3.5.1.5
Potentiometer
Circuits & Potential Dividers — AQA A-Level Physics
- A variable resistorA component whose resistance can be manually adjusted, used to control current in a circuit. replacing R_2 makes V_out adjustable.
- V_out can range from 0 V to V_in.
- A potentiometerA potential dividerA circuit that uses two or more resistors in series to produce a fraction of the source voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference. across one of the resistors. with a sliding contact that allows continuous variation of the output voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference. from zero to the supply voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference.. is a single variable resistor wired as a potential dividerA circuit that uses two or more resistors in series to produce a fraction of the source voltage across one of the resistors..
Key Definition
Potentiometer — A variable potential divider using a single resistor with a sliding contact. Output p.d. varies continuously from 0 to V_in.
PotentiometerA potential divider with a sliding contact that allows continuous variation of the output voltage from zero to the supply voltage. circuit
A single resistor with three terminals: two ends connected across V_in, a sliding contact providing V_out.
- Useful in volume controls and dimmer switches.
- Gives a smoothly variable output, unlike fixed resistor pairs.
Key Definition
Temperature sensor circuit — A potential divider using a thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). and a fixed resistor. As temperature rises, thermistor resistance falls, so V_out across the thermistor falls.
- ThermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient).: resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohms (Ω). decreases as temperature increases (NTC type).
- If V_out is measured across the thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient)., V_out falls when temperature rises.
- If V_out is measured across the fixed resistor, V_out rises when temperature rises.
Key Definition
Light sensor circuit — A potential divider using an LDRLight-dependent resistor. A component whose resistance decreases as light intensity increases. and a fixed resistor. As light intensity increases, LDR resistance falls, so V_out across the LDR falls.
- LDRLight-dependent resistor. A component whose resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω). decreases as light intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. increases.: resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). decreases as light intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. increases.
- If V_out is measured across the LDRLight-dependent resistor. A component whose resistance decreases as light intensity increases., V_out falls in brighter light.
- If V_out is measured across the fixed resistor, V_out rises in brighter light.
ThermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). and LDRLight-dependent resistor. A component whose resistance decreases as light intensity increases. sensor circuits
Two potential dividerA circuit that uses two or more resistors in series to produce a fraction of the source voltage across one of the resistors. circuits side by side. Left: thermistor as R_2 with V_out across it. Right: LDRLight-dependent resistor. A component whose resistance decreases as light intensity increases. as R_2 with V_out across it.
- Sensor circuits can trigger connected devices (e.g. heater, alarm).
- The variable resistor sets the threshold at which the device activates.
Worked Example
A 9.0 V supply is connected to a 1.0 k-ohm fixed resistor (R_1) and a thermistor (R_2). At 20 degrees C, the thermistor has a resistance of 2.0 k-ohm. Calculate V_out across the thermistor. What happens to V_out if the temperature increases to 50 degrees C and the thermistor resistance drops to 500 ohm?
Show Solution
1
At 20 degrees C
$V_out = R_2 / (R_1 + R_2) \times\;\text{V}_in = 2000 / (1000 + 2000) \times 9.0 = 6.0\;\text{V}.$
2
At 50 degrees C
$V_out = 500 / (1000 + 500) \times 9.0 = 500/1500 \times 9.0 = 3.0\;\text{V}.$
3
V_out halves as the thermistor resistance falls.
Answer
6.0 V at 20 degrees C; 3.0 V at 50 degrees C. V_out decreases as temperature increases.
Common Mistake
MEDIUM
Students often: Stating V_out always decreases when temperature increases, without specifying which component V_out is measured across.
Instead: V_out across the thermistor decreases. V_out across the fixed resistor increases. Always state which component.
Instead: V_out across the thermistor decreases. V_out across the fixed resistor increases. Always state which component.
Examiner Tips and Tricks
- AQA commonly asks 'explain how V_out changes when temperature increases.' State the sequence: temperature up, thermistor resistance down, share of p.d. across thermistor decreases, so V_out across thermistor decreases.
- Give all three steps.