What is the primary difference between a potentiometer and a rheostat in terms of circuit connection?

A potentiometer uses three terminals to act as a variable voltage divider, while a rheostat uses only two terminals (one end and the wiper) to act as a variable series resistor for current control.

How does the behavior of an NTC thermistor differ from a PTC thermistor?

An NTC (Negative Temperature Coefficient) thermistor's resistance decreases as temperature increases, while a PTC (Positive Temperature Coefficient) thermistor's resistance increases as temperature increases.

Compare the resistance of an LDR in bright light versus total darkness.

In bright light, an LDR has very low resistance (often a few hundred ohms), whereas in total darkness, its resistance is extremely high (often several mega-ohms).

What common error occurs when calculating $V_{out}$ in a potential divider circuit?

Students often place the wrong resistance in the numerator of the formula. The numerator must always be the resistance of the specific component across which the output voltage is being measured.

Why is it a mistake to assume a linear relationship for sensory resistors like LDRs?

Sensory resistors are non-ohmic and typically follow a non-linear (often logarithmic) curve, meaning a doubling of the stimulus (like light) does not result in a simple halving of the resistance.

What happens if you only connect the two outer terminals of a potentiometer?

The component will function as a fixed resistor with a value equal to the maximum resistance of the track, and moving the wiper will have no effect on the circuit.

Define a 'Potential Divider'.

A potential divider is a simple linear circuit that produces an output voltage ($V_{out}$) that is a fraction of its input voltage ($V_{in}$), determined by the ratio of the resistances in the circuit.

What is the 'Wiper' in a variable resistor?

The wiper is the movable contact that slides along the resistive track of a potentiometer or rheostat, effectively changing the length of the resistive path included in the circuit.

State the standard Potential Divider formula for two resistors in series.

The formula is $V_{out} = V_{in} \cdot \left( \frac{R_2}{R_1 + R_2} \right)$, where $V_{out}$ is measured across $R_2$.

Why does the resistance of a thermistor decrease as it gets hotter?

Increased thermal energy provides enough energy for more electrons to break free from their atoms (liberating charge carriers), which increases the conductivity and thus decreases the resistance.

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4. Electrons, Waves & Photons

Variable Resistance Components

Summary

Variable resistance components are electronic devices whose electrical resistance can be adjusted either manually or in response to environmental stimuli. These components, including potentiometers, thermistors, and light-dependent resistors (LDRs), are fundamental in potential divider circuits to control output voltage, sense physical changes, and provide user-adjustable interfaces in electronic systems.

1. Definition & Core Concepts

Variable Resistors are components designed to provide a range of resistance values rather than a single fixed value. They allow for the dynamic control of current flow and potential difference within a circuit.
The Potentiometer is a three-terminal device used primarily to create a variable potential divider, providing a continuously adjustable output voltage from a fixed input.
Sensory Resistors, such as LDRs and thermistors, are non-ohmic components whose resistance changes automatically based on external physical conditions like light intensity or temperature.
In circuit diagrams, these components are typically represented by a standard resistor symbol with an added arrow, indicating the ability to vary the resistance or the presence of a sliding contact.

Diagram of a potentiometer acting as a potential divider, showing the input voltage across the resistive track and the variable output voltage taken from the sliding wiper contact.

2. Underlying Principles

3. Methods & Techniques

To use a potentiometer as a Rheostat (variable current control), only two terminals are used: one end of the track and the wiper. This varies the total resistance of the circuit branch.
To use it as a Voltage Divider, all three terminals are used. The full supply voltage is connected across the outer terminals, and the variable output is taken between the wiper and one outer terminal.
When designing Sensory Circuits, a sensory resistor is paired with a fixed resistor. If the sensor is placed in the 'lower' position of the divider, $V_{out}$ increases as the sensor's resistance increases.
Calibration is the process of mapping the output voltage of a sensory circuit to specific physical units (like degrees Celsius or Lux) by taking measurements at known reference points.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions