Fixed resistor and metal wire
- The graph for a fixed resistor, or for a metal wire at constant temperature, is a straight line through the origin.
- The line continues through the origin into the negative values, and the graph is symmetrical, because the resistor opposes the current equally in both directions.
- The straight line shows a constant resistance, which is what a resistor is designed for, since it sets a fixed current for a given potential difference.
- A resistor with a larger resistance gives a less steep line, because a smaller current flows for each volt.
- If a wire carries a large current and becomes warm, the ends of its line start to bend towards the potential difference axis, because its resistance rises as it heats up.
Filament lamp
Definition
Filament lamp
A lamp that gives out light because a thin metal filament inside it is heated to a very high temperature by the current.
- The graph for a filament lamp is a curve through the origin that becomes less steep as the potential difference increases in either direction, giving a shallow S-shape.
- The graph is symmetrical, because the filament heats up in the same way whichever direction the current flows.
- At small potential differences, the filament is cool and the graph is almost straight.
- As the current increases, the filament gets hotter, its ions vibrate more, the electrons collide with them more often, and its resistance increases.
- This property is tied to the job of the lamp, because the filament must become very hot to give out light, so its resistance unavoidably rises when it is working.
Diode
Definition
Diode
A component that lets current flow through it in one direction only, because its resistance is very high in the opposite direction.
- A diode only allows current to flow through it in one direction, called the forward direction.
- In the reverse direction, the diode has a very high resistance, so the current is almost zero and the graph lies along the negative potential difference axis.
- In the forward direction, the current stays almost zero until the potential difference reaches about 0.6 V0.6\ \text{V}0.6 V, and then rises very steeply.
- Above this potential difference, the diode has a very small resistance.
- The graph is therefore not symmetrical, which is the clearest sign of a component that treats the two directions differently.
- Diodes are used to make sure current can only flow one way, for example to protect a device from damage if its battery is put in the wrong way round.
NTC thermistor
Definition
NTC thermistor
A resistor whose resistance decreases as its temperature increases.
- The resistance of an NTC thermistor decreases as its temperature increases.
- A graph of resistance against temperature is a curve that falls steeply at low temperatures and then levels off at higher temperatures.
- On a current-potential difference graph, a thermistor held at one temperature gives a straight line, and a thermistor at a higher temperature gives a steeper line because its resistance is smaller.
- If the current is large enough to warm the thermistor, its line curves upwards and becomes steeper, because its resistance falls as it heats itself.
- Because its resistance responds to temperature, a thermistor is used as a temperature sensor in thermostats, oven controls, fire alarms and digital thermometers.
Light-dependent resistor
Definition
Light-dependent resistor (LDR)
A resistor whose resistance decreases as the intensity of the light falling on it increases.
- The resistance of an LDR decreases as the light intensity falling on it increases.
- In darkness, an LDR can have a resistance of around a million ohms, while in bright light its resistance can fall to a few hundred ohms.
- A graph of resistance against light intensity is a curve that falls steeply at low light intensity and then levels off.
- On a current-potential difference graph, an LDR in brighter light gives a steeper straight line, because its resistance is smaller.
- Because its resistance responds to light, an LDR is used as a light sensor, for example in street lights that switch on at dusk and in automatic night lights.
Example
Reading resistance from a lamp graph
- A current-potential difference graph for a filament lamp passes through the points 2.0 V2.0\ \text{V}2.0 V, 0.25 A0.25\ \text{A}0.25 A and 6.0 V6.0\ \text{V}6.0 V, 0.40 A0.40\ \text{A}0.40 A.
- The resistance has nearly doubled between the two points, which matches the curve becoming less steep as the filament heats up.
- At −6.0 V-6.0\ \text{V}−6.0 V, the graph reads −0.40 A-0.40\ \text{A}−0.40 A, so the resistance is again 15 Ω15\ \Omega15 Ω and the lamp behaves the same in both directions.
Practical
Characteristics of a filament lamp and a diode
- Aim: to obtain the current-potential difference characteristics of a filament lamp and a diode and relate their shapes to how each component works.
- Apparatus: a 6 V6\ \text{V}6 V filament lamp in a holder, a diode, a 100 Ω100\ \Omega100 Ω protective resistor, a d.c. power supply set to 6 V6\ \text{V}6 V, a variable resistor, a switch, a digital ammeter, a digital voltmeter and connecting leads.
- Method, the filament lamp:
- Connect the supply, switch, variable resistor, ammeter and lamp in series, with the voltmeter across the lamp only.
- Start with the variable resistor at its largest value, and record the potential difference and current at about eight settings up to 6.0 V6.0\ \text{V}6.0 V, waiting for the lamp to settle each time.
- Reverse the supply connections and repeat to obtain the negative readings.
- Method, the diode:
- Replace the lamp with the diode and the protective resistor in series, keeping the voltmeter across the diode only.
- Record readings in steps of about 0.1 V0.1\ \text{V}0.1 V from 0 V0\ \text{V}0 V to about 0.8 V0.8\ \text{V}0.8 V, because the current changes rapidly in this region.
- Reverse the supply connections and record readings up to −3.0 V-3.0\ \text{V}−3.0 V across the diode.
- Method, processing:
- Repeat each set of readings and calculate means.
- Plot a separate graph for each component with current on the vertical axis and potential difference on the horizontal axis, and draw a smooth curve of best fit through each set of points.
- Results: the lamp gives a symmetrical curve that flattens at higher potential differences, while the diode gives almost no current in reverse and a sharp rise above about 0.6 V0.6\ \text{V}0.6 V in the forward direction.
- Conclusion: the resistance of the lamp increases as it heats up, while the diode has a very high resistance in one direction and a very low resistance in the other once it starts to conduct.
- Watch out: take more readings where the graph changes shape quickly, and draw curves freehand in one smooth stroke rather than as straight segments.
- Safety: the lamp glass becomes hot, so do not touch it during or just after the experiment, and never test the diode without the protective resistor.
Common Mistake
- Do not draw the diode graph as a straight line in the forward direction, because the current stays near zero until about 0.6 V0.6\ \text{V}0.6 V and then rises steeply.
- Do not say that the resistance of an NTC thermistor increases with temperature, because it decreases.
- Do not say that an LDR has a low resistance in the dark, because its resistance is highest in the dark.
Exam technique
Relating graphs to components
- When you are asked to relate a graph to a component, give three linked points, which are the shape, what it shows about the resistance, and the reason or use.
- For a filament lamp, the curve gets less steep, so the resistance increases, because the filament heats up.
- For a diode, there is no current in reverse, so the resistance is very high in one direction, which lets current flow one way only.
- When you sketch a graph, include both positive and negative values unless the question tells you otherwise, because symmetry is often what is being tested.
Self review
- Sketch and describe the current-potential difference graph for a filament lamp.
- Explain why the graph for a diode is not symmetrical.
- State how the resistance of an NTC thermistor changes as its temperature increases.
- Describe one use of an LDR and explain why it suits that use.
- State what happens to the line for a thermistor on a current-potential difference graph when it is warmer.
