Internal Resistance of a cell

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Topic: Internal Resistance of a cell

Aim: To measure the internal resistance and emf (the potential differences across a voltage Source    when no current is flowing) and to observe the combination of cells

Hypothesis: The emf  of the old cell is less than the emf of the new cell but the internal resistance of the old cell is much greater than the new cell.


                     All conductors have resistance, in good conductors only a small voltage is needed cause a current of 1A, so the resistance is low. In poor conductors the resistance is high; therefore a very large voltage is needed to cause a current of 1A.

Until now we have ignored the fact that when there is a current in a circuit, charges also flow through batteries or power supply producing it. The chemicals inside a battery or inside a power supply have resistance and this also affects the currents, this resistance inside a voltage source called the Internal Resistance.

As the cell gets older the internal resistance builds up until it is so large that only very small current can be drawn from the cell. When this happens the battery is ‘flat’ has outlived its useful life. The EMF of a cell remains the same even the cell gets ‘flat’ because the emf is caused by the chemical reactions occurring inside the cell. The increase in the internal resistance is due to the facts that the chemical composition of the cell changes as the reaction inside it proceeds (P.Howson. 1999)

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In this experiment we will compare the internal resistance of the cells using required apparatus mentioned below. Thus internal resistances and other conductor components give a total resistance to the current of electricity in a circuit. We will be able to deduce the EMF of each cell.


  • 2 large dry cells (one new and one old 1.5 volts)
  • Rheostat
  • Voltmeter 0-5 or 0-3 V)
  • Ammeter (0-500mA)
  • Switch
  • Connecting wires
  • Graph paper

Independent variables:

  • current (symbol I)

The currents (amps) is independent because we control/ adjusting it from the ...

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This is a 5 star piece of work investigating the emf and the internal resistance of an old and a new cell. It also investigates the effect of different combinations of cells to get the maximum voltage. Great introduction and excellent scientific knowledge and understanding displayed throughout. A good clear plan of work and hypothesis made. Excellent results but no repeats for reliability. Good graphs and analysis of results using suitable accurate equations. Overall an excellent piece of work achieving good results. A logical order which is easy to follow.