Resistors — AQA GCSE Combined Science

For an ohmic conductor kept at constant temperature, current I is directly proportional to potential difference V. This means doubling V doubles I, so the ratio V ÷ I is constant. Since resistance is defined as R = V ÷ I, the resistance remains constant as current changes. The current–potential difference graph is a straight line through the origin, and its gradient equals 1 ÷ R. A fixed resistor at room temperature is a common example. If the temperature changes, for example in a filament lamp, the straight-line relationship breaks down and the component becomes non-ohmic. To use this idea, check that the graph is a straight line through the origin, then calculate R from V ÷ I at any point on the line.

Ohmic conductors keep a constant resistance when temperature is fixed, giving a straight-line current–potential difference graph through the origin. Many components are non-ohmic: resistance alters as current changes. In a filament lamp, larger currents heat the filament, raising resistance. A diode conducts in one direction, with high reverse resistance and a forward threshold. A thermistor's resistance falls as temperature rises, and an LDR's resistance falls as light intensity rises. To investigate, connect the component in series with an ammeter and variable resistor, and place a voltmeter in parallel across the component. Record paired readings, then calculate R = V ÷ I.

A filament lamp contains a thin metal wire that glows when current passes through it. As the current increases, more energy is transferred to the filament each second, so its temperature rises. In a metal, higher temperature makes the ions vibrate more strongly, which increases the number of collisions with the moving electrons and so increases resistance. This means the current–potential difference graph curves: the gradient decreases as potential difference increases, showing that resistance is not constant. At low potential differences the filament is cool and resistance is lower; at higher potential differences it is hot and resistance is higher. Students should be able to use R = V ÷ I at a chosen point on the graph to show the resistance has increased.

A diode is a non-ohmic component that acts as a one-way valve for electric current. In forward bias, conventional current flows easily from the anode to the cathode, so the diode conducts and its resistance is low. In reverse bias, the diode blocks current because its resistance is very high, so the current is negligible. This behaviour is shown by a current–potential difference graph: the forward curve rises steeply after about 0.6 V for a silicon diode, while the reverse curve stays close to zero. A diode is therefore useful for rectification, converting alternating current into direct current, and for protecting circuits from incorrect polarity.

A thermistor is a temperature-dependent resistor, usually made from a semiconductor oxide. At low temperatures, few charge carriers are available, so its resistance is high. As temperature increases, more charge carriers are released, so the resistance decreases. This means the current through a thermistor increases for a given potential difference as it warms up. Thermistors are used in temperature sensors, thermostats and protection circuits. On a resistance–temperature graph the curve slopes downwards; on a current–potential difference graph the line curves upwards because resistance falls as temperature rises. The thermistor is therefore a non-ohmic component.

A thermistor is a resistor whose resistance changes with temperature. In the negative temperature coefficient (NTC) thermistor used in GCSE courses, resistance decreases as temperature increases, so it is a temperature-sensitive component. In a thermostat circuit, the thermistor forms part of a potential divider or control circuit. At low temperature its resistance is high, so the potential difference across it is large; this can switch on a heater. As the room warms, its resistance falls, the potential difference across it falls, and the heater switches off. The circuit therefore keeps temperature roughly constant. You should be able to describe this application and explain how the changing resistance controls a device, rather than simply naming a thermostat.

An LDR, or light-dependent resistor, is a resistor whose resistance depends on light intensity. As light intensity increases, more charge carriers are released in the semiconductor material, so the resistance of the LDR decreases. In darkness its resistance is high, often very high; in bright light its resistance is much lower. This makes an LDR useful as a light sensor in circuits such as automatic street lights, burglar alarms and light meters. In a potential divider, the changing resistance of the LDR changes the potential difference across it, which can switch a device on or off. You should be able to state the relationship and explain how it is used, not just recall the name.

An LDR (light-dependent resistor) is a resistor whose resistance changes with light intensity: in bright light its resistance falls, and in darkness its resistance rises. This makes it useful as a light sensor. In a potential divider, the LDR is connected in series with a fixed resistor across a supply. The output voltage is taken across one of them. As light intensity changes, the LDR's share of the supply voltage changes. For example, with the output across the LDR, the output voltage rises as it gets darker, because the LDR's resistance increases and takes a larger share of the supply voltage. This changing voltage can drive a switching circuit, such as a transistor, to turn a lamp on when it gets dark.

To find a component's resistance you need its current and its potential difference at the same time, then apply R = V ÷ I. Connect the component in series with a cell, switch and ammeter so the same current flows through all of them; connect a voltmeter in parallel across the component only. The ammeter measures current in amperes, the voltmeter measures potential difference in volts. Close the switch, record both readings, then vary the supply or add a variable resistor and repeat to obtain several pairs. For each pair calculate R = V ÷ I; a roughly constant value indicates ohmic behaviour at constant temperature. This method suits a fixed resistor, filament lamp or diode, and the design must ensure the meters do not alter the very quantities being measured.

A circuit diagram is a standardised drawing that shows how components are connected without showing their physical appearance. Use the recognised symbols: a cell as a long and short parallel line, a switch as a break with a hinged line, a resistor as a rectangle, an ammeter as a circle containing A, a voltmeter as a circle containing V, and a lamp as a circle with a cross. Draw connecting wires as straight lines with right-angled corners, and place the ammeter in the series loop and the voltmeter in parallel across the component being tested. An appropriate diagram for measuring resistance therefore shows the cell, switch, ammeter and component in one loop, with the voltmeter connected across the component. Neat, correctly symbolised diagrams let an examiner or another student build the circuit exactly as intended.

Plot current on the y-axis against potential difference on the x-axis for each component. A straight line through the origin shows a linear element obeying V = IR with constant resistance; its gradient equals 1/R. A fixed resistor at constant temperature gives this straight line. A filament lamp curves because increasing current heats the filament, raising resistance, so the gradient falls. A diode conducts only above a forward threshold, giving near-zero current in reverse and a steep rise forwards, so it is non-linear. Compare gradients and shapes to link each curve to the element's function and properties, such as constant resistance, heating, or one-way conduction.

Draw a circuit diagram with a cell, ammeter in series, voltmeter in parallel with the test component, and a variable resistor to change the potential difference. Connect one component at a time: a fixed resistor kept at constant temperature, a filament lamp, and a diode. Record current and potential difference for several settings, including negative values where possible. For the resistor, keep it cool and note the straight-line graph. For the lamp, observe the curve as heating raises resistance. For the diode, note the forward threshold and near-zero reverse current. Repeat readings and check connections to reduce error.