Resistors: fixed, variable, LDR, thermistor and diodes

What you'll learn

This revision guide covers the different types of resistors and diodes you need to know for AQA GCSE Physics. You'll learn how fixed resistors, variable resistors, light-dependent resistors (LDRs), thermistors and diodes behave in electrical circuits, including their characteristic current-potential difference graphs. Understanding these components is essential for Paper 2 and practical investigations involving circuits.

Key terms and definitions

Resistor — a component that opposes the flow of electric current, causing energy to be transferred to the thermal store of the surroundings

Variable resistor — a resistor whose resistance can be manually adjusted, allowing control of current in a circuit

Light-dependent resistor (LDR) — a resistor whose resistance decreases as light intensity increases

Thermistor — a temperature-dependent resistor whose resistance decreases as temperature increases (for negative temperature coefficient types used at GCSE)

Diode — a component that allows current to flow in one direction only, with very high resistance in the reverse direction

Ohmic conductor — a component that obeys Ohm's law, where current is directly proportional to potential difference at constant temperature

I-V characteristic — a graph showing how current through a component varies with potential difference across it

Forward bias — the direction in which a diode allows current to flow freely

Core concepts

Fixed resistors

Fixed resistors have a constant resistance value that does not change during normal operation. They are used to control the amount of current flowing through different parts of a circuit.

Circuit symbol: A rectangle (or sometimes a zigzag line in older diagrams)

Behaviour:

• At constant temperature, fixed resistors are ohmic conductors
• Current is directly proportional to potential difference (I ∝ V)
• They obey Ohm's law: V = IR
• Resistance remains constant regardless of current or voltage

I-V characteristic graph:

• Produces a straight line through the origin
• Gradient = 1/R (where R is resistance)
• The steeper the line, the lower the resistance
• Line passes through both positive and negative quadrants (works in both directions)

Energy transfers: When current flows through a fixed resistor, electrical energy is transferred to the thermal energy store of the resistor and surroundings. This is why resistors become warm during operation.

Common uses:

• Current limiting in LED circuits
• Voltage dividers
• Setting bias points in transistor circuits

Variable resistors

Variable resistors allow manual adjustment of resistance, giving control over current and potential difference in circuits.

Circuit symbol: Rectangle with arrow through it (or rheostat symbol for high-power applications)

Key features:

• Resistance can be changed by moving a sliding contact or rotating a dial
• Range typically from 0 Ω to maximum value (e.g., 100 Ω)
• Three terminals (though often only two are used in GCSE circuits)

Practical applications:

• Volume controls in audio equipment
• Dimmer switches for lighting
• Temperature controls in electric heaters
• Lab experiments investigating current and voltage relationships

In circuits: When resistance is increased, current decreases (assuming constant voltage supply). This allows precise control of circuit behaviour without changing components.

Light-dependent resistors (LDRs)

LDRs are input sensors whose resistance depends on light intensity falling on their surface.

Circuit symbol: Rectangle with two arrows pointing inward (representing incoming light)

Behaviour pattern:

• High light intensity → low resistance (can drop to around 100 Ω in bright light)
• Low light intensity → high resistance (can rise to several megohms in darkness)
• Change is continuous and reversible
• Response time typically a few milliseconds

Physical explanation: LDRs are made from semiconductor materials (often cadmium sulphide). When light photons strike the material, they provide energy to release charge carriers, increasing conductivity and decreasing resistance.

Real-world applications:

• Automatic street lighting that switches on at dusk
• Burglar alarms detecting torch light
• Light meters in cameras
• Garden solar lights
• Automatic car headlights

In sensing circuits: LDRs are commonly used in potential divider circuits to create outputs that vary with light level. This variable voltage can trigger other components or be monitored by data logging equipment.

Thermistors

The thermistors used at GCSE level are negative temperature coefficient (NTC) thermistors, where resistance decreases as temperature increases.

Circuit symbol: Rectangle with a horizontal line through it and temperature symbol (t°)

Behaviour pattern:

• High temperature → low resistance
• Low temperature → high resistance
• Relationship is non-linear (exponential decay)
• Highly sensitive to small temperature changes

Typical values:

• At 25°C: around 10 kΩ
• At 100°C: might drop to 500 Ω
• Exact values depend on thermistor specification

Physical explanation: Made from semiconductor ceramics containing metal oxides. As temperature rises, more charge carriers are released within the material, increasing conductivity and reducing resistance.

Real-world applications:

• Temperature sensors in car engines
• Thermostat controls in heating systems
• Fire alarm sensors
• Temperature compensation in electronic circuits
• Refrigerator temperature monitoring

In sensing circuits: Like LDRs, thermistors are typically used in potential divider arrangements to create temperature-dependent voltage outputs. These can activate switching circuits or provide temperature readings.

Diodes

Diodes are components that allow current to flow in only one direction, acting as a one-way valve for electricity.

Circuit symbol: Triangle pointing to a line (arrow shows direction of conventional current flow)

Forward bias operation:

• Current flows when anode (positive terminal) is connected to positive supply
• Very low resistance (close to zero after threshold voltage)
• Threshold voltage approximately 0.6-0.7 V for silicon diodes
• Below threshold voltage, minimal current flows

Reverse bias operation:

• Current prevented from flowing in opposite direction
• Extremely high resistance (effectively infinite at GCSE level)
• Only tiny leakage current (microamps) — negligible for GCSE purposes

I-V characteristic graph:

• Forward bias: Flat near zero until threshold voltage, then current increases rapidly
• Reverse bias: Remains at zero current (flat line along negative voltage axis)
• Graph is NOT symmetrical — distinctive feature of diodes

Light-emitting diodes (LEDs):

• Special type of diode that emits light when current flows
• Require protection resistor in series to limit current
• More efficient than filament bulbs
• Available in various colours (red, green, blue, white)
• Indicate power on/off status in electronic devices

Real-world applications:

• Rectification (converting AC to DC) in power supplies
• Protecting circuits from reverse polarity
• LEDs for lighting and displays
• Preventing current flowing wrong way in solar panel installations

Comparing component behaviours

Ohmic vs non-ohmic components:

Ohmic (at constant temperature):

• Fixed resistors
• Wires
• Produce straight-line I-V graphs through origin

Non-ohmic:

• Filament lamps (resistance increases with temperature)
• Diodes
• LDRs (when light level changes)
• Thermistors (when temperature changes)
• Produce curved or non-linear I-V graphs

Worked examples

Example 1: Thermistor in a temperature sensor circuit

Question: A thermistor is connected in series with a fixed 1000 Ω resistor and a 6 V battery. At room temperature, the thermistor has a resistance of 2000 Ω. Calculate: (a) The total circuit resistance [1 mark] (b) The current in the circuit [2 marks] (c) The potential difference across the thermistor [2 marks]

Solution:

(a) Total resistance = 1000 Ω + 2000 Ω = 3000 Ω ✓

(b) Using V = IR, rearranged: I = V/R ✓ I = 6 V ÷ 3000 Ω = 0.002 A (or 2 mA) ✓

(c) V = IR for thermistor ✓ V = 0.002 A × 2000 Ω = 4 V ✓

Example 2: LDR behaviour

Question: An LDR has a resistance of 5000 Ω in dim light and 500 Ω in bright light. It is connected in series with a 1500 Ω fixed resistor and a 9 V supply.

(a) Calculate the current when the LDR is in bright light. [2 marks] (b) Explain why the current increases when light intensity increases. [2 marks]

Solution:

(a) Total resistance in bright light = 500 Ω + 1500 Ω = 2000 Ω ✓ Current = V/R = 9 V ÷ 2000 Ω = 0.0045 A (or 4.5 mA) ✓

(b) When light intensity increases, the resistance of the LDR decreases ✓ Lower total circuit resistance means greater current can flow (I = V/R) ✓

Example 3: Diode I-V characteristic interpretation

Question: A student investigates the I-V characteristic of a diode. She obtains these results:

(a) Describe the pattern shown in the results. [2 marks] (b) State the approximate threshold voltage for this diode. [1 mark] (c) Explain why the diode does not conduct when the potential difference is negative. [2 marks]

Solution:

(a) When potential difference is negative, no current flows ✓ Above approximately 0.5 V, current increases rapidly with small increases in voltage ✓

(b) Approximately 0.6 V ✓ (Accept 0.5-0.6 V range)

(c) The diode is in reverse bias ✓ It has very high resistance in the reverse direction, preventing current flow ✓

Common mistakes and how to avoid them

• Confusing which way thermistor resistance changes: Remember NTC thermistors (GCSE standard) have lower resistance at higher temperature. Think: "hot = low R"

• Drawing I-V graphs with incorrect axes: Current (I) always goes on the y-axis, potential difference (V) on the x-axis. Label axes clearly with units (A and V)

• Assuming all resistors obey Ohm's law: Only fixed resistors at constant temperature are ohmic. Filament lamps, diodes, LDRs and thermistors are all non-ohmic components

• Getting diode direction wrong in circuit diagrams: Current flows from anode to cathode (in direction of triangle in symbol). The line in the symbol acts as a "barrier" to reverse current

• Forgetting to include units in calculations: Always state units (Ω, A, V) in final answers. Marks are often lost for missing units even when the numerical value is correct

• Misinterpreting LDR behaviour: More light = lower resistance (not higher). Students often reverse this relationship in exam answers

Exam technique for "Resistors: fixed, variable, LDR, thermistor and diodes"

• Command word "explain": You must give reasons or mechanisms, not just describe what happens. For example, explain why an LDR's resistance changes with light, mentioning charge carriers or conductivity

• Drawing circuit symbols: Use a ruler for straight lines. Standard symbols must be recognizable — learn the exact AQA-approved versions, particularly for LDR and thermistor which students often confuse

• Graph questions: When sketching I-V characteristics, show clear differences between components. Fixed resistor = straight line; diode = flat then steep curve in forward bias only; filament lamp = symmetric curve through origin

• Application questions: Questions often ask about real devices (street lights, thermostats). Identify which component is relevant (LDR for light-sensing, thermistor for temperature) and explain using potential divider principles if needed

Quick revision summary

Fixed resistors maintain constant resistance and obey Ohm's law at constant temperature, producing straight-line I-V graphs. Variable resistors allow manual resistance adjustment. LDRs decrease in resistance as light intensity increases; thermistors (NTC type) decrease in resistance as temperature rises — both are used as sensors in potential divider circuits. Diodes permit current flow in forward bias only (above ~0.6 V threshold) and block reverse current, creating asymmetric I-V characteristics. Understanding component behaviour and I-V graphs is essential for circuit analysis and applications questions.