential difference of 4 V.
Step 1: Write down the known quantities
- Time, t = 1 minute = 60 s
- Current, I = 0.7 A
- Potential difference, V = 4 V
Step 2: Write down the relevant equation
E = I × V × t
Step 3: Substitute in the values
E = 0.7 × 4 × 60 = 168 J
Examiner Tip
'Energy transferred' and 'work done' are often used interchangeably in equations, for example in the previous topic on PowerAlways remember that the time t in the above equations must always be converted into seconds
