Topic B.5 โ Current & CircuitsPaper 1 & 2Thermistors, LDRs, potentiometers~6 min read
Variable Resistors
Not every resistor keeps a fixed value. Some are built to change on purpose โ either because we turn a dial ourselves, or because they respond automatically to what’s happening around them, like temperature or light.
๐ What You Need to Know
A thermistor changes resistance with temperature โ for most, resistance falls as it heats up
A light-dependent resistor (LDR) changes resistance with light โ resistance falls as light gets brighter
A potentiometer is a resistor with a sliding contact, letting you dial in any output voltage you like between two limits
These components are all non-ohmic โ their resistance isn’t a fixed number, it depends on conditions
Thermistors โ sensing temperature
A thermistor is a resistor that reacts to heat. In most thermistors, warming it up gives the charge carriers more energy to move about, so resistance actually drops as temperature rises โ and rises again as it cools. This makes thermistors brilliant temperature sensors, and you’ll find them inside ovens, fire alarms, and digital thermometers.
A thermistor’s resistance falls sharply as it warms up โ the colder it is, the more it resists current.
An LDR works the same way, but responds to light instead of heat. In the dark, an LDR’s resistance is huge โ often millions of ohms. Shine a bright light on it, and that resistance collapses down to just a few tens of ohms. This is exactly how automatic street lights know when to switch on: as it gets dark, the LDR’s resistance climbs, and a circuit uses that change to trigger the lights.
Just like a thermistor, an LDR’s graph starts high and falls away โ but the trigger here is light, not heat.
Potentiometers โ dialling in a voltage
A potentiometer is a variable resistor with a sliding contact that splits it into two parts. Moving the slider changes how much resistance is on each side โ which in turn changes how the supply voltage is shared between them. This lets you dial in any output voltage between zero and the full supply, just by sliding the contact up and down. It’s exactly how a volume dial or a light dimmer switch works.
Sliding the contact up and down shares the supply voltage differently between the two parts of the resistor โ the voltmeter reads whatever fraction sits below the slider.
Quick recap: thermistors sense temperature, LDRs sense light โ both usually drop in resistance as the trigger increases; potentiometers let you dial in a chosen fraction of the supply voltage.
WE 1
A thermistor is connected in series with a fixed resistor and a battery. If the room cools down, what happens to the potential difference across the thermistor?
Think about the thermistor’s resistance:
Cooling down means its resistance increases.
Think about the current:
They’re in series, so the same current flows through both โ and Ohm’s law says V = IR.
The potential difference across the thermistor increases, since its resistance has gone up while the current is the same
WE 2
Why does an automatic street light stay off all day and only switch on at night?
Think about the LDR during the day:
Bright daylight gives it a very low resistance.
Think about the LDR at night:
In the dark, its resistance climbs to a very high value.
The circuit is designed to detect that big rise in resistance, and uses it to trigger the light switching on as it gets dark
WE 3
A potentiometer with a total resistance of 12 ฮฉ is connected across a 9.0 V supply. The slider is positioned so the upper part has a resistance of 5.0 ฮฉ. What is the potential difference across that upper part (assuming an ideal, high-resistance voltmeter draws no current)?
Since it’s a series arrangement, the p.d. splits in proportion to resistance:V(upper) = V(supply) ร R(upper) / R(total)
Substitute the values:V = 9.0 ร (5.0 รท 12)V = 3.75 V
๐ก Top Tips
Link “hotter/brighter” to “lower resistance” for both thermistors and LDRs โ it’s the same falling pattern for both
Remember these components are non-ohmic โ don’t expect their I-V graph to be a straight line
For potentiometer questions, treat the two halves as resistors in series and share the voltage in proportion to resistance
โ Common Mistakes
Mixing up which direction resistance changes โ remember it falls as temperature or light increases, for both thermistors and LDRs
Forgetting that a thermistor or LDR in series still obeys the series circuit rules we already know โ same current, shared p.d.
Assuming a potentiometer’s slider changes the total resistance โ it only changes how that fixed total is split between the two parts
That brings us to the end of Current & Circuits โ from reading a circuit diagram all the way through to variable resistors, you now have the full toolkit to tackle any B.5 question with confidence.
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