Variable Potential Divider

Objectives

Lesson outcomes

Explain why increasing the resistance in one part of a potential divider increases the p.d. across that part.
Describe how a variable potential divider changes its output when a resistor value changes.
Interpret circuit diagrams that use variable resistors, LDRs, or thermistors in potential dividers.

Syllabus

CIE 0625 syllabus points

3 linked

4.3.1.1 Draw and interpret circuit diagrams containing cells, batteries, power supplies, generators, potential dividers, switches, resistors (fixed and variable), heaters, thermistors (NTC only), light- dependent resistors (LDRs), lamps, motors, bells, ammeters, voltmeters, magnetising coils, transformers, fuses and relays and know how these components behave in the circuit
4.3.3.1 Know that the p.d. across an electrical conductor increases as its resistance increases for a constant current
4.3.3.2 Describe the action of a variable potential divider

Lesson Notes

Student guidance and lesson notes

Overview

This lesson extends the basic potential-divider idea into circuits that respond to changing conditions. You are using the same rule as before, but now one resistor can change, so the output p.d. changes too.

What You Need to Know

In a variable potential divider, the output p.d. changes because one resistance changes.
If the current stays the same, a larger resistance gets a larger share of the total p.d.
A variable resistor changes when you adjust it directly.
An LDR changes resistance with light level, and a thermistor changes resistance with temperature.
You should be able to explain how the output changes when the sensor resistance goes up or down.

How to Work Through It

Start with a two-resistor potential divider and decide which resistor gets the larger p.d.
Replace one resistor with a variable resistor, LDR, or thermistor and predict what changes.
Compare a few sensor-circuit examples and explain how the output responds to light or temperature.
Finish by linking the circuit diagram to a real control use, such as an automatic light or alarm.

Check Your Understanding

If the resistance of one part of the divider increases, what happens to the p.d. across it?
In an LDR circuit, how would the output change when the light level changes?
Can you explain the difference between a fixed divider and a variable divider in one sentence?

Common Mistakes

Treating the output as if it changes randomly. It changes because the resistance ratio changes.
Forgetting that the current is the same in the series divider while the p.d. splits unequally.
Reversing the effect of a sensor because the position of the output point in the circuit has not been checked carefully.

Next Steps

Practise predicting output changes from circuit diagrams before you calculate anything.
Carry this circuit-control idea into the next lesson on relays and diodes.