Two resistors in series share the supply voltage in proportion to their resistances. Make one of them a sensor and the tapped-off voltage becomes a signal that responds to temperature or light.
A potential divider uses two resistors in series to split a supply voltage. The output across R₂ is Vₒᵤₜ = Vᵢₙ × R₂ / (R₁ + R₂). A potentiometer is a continuously variable divider, used in null methods to compare potential differences. Replacing one resistor with a thermistor or LDR makes a sensor circuit whose output voltage changes with temperature or light.
Move the divider tap, or swap in a thermistor or LDR, and watch the output voltage follow V × R₂ / (R₁ + R₂) as the resistances change.
The output is the supply scaled by the resistance ratio.
| Relation | Use |
|---|---|
| Vₒᵤₜ = V R₂/(R₁+R₂) | potential divider output |
| potentiometer | a variable divider for null methods |
| sensor divider | thermistor or LDR sets the output |
Four quick checks tied to this lesson. Each correct answer earns XP and lights this skill on your star map.
In a potential divider, the output voltage across R₂ is:
A 12 V supply is divided by two equal resistors. The output across one of them is:
A potentiometer is used to:
In a potential divider, a 4.0 V output is wanted from a 12 V supply across R₂ = 2.0 kΩ. The value of R₁ is:
Sensor dividers turn a physical change into a voltage.
Use the resistance of the component you take the output across in the numerator: Vₒᵤₜ = V R₂/(R₁ + R₂). When a sensor replaces one resistor, decide carefully which resistor the output is measured across, because a falling sensor resistance can make the output rise or fall depending on the arrangement. The unloaded divider equation assumes negligible current is drawn from the output.
Unlocks once the checks above are done. Worth more XP, written to AS Paper 1 and 2 standard.
A 9.0 V supply is connected across a 1.0 kΩ and a 2.0 kΩ resistor in series. The p.d. across the 2.0 kΩ resistor is:
In a thermistor potential divider, the temperature rises. The thermistor resistance falls, so the p.d. across the thermistor:
A potentiometer compares two potential differences by adjusting until the galvanometer reads:
This skill is now lit gold on your star map. You have finished the lessons of Topic 10; the Paper 1 set awaits.