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Calculating Energy Transfers (AQA GCSE Physics: Combined Science)
Revision Note
How to Calculate Energy Transferred
- 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 energy transferred electrically 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 transferred from the cell or 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 Power. Always remember that the time t in the above equations must be converted into seconds
Students often find it really confusing that there are so many different equations for energy.
When you begin a calculation, start by writing out the known variables. These are the values you are given in the question. Once you know which variables you have and which variable you are calculating, you can use these to select the correct equation. So if you are given power and time, you would use E = Pt, but if you are given charge and potential difference, you would use E = QV.
Sometimes, there may be more than one variable missing, and you may have calculated a value for it in an earlier part of the question. For harder questions, you may need to use another equation to calculate the missing variable. By listing the known quantities at the start of your calculation, you plan how to get to your answer and choose the most appropriate equations for the task.
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