1. 5.3 Electric cells

2. Primary cell Designed to be used once and thrown away – the electrochemical reaction in the cell is not easily reversible.

3. Primary cell

5. 5.3 Discharge characteristics.

6. Terminal p.d. loses its initial value quickly, has a stable and constant value for most of its lifetime, followed by a rapid decrease to zero as the cell discharge completely.

7. Secondary cell The reaction can be reversed by running a current into the cell with a battery charger to recharge it, regenerating the chemical reactants.

8. Secondary cell Have quite a high self-discharge where chemical reactions inside the battery reduce the energy available. This is why primary cells are used in many devices instead.

9. Re-charging a cell

11. Internal resistance

12. We have assumed so far that the power source has no resistance…….not a good assumption!

13. Internal resistance In actuality the p.d. across a cell is less than the EMF due to energy lost in the INTERNAL RESISTANCE

14. Internal resistance To help us visualize this, a cell is represented as a “perfect” cell of emf ε attached in series to the internal resistance, given the symbol r.

15. Internal resistance Using Kirchoff’s 2 nd law, the emf ( ε ) of the “perfect” cell is equal to the sum of the p.d.s around the circuit. ε = IR + Ir ε = I(R + r)

16. Example A cell of emf 12V and internal resistance 1.5 Ω produces a current of 3A. What is the p.d. across the cell terminals? ε = I(R +r) = V + Ir V = ε - Ir V = 12 – 3x1.5 V = 7.5 V

17. Internal resistance Connecting a voltmeter (VERY high resistance) across the terminals of a cell measures the EMF of the cell (no current flowing) V

18. 5.3 Measuring internal resistance

19. 5.3 Internal resistance questions