Potential divider (AQA A-Level Physics): Revision Notes

5.1.5 Potential divider

Potential Divider Basics

A potential divider is a circuit containing multiple resistors in series across a voltage source. Its main purpose is to produce a specific fraction of the source voltage as an output, which remains stable.

• By altering the resistance values within the circuit, the potential divider can vary the output potential difference.
• One common approach is to use a variable resistor within the potential divider circuit. Adjusting this resistor changes the output voltage by modifying the current and, consequently, the potential difference across each component.

Potential Divider Formula:

The output potential difference, $V_{out}$ , across a resistor $R_2$ in a series arrangement with another resistor $R_1$ is given by:

$$V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$$

Where:

• $V_{in}$ is the input (source) voltage.
• $R_1$ and $R_2$ are the resistances of the resistors in series.

Using Light-Dependent Resistors (LDRs) and Thermistors in Potential Dividers

Potential dividers can be adapted for sensing applications by using a light-dependent resistor (LDR) or a thermistor:

• LDR (Light Dependent Resistor): Its resistance decreases as light intensity increases.

• Thermistor: Its resistance changes with temperature (in most cases, decreases as temperature rises). When an LDR or thermistor is used in a potential divider circuit:

• Changes in light intensity or temperature directly affect the resistance of the LDR or thermistor.

• This in turn changes the output voltage, which can be used to trigger specific responses in a circuit (e.g., turning on a light when it gets dark).

Applications of Potential Dividers in Real-World Circuits

• Light Sensors: Automatically turn on street lamps as it gets dark.
• Temperature Sensors: Use thermistors to regulate heating systems. By carefully choosing components and their arrangement, potential dividers provide an effective way to control and adjust voltage outputs in response to environmental changes, making them essential in various automatic control systems.