Energy Transfers (AQA GCSE Physics Combined Science): Revision Notes
Electrical energy
What is electrical energy?
Electrical appliances are designed to transfer energy from one form to another. When you use any electrical device, energy gets transferred through the flow of electric charge.
Power of a device
Power tells us how quickly energy is transferred. The power of any electrical device depends on:
- The potential difference (voltage) across it
- The current flowing through it
A device that transfers more energy in a shorter time has higher power.
Power in Practice:
- A kettle with 2 kW transfers much more energy to heat water than a 10W light bulb in the same time
- Higher power devices work faster but use more electricity
Energy, charge and potential difference
These three quantities are connected by this important equation:
Where:
- E = energy transferred (measured in joules, J)
- Q = charge flow (measured in coulombs, C)
- V = potential difference (measured in volts, V)
This means work is done when charge flows in a circuit.
Using the formula triangle
You can rearrange the formula using a triangle method:
- Cover up the quantity you want to find
- The position of the other two tells you the calculation
Measuring in circuits
Voltmeters measure potential difference and are always connected in parallel with components.
Ammeters measure current and are always connected in series with components.
Calculating Charge:
You can calculate charge using:
Where I = current (amps) and t = time (seconds)
Working out electrical energy
Worked Example: Calculating Electrical Energy
Step 1: Find the charge that flows Use if you know current and time
Step 2: Use Multiply charge by potential difference
Sample Calculation:
- A charge of 25C passes through a motor
- Potential difference across motor = 6V
- Energy transferred =
Different ways to calculate energy
Alternative Formula:
You can also use:
This combines the two formulas when you know current, voltage and time.
Key Points to Remember:
- Electrical energy = charge × potential difference ()
- Higher power devices transfer more energy per second
- Voltmeters go in parallel, ammeters go in series
- Work is done when electric charge flows through circuits
- You can calculate charge from current and time ()