4.3.2 Internal resistance

It is important to understand that different power sources have different internal resistances. E.g if a large current is needed, a power supply with a small internal resistance is used. If a small current is needed, a power supply with a large internal resistance is used.

So, what is internal resistance? Basically, when there is a current in a power source work has to be done by the charges to move through the power source. The p.d. measured at the terminals of the power source (the terminal p.d.) is always less than the actual emf (provided there is a current in the circuit) - the difference is the lost volts.

From K2 we can determine this equation:

electromotive force (emf) = p.d. + lost volts

It is important to realise that changing the current will change the lost volts and consequently the terminal p.d.. An increase in the current means that more charges travel through the cell each second meaning overall more work is done by the charges. This increases the lost volts which subsequently reduces the terminal p.d.

We can apply V=IR to lost volts too (lost volts = Ir, where r is the internal resistance). This means that total emf is equal to IR + Ir...

emf = I(R+r)

We also need to be able to determine the internal resistance of a cell/source of emf:

• Create a series circuit with a variable resistor
• Introduce a voltmeter across a power supply
• Have an ammeter in the circuit (it doesn't matter where as it is a series circuit)
• Change the resistance of the circuit - this will draw different currents from the power source.
• Rearrange E= V+Ir to give V = -Ir + E
• Plot a graph of V against I
• The gradient will be -r, the y intercept will be the emf