11 November Part 1 of Internal Resistance (AKA "the mystery of the lost volts")

Internal resistance - the mystery of the lost volts.

Extract from the revision guide (slightly edited) : 

The energy provided by a source (e.g. a cell or battery) is delivered to the components of the circuit by charge flowing round the circuit.  However for most sources some of the energy is dissipated inside the source due to the source's internal resistance. When a current is drawn from the source this internal resistance causes the potential difference across the terminals of the source to be less than the emf of the source. 

The lost p.d. in the source is the energy dissipated per unit charge (i.e. per coulomb) inside the source due to its internal resistance. The lost p.d. depends on the current and on the internal resistance of the source.    Lost p.d. = current x internal resistance
For a source of emf ε with internal resistance r connected to a load of resistance R, as shown in the circuit below  ε = I R + I r  where IR is the potential difference across the load resistance and Ir is the lost p.d. 

The external p.d. V = I R = ε – Ir. The graph below shows how the external p.d. V varies with the current drawn. This graph has a gradient of  –r and a y-intercept equal to ε.   The intercept on the x-axis is the short circuit current, with a terminal p.d. of zero volts.

Note that the p.d. V falls as the current increases. This is why the output potential difference of an electrical source of energy (including a laboratory power supply unit) falls if more current is drawn from the source. The headlights of a car often dim for a moment as you operate the starter motor. 

The videos

Here are a number of introductory videos - watch them and add to  your  notes.

Also make sure you go over the topic in the Chapter 2 revision guide and in the textbook chapter 2.


Useful introduction from www.Fizzics.org.uk



Further introduction and explanation of the graph of V against I -



Quite a good lecture on the theory



Demonstration of a potato cell.  Different metal plates are inserted into a potato to make a cell that produces about one volt.  However it has a high resistance and can only provide very small current - in micro amps.  The current is varied by switching a resistance box to different values.  No sound track - just watch the meters.




This final video shows some practical work - the image is bit fuzzy but the physics explanations are clear.  Note the results plotted on the graph of V against I