Sometimes exam questions about circuits will use resistors instead of lamps. Lamps are just like resistors except that they glow white-hot whereas resistors do their resisting more discreetly. Solving problems with resistors instead of lamps uses all the same principles. Often they include ammeters to show the current flowing in a particular part of the circuit and ask you to deduce the current in another part of the circuit. This uses the same skills as deciding whether a lamp is ‘bright’, ‘dim’, ‘normal’ or ‘out’.
The circuit below has a number of ammeters. The current leaving and returning to the cells must be the same so you can see that the current flowing down the different branches all adds up to the reading on A1.
What current will be passing through resistor ‘x’?
It must add to A2 to make 0.3A. Obviously it is 0.2A.
The 0.1A that passes through A2 joins up with the 0.2A passing through ‘x’ to make 0.3A and return to the cells.
The electrons lose energy in the form of heat as they pass through the resistors but they keep moving at the same rate.
Now you can try a slightly more difficult example for yourself – what fun! Look at the diagram below. All the resistors are identical so try to work out what current would show on A2, A3, A4, A5 and A7. You can leave your answers in the comments section.
Questions…
- What are the readings on A2,A3, A4, A5 & A7?
- What would the voltage be in the two circuits above? (No tricky calculation required!)
- By convention, which way does current flow in the circuit?
- What can be said about the current that leaves and returns to the cell?
- Suggest an equation that links voltage, current and resistance.
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