Energy and Voltage in Circuits » Investigating Current and Voltage

The circuit you need

You have seen the current-voltage graphs for a wire, a resistor, a metal filament lamp and a diode. The spec also asks you to know how to investigate this experimentally. In other words, how are the readings behind those graphs collected? The same basic circuit works for all four components, with a few changes for the diode, so you only need to learn it once.

You do not have to have done this experiment yourself. You do need to be able to describe it clearly enough for somebody else to follow.

Build the circuit like this, or draw it in this order in the exam:

1. Start with a low-voltage d.c. power supply or a battery.
2. Connect a switch, an ammeter, a variable resistor and the component you are testing in series, in one loop back to the supply.
3. Connect a voltmeter in parallel with the component, with one lead touching each end of it.

Part	Its job
Power supply or battery	Provides the voltage that drives the current
Ammeter, in series	Measures the current in the component
Voltmeter, in parallel across the component	Measures the voltage across the component only
Variable resistor, in series	Changes the current in the circuit, and so the voltage across the component
Switch	Lets you turn the current off between readings

How does the variable resistor change the voltage? When you increase its resistance, the current in the loop gets smaller, so the voltage across the component falls. When you decrease its resistance, both go up. Some power supplies have a dial that changes the voltage directly, and that works just as well.

Key terms:

• Anomalous result: a reading that does not fit the pattern of the others.
• Line of best fit: a straight line or smooth curve drawn through the pattern of the plotted points, with points that miss it spread evenly on both sides.
• Zero error: when a meter does not read zero although there is nothing to measure.
• Protective resistor: a fixed resistor put in series with a component to stop the current becoming large enough to damage it.

The method, step by step

In this investigation you choose the voltage, so the voltage across the component is the independent variable. The current in the component is the dependent variable. The component itself must not be swapped part way through.

1. Set up the circuit with the variable resistor at its highest resistance. Check the circuit before closing the switch.
2. Close the switch. Adjust the variable resistor until the voltmeter reads the first voltage, for example 1.0 V.
3. Read the ammeter and record the voltage and the current in a table.
4. Open the switch.
5. Repeat for at least five different voltages, going up in equal steps.
6. Reverse the connections to the power supply, so the current is in the opposite direction. Take the same set of readings again and record them as negative values.
7. Repeat the whole experiment and calculate a mean current for each voltage.
8. Plot a graph of current against voltage.

A digital ammeter or voltmeter shows a minus sign by itself when the supply is reversed. Step 6 is what gives you the left-hand half of the graph.

What changes for each component

The diode needs small steps because its current changes very quickly over a very small range of voltage. With steps of 1 V you would miss the part of the graph where it turns upwards. A variable resistor in series cannot set these small voltages, because a diode that is passing almost no current takes nearly all of the supply voltage. That is why the dial on the supply is used here. With the supply reversed, expect the milliammeter to stay at zero.

Recording the results

Draw the table before you start, with the quantity and its unit in every heading. Here is a set of results for a fixed resistor.

Voltage in V	Current in A, first reading	Current in A, second reading	Current in A, third reading	Mean current in A
1.0	0.05	0.05	0.05	0.05
2.0	0.10	0.11	0.09	0.10
3.0	0.15	0.15	0.15	0.15
4.0	0.20	0.21	0.19	0.20
5.0	0.25	0.31	0.25	0.25

Repeating the readings is what lets you spot an anomalous result. Without repeats, the 0.31 A would have gone straight onto the graph.

Plotting the graph

1. Put voltage on the horizontal axis and current on the vertical axis. Label both with the quantity and unit.
2. If you have negative readings, draw the axes so they cross in the middle of the grid.
3. Choose simple, even scales that spread the points over more than half of the grid.
4. Plot each point as a small, neat cross.
5. Draw a line of best fit. For a wire or resistor, use a ruler. For a lamp or a diode, draw one smooth curve by hand.

Never join the points dot to dot. The shapes you should get, and what they mean, are explained in the lesson Current-Voltage Graphs.

Sources of error and safety

Exam-style question

A student investigates how the current in a length of metal wire varies with the voltage across it. The student has a battery, a switch, an ammeter, a voltmeter, a variable resistor and connecting leads.

(a) State how the ammeter and the voltmeter should be connected. [2 marks]

(b) Describe how the student should use this equipment to obtain the readings needed for a current-voltage graph. Include both positive and negative voltages. [5 marks]

(c) Explain why the student should open the switch between readings. [2 marks]

(d) At 3.0 V the student records three currents: 0.24 A, 0.26 A and 0.25 A. Calculate the mean current. [2 marks]