Circuits

A circuit needs a source of potential difference, such as a cell or power supply, and a complete conducting path. Closing a switch completes that path; opening it breaks the path.

Current is the rate at which electric charge flows, measured in amperes (A). Potential difference, also called voltage, is the energy transferred per unit charge between two points, measured in volts (V). Resistance describes how strongly a component opposes current, measured in ohms (Ω).

These quantities are connected by:

Measuring the potential difference across a component and the current through it lets you calculate its resistance:

For example, suppose a resistor has a potential difference of 3.0 V across it and a current of 0.20 A through it. Its resistance is $3.0/0.20 = 15\,\Omega$. For a lamp or diode, the calculated resistance can change as the operating conditions change: one reading does not necessarily describe the component at every voltage.

Reading a circuit diagram

Circuit diagrams show electrical connections using standard symbols, rather than showing what the apparatus looks like on the bench. Follow the wires and junctions to decide which components share a path and which are on separate branches.

A cell supplies potential difference; a battery contains two or more cells. A switch opens or closes a path. A fixed resistor limits current, while a variable resistor has an adjustable resistance. A filament lamp transfers electrical energy to light and heating.

A diode allows current in one direction only. An LED is a light-emitting diode. Conventional current travels from the positive terminal towards the negative terminal through the external circuit, so diode orientation matters.

Some components respond to their surroundings: an LDR’s resistance decreases as light intensity increases, and the thermistor used in school circuits has a resistance that decreases as temperature increases. These provide other ways to explore changing resistance.

Connecting the meters correctly

An ammeter goes in series with the component: charge passing through the component must also pass through the ammeter. A voltmeter goes in parallel across the component, with one lead connected to each end. It compares those two points rather than measuring along the whole circuit.

The variable resistor changes the current and therefore the potential difference across the test component. The voltmeter measures the actual potential difference across that component; it need not equal the power supply setting.

Build the circuit with the supply switched off. Trace the complete main loop, then check that the voltmeter leads connect to opposite ends of the test component. Check meter polarity, suitable ranges and whether the meters read zero before taking measurements. Start with a low supply setting and have the connections checked before switching on.

Investigating a component’s I–V characteristic

Use a low-voltage d.c. supply, connecting leads, an ammeter, a voltmeter, a variable resistor and the component under investigation. A fixed resistor, filament lamp and diode provide useful contrasting examples.

1. Connect the fixed resistor as the test component in the measuring circuit.
2. Adjust the variable resistor to obtain a small potential difference across it. Record both potential difference and current.
3. Repeat the reading and calculate a mean current. If readings differ noticeably, check connections and whether the component is heating.
4. Change the potential difference in small steps, collecting a spread of readings. A table headed potential difference / V and mean current / A keeps the quantities and units clear.
5. Switch off, reverse the supply connections and collect readings with the opposite polarity. Record these as negative potential differences and currents.
6. Switch off before replacing the resistor with the filament lamp, then the diode. Repeat the measurements within each component’s safe operating limits.

Keep the same component and connecting equipment throughout each set of readings. For the fixed resistor, keep its temperature as constant as possible by using small currents and switching off between readings.

Plot mean current on the vertical axis against potential difference on the horizontal axis. The investigation is complete when you have enough readings across both polarities to identify the characteristic shape, rather than just a single resistance value.

Comparing the component characteristics

A fixed resistor at constant temperature gives a straight line through the origin. Doubling its potential difference doubles its current, so $V/I$ remains constant. This is Ohm’s law. On the same axis scales, a steeper straight line represents a smaller resistance because more current flows at a given voltage.

A filament lamp gives a curved graph. Current is not directly proportional to potential difference, so its resistance is not constant across the readings. It is a non-ohmic conductor.

A diode also has a non-ohmic characteristic: its graph is not a straight line through the origin. Unlike the resistor and lamp, it allows current in only one direction. Reversing the supply therefore lets you investigate its direction-dependent behaviour as well as how its current changes with potential difference.

Constructing and checking series and parallel circuits

Begin with one fixed resistor, measure its current and potential difference, and calculate its resistance. Repeat for a second resistor separately. You can then compare their individual resistances with the resistance of the pair.

For a series circuit, connect the two resistors end to end in one main loop. Put the ammeter in that loop and the voltmeter across the whole pair to calculate total resistance. You can then move the voltmeter, with the supply switched off, to measure across each resistor separately.

The same current passes through both resistors. Their potential differences add to the potential difference across the pair, and their resistances add:

For a parallel circuit, connect each resistor in a separate branch between the same two junctions. An ammeter before the junction measures total current; an ammeter placed within a branch measures only that branch’s current. Each branch has the same potential difference, and the branch currents add to the total current.

Calculate the pair’s resistance using the potential difference across the branches divided by the total current. It should be less than either resistor’s individual resistance: the extra branch gives charge another route.

Check the physical circuit against the diagram by tracing each path from one supply terminal to the other. A component is not in parallel merely because it sits beside another on the bench: both ends must connect to the same two junctions. Repeat the comparison using filament lamps, measuring resistance at each operating condition rather than assuming it is constant.

Safe working and dependable results

Keep liquids away from electrical apparatus. Avoid touching wires or components while the circuit is operating: wires carrying large currents and filament lamps can become hot. Switch off between readings and before rewiring, and allow hot components to cool before handling them. If you smell burning or see unexpected overheating, switch off immediately and tell the teacher.

Avoid a direct wire connection across the supply, which could allow a dangerously large current. Stay within the ratings of the supply, meters and components.

Use repeated readings to identify inconsistent results and reduce random uncertainty. Check for zero errors before starting. Unwanted heating can change resistance during a measurement, while real meters can slightly affect the circuit. These effects help explain why measured values may not agree perfectly with the simple circuit model.