Most components have a fixed resistance. But some are designed to change theirs — automatically, in response to the world around them. A thermistor reacts to heat, a light-dependent resistor reacts to brightness, and a potentiometer is a dial you turn by hand. These are the components that let a circuit sense its surroundings, and they’re behind everything from street lights to thermostats.
📘 What you need to know
A variable resistor is one whose resistance can change, rather than staying fixed
A thermistor is a temperature sensor: as temperature rises, its resistance falls
A light-dependent resistor (LDR) is a light sensor: as light rises, its resistance falls
Both are non-ohmic — their resistance is not constant
A potentiometer is a variable resistor used to give an adjustable output voltage
Sensors are usually placed in a potential divider so a changing resistance gives a changing output voltage
Thermistors — sensing heat
A thermistor is a resistor whose resistance depends on its temperature. The type you’ll meet in IB is called an NTC thermistor (negative temperature coefficient), and its behaviour is simple to remember:
Thermistor rule (NTC)temperature up → resistance downtemperature down → resistance up
The hotter the thermistor gets, the lower its resistance drops. A steep fall that levels off — the classic thermistor curve.
This makes thermistors perfect temperature sensors. They’re used in ovens, fire alarms, digital thermometers and fridges — anywhere a circuit needs to react to how hot or cold things are.
Here’s the intuition for why heat lowers the resistance: warming the material frees up more charge carriers to move, so charge flows more easily. It’s the opposite of a plain metal wire, where heating raises resistance. That flip is exactly why a thermistor counts as non-ohmic — and why examiners like testing it. Just lock in the rule: NTC thermistor, hotter means lower resistance.
Light-dependent resistors (LDRs) — sensing light
A light-dependent resistor (LDR) works the same way, but responds to light instead of heat. Its resistance depends on how much light falls on it (the light intensity):
LDR rulelight up (brighter) → resistance downlight down (darker) → resistance up
Brighter light means lower resistance. In the dark an LDR’s resistance is huge (millions of ohms); in bright light it drops to just tens of ohms.
LDRs make great light sensors. The classic use is automatic street lights: in daylight the LDR’s low resistance keeps the lights off; as dusk falls and the light fades, its resistance shoots up, and the circuit switches the lights on.
Notice the pattern — both the thermistor and the LDR do the same thing: “more of the stimulus, less resistance.” More heat, less resistance. More light, less resistance. If you remember one, you remember both. Just swap “temperature” for “light intensity” on the axis.
The potentiometer
The third variable resistor is one you control by hand: the potentiometer. It’s like a variable resistor with a sliding contact, wired so that it acts as a potential divider — giving a continuously adjustable output voltage. Think of the volume dial on a speaker: sliding the contact changes how the voltage is split, so you can dial the output anywhere between a maximum and zero.
Sensors in a potential divider
On their own, a thermistor or LDR just changes its resistance — but circuits respond to voltage, not resistance. To turn a changing resistance into a changing voltage, we put the sensor in a potential divider: two resistors in series across a supply, with the output voltage tapped off across one of them.
Two resistors share the supply voltage. The output is the slice taken across R₂ — and it depends on the ratio of the two resistances.
The output voltage is the share taken by the lower resistor. Since the current is the same through both (they’re in series), the voltage splits in proportion to resistance:
Now swap one of those resistors for a sensor. If R1 is a thermistor, then as it warms up its resistance falls — so it takes a smaller share of the voltage, and R2 gets a bigger share. The output voltage rises with temperature. That rising voltage can then switch on a fan, sound an alarm, or feed a controller.
WE 1
A potential divider has a 12 V supply. R1 = 4.0 kΩ is on top and R2 = 8.0 kΩ is on the bottom. The output is taken across R2. Calculate the output voltage.
Step 1 — use the divider equationVout = Vin × R2 / (R1 + R2)
Step 2 — substitute (units of kΩ cancel)Vout = 12 × 8.0 / (4.0 + 8.0) = 12 × 8/12Vout = 8.0 VR₂ is twice R₁, so it grabs two-thirds of the 12 V. Bigger resistance = bigger voltage share.
WE 2
A thermistor (R1) is in series with a fixed 2.0 kΩ resistor (R2) across a 9.0 V supply, with the output taken across R2. When cold, the thermistor is 4.0 kΩ; when hot, it drops to 1.0 kΩ. Find the output voltage in each case, and describe the change.
Cold: thermistor = 4.0 kΩVout = 9.0 × 2.0 / (4.0 + 2.0) = 3.0 VHot: thermistor drops to 1.0 kΩVout = 9.0 × 2.0 / (1.0 + 2.0) = 6.0 VOutput rises from 3.0 V to 6.0 V as it heatsAs the thermistor heats, its resistance falls, so it takes less voltage — leaving more for the fixed resistor. The output climbs, and that rising voltage can trigger a fan or alarm.
Stimulus changes heat / light
sensor resistance changes
Divider output voltage changes
triggers a response
Circuit reacts
The three variable resistors at a glance
Component
Responds to
Behaviour
Thermistor (NTC)
Temperature
Hotter → lower resistance
Light-dependent resistor (LDR)
Light intensity
Brighter → lower resistance
Potentiometer
A hand-turned slider
Gives an adjustable output voltage
💡 Top tips
Thermistor: hotter → lower R. LDR: brighter → lower R. Same idea, different stimulus.
Both are non-ohmic — their resistance isn’t constant, so no straight-line I–V graph.
Sensors go in a potential divider to turn changing resistance into changing voltage.
Divider rule: the bigger resistance takes the bigger share of the voltage.
In the dark an LDR is huge (millions of Ω); in bright light it’s tiny (tens of Ω).
⚠ Common mistakes
Getting the thermistor the wrong way round — for NTC, hotter means lower resistance
Thinking an LDR’s resistance rises in bright light — it falls
Putting the output across the wrong resistor in a divider — check which one you’re tapping
Assuming the bigger resistor gets the smaller voltage — it gets the bigger share
Treating a thermistor or LDR as ohmic — their resistance changes, so they’re non-ohmic
Quick recap: Variable resistors change their resistance. A thermistor’s resistance falls as it heats; an LDR’s falls as light brightens — both are non-ohmic sensors. A potentiometer gives an adjustable voltage by hand. Placing a sensor in a potential divider (Vout = Vin × R2/(R1+R2)) turns its changing resistance into a changing output voltage that a circuit can act on.
That wraps up the whole Current & Circuits unit — from reading a circuit diagram all the way to sensors that let a circuit feel the world. You now have the full toolkit: current, voltage, resistance, power, real batteries, and the components that respond to their surroundings. Head back to the unit hub to revise any topic, and try a few past-paper questions to lock it all in.
Finished the unit? Let’s make it stick.
Book a free meeting and we’ll run through sensors, potential dividers and anything else that’s still fuzzy.
