An LDR is a light-dependent resistor: the more light falls on it, the smaller its resistance. An NTC gets smaller as it gets warmer, a PTC gets bigger. In a variable resistor you move a sliding contact and so change the length the current goes through, and with it the resistance.
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There is a resistor that knows whether it's light or dark, and one that feels whether your e-bike battery is getting too hot. Those are the LDR and the NTC: resistors that change by themselves, through light or through temperature.
First the variable resistor. Inside is a long wire or a track, and a sliding contact that you move with a knob. If the current has to go through a longer piece, the resistance gets bigger and the current smaller. An old-fashioned volume knob works like that. In a circuit diagram you draw it as a resistor with a diagonal arrow through it.
Then two resistors that respond to temperature. In a PTC the resistance increases as it gets warmer. Many USB ports have one inside as a fuse: if the current gets too high, it heats up, its resistance shoots up and it chokes off the current. An NTC does the opposite: the warmer, the smaller the resistance. In an e-bike battery there's one right against the cells, and if its resistance drops too far, charging stops. In the symbol there's a plus for the PTC and a minus for the NTC.
And then light. An LDR is a light-dependent resistor: the more light falls on it, the smaller its resistance. In the dark that can be a million ohms, in bright light a few hundred ohms. A plug-in night light uses that. In the symbol two little arrows point towards the resistor: the light falling on it.
For upper secondary physics, on electricity.
In this video
The full explanation, in writing
A resistor that knows something
0:00There is a resistor that knows whether it's light or dark. And one that feels whether your e-bike battery is getting too hot.
The variable resistor
0:14First the simplest: the variable resistor. Inside is a long wire or a track, and a sliding contact that you move with a knob. If the current has to go through a longer piece, the resistance gets bigger and the current smaller. An old-fashioned volume knob works with a sliding contact like that.
0:31In a circuit diagram you draw the variable resistor as a resistor with a diagonal arrow through it.
Resistors that respond to temperature
0:39Then two resistors that respond to temperature. In a PTC the resistance increases as it gets warmer. Metals already do that a little; a PTC does it strongly. Many USB ports have one inside as a fuse: if the current gets too high, it heats up, its resistance shoots up and it chokes off the current.
1:01Once it has cooled down, the port works again. An NTC does the opposite: the warmer, the smaller the resistance. In an e-bike battery there's one right against the cells. The electronics measure its resistance, and if it drops too far, the battery is too hot and charging stops.
1:21You can see the difference in the symbol: a resistor with a diagonal line through it, with a plus for the PTC and a minus for the NTC. Your turn. The same NTC lies first in a cold room and then in your warm hand. Where is its resistance greatest?
The light-dependent resistor
1:42In the cold room: the colder, the greater the resistance of an NTC. And then light. An LDR is a light-dependent resistor: the more light falls on it, the smaller its resistance. In the dark that can be a million ohms, in bright light a few hundred ohms.
1:59A plug-in night light uses that. When it gets dark, the resistance of the LDR shoots up, and the circuit behind it switches the lamp on. In the symbol there are two little arrows pointing towards the resistor: that's the light falling on it.
Three things to remember
2:17Three things. In a variable resistor you use a sliding contact to change the length the current goes through, and with it the resistance. Symbol: a resistor with a diagonal arrow through it. In a PTC the resistance increases as it gets warmer, in an NTC it decreases.
2:35The symbol has a plus or a minus. And in an LDR the resistance gets smaller the more light falls on it. In the symbol two little arrows point at the resistor.
Two questions
2:48Two questions. One. A battery, a lamp and an LDR are together in one circuit. You hold your hand above the LDR. What does the lamp do? Two. You need a resistor whose resistance gets smaller as it gets warmer. Which component is that, and what is in its symbol?
The answers
3:09The first: less light falls on the LDR, so its resistance gets bigger. The current in the circuit gets smaller, and the lamp glows more dimly. The second: an NTC. In the symbol there's a minus by the diagonal line.
Ohm's law and the filament lamp
3:24What Ohm's law says exactly, and why an ordinary lamp doesn't have a fixed resistance either: on the channel there's a separate video about resistance and Ohm's law.
Frequently asked questions
What is the difference between an NTC and a PTC?
In an NTC the resistance gets smaller as it gets warmer, in a PTC it gets bigger. You can see it in the symbol by the sign: a minus for the NTC and a plus for the PTC, next to a resistor with a diagonal line through it.
How does an LDR work?
An LDR is a light-dependent resistor: the more light falls on it, the smaller its resistance. In the dark it can be a million ohms, in bright light a few hundred ohms. A night light uses that: when it gets dark, the resistance shoots up and the circuit behind it switches the lamp on.
How does a variable resistor work?
Inside is a long wire or track, with a sliding contact that you move. If the current has to go through a longer piece, the resistance gets bigger and the current smaller. In a circuit diagram it is a resistor with a diagonal arrow through it.
What does the lamp do if you hold your hand above an LDR?
It glows more dimly, if the LDR is in one circuit with the lamp and a battery. Less light falls on the LDR, so its resistance gets bigger, and then the current in the circuit gets smaller.
What is an NTC used for?
To measure temperature. In an e-bike battery there's one right against the cells: the electronics measure its resistance, and if it drops too far, the battery is too hot and charging stops.
Before this came the resistance of a wire: why a long, thin cable has more resistance than a short, thick one, and what resistivity is. Next: circuits, with resistors and lamps in series and in parallel.
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Physics with Thomas: physics explained with pictures where they add something, and without them where they distract.
In this series
Electric current6:12
Voltage and capacity4:43
Connecting ammeter and voltmeter4:10
Resistance and Ohm's law4:21
Resistance of a wire4:40
Variable resistor, NTC, PTC and LDR3:40
Series circuit3:32
Parallel circuit3:56
Series-parallel circuit3:37
Circuits practical test5:13
Power2:58
Kilowatt-hour3:05
About me
My name is Thomas Schuurmans, and I have a passion for physics and for understanding why things work the way they do. What I want is to get young people asking “why is that?” and “how does that work?” more often, and to hand them the tools of physics in a way that is simple and that you can see.
I graduated in applied physics at Delft University of Technology in 2003. After that I spent more than twenty years outside education: first at TNO, the Dutch applied research institute, then at a design agency, and eventually founding Proportion Global, through which I work on innovation questions in Africa, Latin America and South Asia. That work resembles physics more than you would expect: don't start from a solution, first understand what is going on, try something, be wrong, and look again.
Since 2026 I have been teaching physics to both lower and upper secondary classes, and I started a master's at the University of Amsterdam for my full teaching qualification. That is where I learned that a secondary school pupil's real attention for new material lasts about seven minutes. And my own weakness happens to be telling too many side stories.
That is where the idea came from: videos that explain one topic sharply and visually, inside those seven minutes. I make them for my own pupils. Then I publish them, because good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.