E.M.F. and Internal Resistance

Overview

Real sources do not behave like perfect batteries. This lesson separates the energy supplied by a source from the energy available across its terminals, then uses internal resistance to explain why terminal potential difference falls when current is drawn.

What You Need to Know

• Circuit diagrams use standard symbols so that cells, resistors, switches, meters, and sensors can be interpreted without ambiguity.
• Use e.m.f. for the energy supplied by the source to each unit charge around the complete circuit.
• Use potential difference for the energy transferred by each unit charge in a component.
• A source with internal resistance transfers some energy to charge inside the source itself. This reduces the terminal potential difference available to the external circuit.
• The terminal p.d. is equal to the e.m.f. when no current is being drawn, but it decreases when the current through the internal resistance increases.
• The energy transferred inside the source is often described as lost volts.

How to Work Through It

1. Start by reviewing circuit symbols and drawing simple circuits cleanly.
2. Use energy-per-charge language to define e.m.f. and compare it with p.d. across a component.
3. Add an internal resistor to the source model and identify where energy is transferred inside and outside the source.
4. Explain how increasing current changes the terminal p.d. and the lost volts.

Check Your Understanding

• Why is e.m.f. not simply another name for terminal p.d.?
• What happens to terminal p.d. when the current supplied by a real cell increases?
• Where is energy transferred when charge passes through internal resistance?
• Why is the open-circuit terminal p.d. close to the e.m.f. of the source?

Common Mistakes

• Defining e.m.f. as a force rather than energy transferred per unit charge.
• Treating the internal resistance as an extra external resistor instead of part of the source model.
• Assuming terminal p.d. is always equal to e.m.f.
• Explaining lost volts as charge disappearing rather than energy being transferred inside the source.

Next Steps

• Practise explaining source behaviour using energy-per-charge language.
• Use the circuit-symbol and energy ideas when applying Kirchhoff’s laws in the next lesson.