2.1.4 Resistors

In this lesson, we will explore the properties and behaviour of resistors, as well as other components whose resistance can vary. We will discuss how the resistance of different components can change with current, temperature, or light intensity. Additionally, we will examine the applications of thermistors and LDRs in circuits.

Resistors and Variable Resistance

Let's begin by understanding the difference between resistors and components with variable resistance.

Resistors:

• Constant Resistance: Resistors are components designed to have a constant resistance (R) that remains unchanged as the current flowing through them changes.
• Ohmic Conductors: Ohmic conductors, such as metallic wires, exhibit a linear relationship between current and potential difference, meaning their resistance remains constant.

Variable Resistance Components:

• Non-Constant Resistance: Some components, such as lamps, diodes, thermistors, and LDRs (Light Dependent Resistors), have resistance that can change with current, temperature, or light intensity.
• Changing Resistance: The resistance of these components can either increase or decrease depending on the specific characteristics of the component.

Behaviour of Variable Resistance Components

Let's explore the behaviour of specific variable resistance components.

Filament Lamp:

• Resistance and Temperature: The resistance of a filament lamp increases as the temperature of its filament increases. This behaviour is due to the increased resistance of the filament material at higher temperatures.
• Applications: Filament lamps are commonly used in lighting applications, where their variable resistance characteristics are utilised.

Diode:

• One-Way Current Flow: The current through a diode flows in one direction only, from the anode to the cathode. In the reverse direction, the diode has a very high resistance.
• Applications: Diodes are used in rectifier circuits and as electronic switches due to their one-way current flow behaviour.

Thermistor:

• Resistance and Temperature: The resistance of a thermistor decreases as the temperature increases. This behaviour is due to the change in the electrical properties of the thermistor material with temperature.
• Applications: Thermistors are used in circuits where temperature sensing and control are required, such as thermostats.

LDR (Light Dependent Resistor):

• Resistance and Light Intensity: The resistance of an LDR decreases as the intensity of incident light increases. This behaviour is due to the change in the conductivity of the LDR material when exposed to light.
• Applications: LDRs are used in light-sensitive circuits, such as street lights that automatically switch on when it gets dark.

Measuring Resistance in a Circuit

To measure the resistance of a component, we need to measure the current flowing through it and the potential difference across it. An appropriate circuit can be designed using a power supply, an ammeter to measure current, and a voltmeter to measure potential difference.

Draw a circuit diagram using the correct circuit symbols, including the component to be measured, the ammeter, and the voltmeter.

Conclusion

We explored the characteristics of resistors and variable resistance components. We learned that resistors have a constant resistance, while other components exhibit variable resistance that can change with current, temperature, or light intensity. We discussed the behaviours of filament lamps, diodes, thermistors, and LDRs, as well as their applications in various circuits. Additionally, we explored the design and use of a circuit to measure the resistance of a component by measuring current and potential difference.