Recall and use the equation for p.d. V = W / Q

4.2.3 Electromotive Force and Potential Difference

What is Electromotive Force (EMF)?

EMF is the energy supplied by a source (like a battery) to move charge around a circuit. Think of it as the “push” that gets electrons moving, just like a water pump pushes water through a pipe. 🚰

Potential Difference (Voltage)

The potential difference between two points is the work done per unit charge to move a test charge between those points. It’s measured in volts (V). 1 V = 1 J/C. ⚡️

The Key Equation

The relationship between work, charge and voltage is:
$$V = \frac{W}{Q}$$
where V is the potential difference, W is the work done (in joules) and Q is the charge (in coulombs). 📐

Water‑Pump Analogy

- Battery = water pump - Electrons = water molecules - Voltage = water pressure - Current = flow rate of water If the pump (battery) is strong, it creates a high pressure (high voltage) that pushes more water (current) through the pipes (circuit). 💧

Example Calculation

Exam Tips

• Remember the formula: V = W/Q – it’s a simple fraction.
• Check units: J/C = V. If you get volts, you’re on the right track.
• When a question asks for “potential difference”, they mean voltage.
• Use the water‑pump analogy to explain EMF in words.
• Show all steps and include units in your answer.

Practice Questions

1. Calculate the voltage if 150 J of work is done on 0.3 C of charge.
2. Explain why a battery can be considered a source of EMF.
3. A circuit has a voltage of 12 V and a current of 2 A. What is the power supplied by the battery? (Use P = VI)
4. In a 9 V battery, 0.5 C of charge moves through the circuit. How much work is done by the battery?