Using D.C. Circuits to Investigate Components

A test circuit that measures two things

To investigate a component, we need to measure the current through it and the potential difference across it. Current is the rate of flow of charge; potential difference describes the energy transferred per unit charge. Together, these readings allow us to calculate resistance:

$$R = \frac{V}{I}$$

Here, $R$ is resistance in ohms ($\Omega$), $V$ is potential difference in volts (V), and $I$ is current in amperes (A).

A low-voltage direct current supply drives charge around the test circuit in one direction. The test component, variable resistor and ammeter are connected in series. Because the same current passes through these components, the ammeter measures the current through the test component. The voltmeter connects in parallel across only the test component, measuring the potential difference between its two ends.

Increasing the variable resistor’s resistance reduces the current. Adjusting it gives a range of current and potential difference readings without rebuilding the circuit. An adjustable d.c. supply can also be used to change the readings.

Collecting an I–V characteristic

An I–V characteristic shows how current changes with potential difference. Use a low-voltage d.c. supply, connecting leads, an ammeter, a voltmeter, a variable resistor and the component being tested.

1. With the supply switched off, assemble the circuit. Set the variable resistor to a high resistance to limit the starting current.
2. Switch on and record the current through the component and the potential difference across it.
3. Adjust the variable resistor or supply in small steps. At each setting, wait for the meter readings to settle and record a pair of readings.
4. Switch off before reversing the supply connections. Repeat to investigate current in the opposite direction, recording these readings with negative signs.
5. Repeat measurements, for example three times at each setting, and calculate mean readings. Plot current against potential difference and draw a suitable best-fit line or curve.
6. Switch off before replacing the test component with a filament lamp, diode, thermistor or .

For a suitable fixed resistor, a school investigation might use supply settings from 0 to 6 V. The safe range depends on the component: do not assume that every component, especially a diode, can withstand the same settings. Keep current within the component’s rating, use the variable resistor to limit it, and avoid short circuits. Filament lamps and heated components can become hot; let them cool before touching them.

You have enough readings when they cover the chosen safe range and show the relationship clearly. Calculate $V/I$ at non-zero readings to see whether resistance stays constant or changes. A fixed resistor kept at constant temperature provides a useful comparison: its I–V graph is a straight line through the origin.

Filament lamps: current changes temperature

A filament lamp contains a thin wire that becomes hot enough to glow when current flows through it. As current increases, the filament gets hotter and the lamp becomes brighter. The higher temperature increases the filament’s resistance.

Consequently, doubling the potential difference does not double the current. On an I–V graph, the curve becomes less steep at larger potential differences: current still increases, but by progressively smaller amounts for equal increases in potential difference. Reversing the supply gives a corresponding curve on the negative side because the filament heats whichever way the current flows.

Diodes: direction matters

A diode behaves differently in its two directions. In the forward direction, little current flows at small potential differences; once the potential difference is sufficient, current rises rapidly. In the reverse direction, its resistance is very high and current is negligible within the safe testing range.

Reversing the supply therefore reveals a strongly asymmetric I–V characteristic. Unlike a filament lamp, a diode does not produce matching positive and negative curves. Limit the current carefully when testing the forward direction, where a small increase in potential difference can cause a large increase in current.

LDRs: separate light from current

An LDR’s resistance is high in dim light and lower in bright light. To investigate this, replace the test component with an LDR and illuminate it using a lamp whose brightness can be adjusted.

Begin with the lamp off in a darkened setting. Record current and potential difference, then increase the illumination in stages. Keep the lamp’s position fixed, keep other lighting unchanged, and avoid warming the LDR. Place the lamp close enough to illuminate it, but not touching it. After each change, wait for the readings to settle and calculate the LDR’s resistance.

The calculated resistance should decrease as illumination increases. At the same potential difference across the LDR, a lower resistance means a larger current. This response makes LDRs useful for sensing light levels, such as in automatic outdoor lighting.

When investigating the effect of changing current through an LDR instead, keep its illumination and temperature constant. Otherwise, a resistance change caused by brighter light could be mistaken for an effect of changing current.

NTC thermistors: separate temperature from current

An NTC thermistor has a high resistance when cold and a lower resistance when hot. To investigate temperature, use it as the test component and place a controlled heater nearby, without touching it. Use a thermometer to record the temperature near the thermistor.

Start with the heater off. Increase the heating in stages, allowing the temperature and meter readings to settle before recording temperature, current and potential difference. Calculate resistance for each temperature, then compare the results or plot resistance against temperature. Keep the electrical testing current small so that the thermistor is not significantly warmed by the measurement itself.

As temperature rises, the calculated resistance decreases. At the same potential difference across the thermistor, current therefore increases. This makes thermistors useful as temperature sensors, including in electronic thermostats. Handle the heater carefully, keep leads away from hot surfaces and allow the apparatus to cool afterwards.

Increasing current through a thermistor can itself cause heating. If this raises its temperature, its resistance falls. To distinguish an effect of current from an effect of temperature, monitor or control its temperature. The key contrast is that heating increases a filament’s resistance but decreases an NTC thermistor’s resistance.

Test circuit

• Low-voltage d.c. supply; test component, ammeter and variable resistor in series.
• Voltmeter in parallel across the test component.
• Change current, record paired $I$ and readings, repeat and calculate means.