GCSE Science/Circuits Part2

Now we have learned Ohm's Law we can start applying it to some circuits.

The test circuits below can be used to investigate how the voltage across a component varies with the current flowing through the component. Either circuit can be used. The one on the left is easiest to set up, the one on the right is easiest to use once it is set up, but does require you to use the rheostat as a potential divider.

Note that the voltmeter is in parallel with the component and the ammeter is in series with it.This is necessary so that the ammeter and voltmeter do not interfere with the circuit in anyway. An ideal meter does not change either the current or the voltage.

The wire should be resistance wire, such as nichrome. Ordinary copper wire would short circuit the power pack and blow the pack's fuse.


A test circuit	A better test circuit

Set up either circuit and then by turning the voltage setting on the power pack and the slider on the rheostat, adjust the current to read 0.1 A. Take a reading of the voltage. Repeat for 0.2 A, 0.3 A up to 1.0 A. Repeat the readings but this time go from 1.0 A to 0.9 A and so on down to 0.1 A.

Set your results out in a table like this.

Current (A)	Voltage going up (V)	Voltage going down (V)	Average voltage (V)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0

Now plot the average voltage against the current on graph paper. You should find the points all fall approximately on a straight line. The slope of the line =Voltage/Current. This is the resistance.

Below is a typical voltage/current graph for a wire. Note that all the points fall approximately on a straight line.

The slope of the line is 1.1 V/A.

Therefore the resistance of this wire is 1.1 Ω

All Ohmic components have a constant resistance like this. However, non-Ohmic components (that is, components that do not obey Ohm's law) do not have such a resistance. A bulb, for example, has a resistance that increases as the current flowing through it goes up, because the filament is heating up. A current-voltage graph for a bulb would be a curve.

Q1) Plot a current/voltage graph for the following set of data for a 12V filament bulb.

Summary

Resistance is the gradient of a current-voltage graph.
Ohmic conductors, such as a wire, will have a constant resistance (straight line on the graph).
Non-Ohmic conducts, such as a bulb, will have a changing resistance due to temperature (curved line on the graph).

Answers | <<Current, Voltage, Resistance | Parallel and series circuits>>