Types of Resistor

For some resistors, the value of the resistance remains constant but that in others it can change as the current changes.

The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.

The resistance of a filament lamp increases as the temperature of the filament increases. The resistance increases because as the ions in the metal warm up they also vibrate more.

PD – Current graph for a filament lamp

The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

PD – current graph for a diode

The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.

PD – current graph for a fixed resistor

On the graphs above, the gradient is equal to 1/resistance. So a steep line is a low resistance.

The resistance of a thermistor decreases as the temperature increases. Thermistors are used in temperature sensing circuits, like thermostats.

The resistance of an LDR decreases as light intensity increases. LDRs are used in light sensing circuits.

The potential difference is directly proportional to the current

The graph is a straight line through the origin, which has a constant gradient

metals contain free electrons (and ions)

as temperature of filament increases ions vibrate faster

so electrons collide more (frequently) with the ions

Figure 2

Circuit for investigating current-potential characteristics of a bulb
Circuit for investigating current-potential characteristics of a bulb

To investigate how the potential difference across a component changes when you change the current through the component:

Use the variable resistor to change the current in the circuit

Record the potential difference and the current

Plot a graph of potential difference on the x-axis and current on the y-axis

The resistance at any point is 1/gradient

Circuit to investigate current-PD characteristics of a lamp

Use the ammeter to measure the current

Use the voltmeter to measure the potential difference

Use the variable resistor to vary the current in the circuit, record current and PD for at least 6 pairs

Plot a graph with potential difference on the x-axis and current on the y-axis.

The resistance at any point on the graph is 1/gradient.