Retrieval Practice
EMF & Internal Resistance — AQA A-Level Physics
Q1. Define the electromotive force (EMF) of a power supply and state its unit.
- EMF is the electrical energy per unit charge produced by the source.
- Unit: volt (V).
Q2. What causes a cell to have internal resistance?
Collisions between charge carriers (electrons) and atoms inside the cell as current flows through it.
Q3. Write the EMF equation relating epsilon, I, R, and r.
epsilon = I(R + r), which expands to epsilon = IR + Ir.
Q4. What are 'lost volts' and how are they calculated?
- Lost volts (v) are the p.d. dropped across the internal resistance.
- Calculated as v = Ir.
Q5. How does terminal p.d. relate to EMF and lost volts?
- V = epsilon - Ir.
- Terminal p.d. equals the EMF minus the lost volts.
Q6. On a V-I graph for a cell, what do the y-intercept and gradient represent?
- y-intercept = EMF (epsilon).
- Gradient = negative of internal resistance (-r).
Q7. What is the total EMF and total internal resistance when identical cells are connected in series (same direction)?
- Total EMF = sum of individual EMFs.
- Total internal resistance = sum of individual internal resistances.
Q8. What happens to EMF and internal resistance when n identical cells are connected in parallel?
- EMF stays the same as one cell.
- Total internal resistance = r/n.
Q9. When does the terminal p.d. equal the EMF?
- When current I = 0 (open circuit).
- V = epsilon - Ir becomes V = epsilon.
Q10. Why should the circuit be connected for as short a time as possible when measuring EMF and r?
To prevent the cell heating up, which would change its internal resistance and make results unreliable.