GCSE — physicsMedium4 min read

Resistors, LDRs, and Thermistors

I-V characteristics; factors affecting resistance; required practical

Tutor AI Team2026-02-25T05:09:48.322Z

# Resistors, LDRs, and Thermistors — GCSE Physics

Not all resistors behave the same way. Some change their resistance depending on external conditions like temperature or light. Understanding these components is key for designing sensor circuits.

1. Fixed Resistors

A fixed resistor has a constant resistance (at constant temperature). It obeys Ohm's Law: the I-V graph is a straight line through the origin.

2. Filament Lamp

A filament lamp's resistance increases as current increases because the filament gets hotter. Hotter metal ions vibrate more → more collisions with electrons → higher resistance. The I-V graph is a curved S-shape through the origin.

3. Thermistors (NTC)

A thermistor (Negative Temperature Coefficient) is a resistor whose resistance decreases as temperature increases.

Temperature	Resistance	Current
Cold	High	Low
Hot	Low	High

How it works: At higher temperatures, more charge carriers are released in the semiconductor material, reducing resistance.

Applications:

Temperature sensors (thermostats, car engines, fire alarms)
In potential divider circuits to control devices based on temperature

4. Light-Dependent Resistors (LDRs)

An LDR is a resistor whose resistance decreases as light intensity increases.

Light Level	Resistance	Current
Dark	High	Low
Bright	Low	High

How it works: Light energy frees more charge carriers in the semiconductor, reducing resistance.

Applications:

Automatic street lights (turn on when dark)
Camera light meters
Security lights

5. Diodes

A diode allows current to flow in one direction only.

Forward bias: current flows (very low resistance after threshold ~0.7 V)
Reverse bias: no current flows (very high resistance)

LED (Light-Emitting Diode): A diode that emits light when forward-biased. Used in displays, indicators, and lighting.

6. Sensor Circuits with Potential Dividers

A potential divider is two resistors in series. The voltage is shared proportionally.

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

Thermistor in a Potential Divider

When temperature rises → thermistor resistance falls → $V_{\text{out}}$ across the thermistor falls (and across the fixed resistor rises)
Can be used to trigger a fan or alarm when temperature exceeds a threshold

LDR in a Potential Divider

When light increases → LDR resistance falls → $V_{\text{out}}$ across the LDR falls
Can be used to trigger lights when it gets dark

7. Required Practical: I-V Characteristics

Method

Connect the component (resistor, lamp, or diode) with an ammeter in series and voltmeter in parallel
Include a variable resistor to change the p.d.
Record pairs of I and V values
Reverse the battery and repeat for negative values
Plot I (y-axis) against V (x-axis)

Expected Results

Resistor: Straight line through origin (constant resistance)
Filament lamp: S-curve (resistance increases with temperature)
Diode: No current in reverse; sharp increase in forward direction after ~0.7 V

Worked Example: Example 1

Problem

Question: An LDR has a resistance of 500 kΩ in the dark and 1 kΩ in bright light. It's in series with a 10 kΩ resistor and a 5 V supply. Calculate the current in: (a) dark, (b) bright light.

(a) $R_T = 500{,}000 + 10{,}000 = 510{,}000$ Ω. $I = 5/510{,}000 = 9.8 \times 10^{-6}$ A = 9.8 μA

(b) $R_T = 1000 + 10{,}000 = 11{,}000$ Ω. $I = 5/11{,}000 = 4.5 \times 10^{-4}$ A = 0.45 mA

Solution

Worked Example: Example 2

Problem

Question: A thermistor has 50 kΩ resistance at 20°C and 2 kΩ at 80°C. It's connected to a 6 V battery in series with a 3 kΩ resistor. Calculate the voltage across the 3 kΩ resistor at each temperature.

At 20°C: $I = 6/(50{,}000 + 3000) = 1.13 \times 10^{-4}$ A. $V = 1.13 \times 10^{-4} \times 3000 = 0.34$ V

At 80°C: $I = 6/(2000 + 3000) = 1.2 \times 10^{-3}$ A. $V = 1.2 \times 10^{-3} \times 3000 = 3.6$ V

Solution

9. Practice Questions

1. Describe how the resistance of a thermistor changes as temperature increases. (1 mark)
1. State one application of an LDR. (1 mark)
1. A diode is connected in reverse bias. What happens to the current? (1 mark)
1. Sketch the I-V characteristic of a filament lamp. Explain its shape. (4 marks)
1. Design a circuit using a thermistor that could switch on a fan when the temperature gets too high. (4 marks)
Answers

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Summary

Thermistor: resistance decreases as temperature increases
LDR: resistance decreases as light intensity increases
Diode: allows current in one direction only
Filament lamp: resistance increases as it heats up
These components are used in sensor circuits with potential dividers

Tags:

#gcse#physics#electricity#resistors#ldr#thermistor

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