# Question Video: Identifying Circuit Symbols Physics • 9th Grade

The diagram shows an electrical circuit. How many light-emitting diodes does the circuit contain? How many thermistors does the circuit contain? How many voltmeters does the circuit contain?

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### Video Transcript

The diagram shows an electrical circuit. How many light-emitting diodes does the circuit contain? How many thermistors does the circuit contain? How many voltmeters does the circuit contain?

Looking at this diagram, we see indeed it represents an electrical circuit with a number of electrical components in it. In order to answer these questions about how many of certain components are in the circuit, we’ll need to know what those components look like as symbols. In other words, in order to answer how many light-emitting diodes are in this circuit, we’ll need to know the symbol for a light-emitting diode. And likewise for thermistors as well as voltmeters, we’ll need to know the symbols for each of these components. Once we recall the symbols, it’s just a matter of counting the number of times that symbol appears in the circuit in the diagram.

So starting with our first question, how many light-emitting diodes does the circuit contain? A light-emitting diode is, first of all, a diode whose symbol looks like this. And when that diode emits light, we modify the symbol to have a circle around it with two arrows leaving the circle. So the question becomes, looking over at our circuit diagram, how many times do we see this symbol appear? Looking the circuit over, we see it appears here, that’s one time, and then here. That’s another time. Now, this symbol over here looks a bit like a light-emitting diode. But it’s lacking the arrows leaving the circle. Those arrows indicate the light being emitted. So this doesn’t count as a light-emitting diode in the circuit. So then we’ve got two light-emitting diodes in this circuit. And we’ll write that down as our answer.

The next question asks about the number of thermistors contained in the circuit. A thermistor, remember, is a resistor whose resistance depends upon its temperature. A common way to symbolize a thermistor is like this. It starts out with a basic resistor symbol and then adds this diagonal line with a little slope to it. Looking back to our circuit, we see a somewhat similar-looking symbol right here. But that symbol has a diagonal arrow over a resistor whereas the thermistor symbol is a little bit different. Overall then, we don’t see any thermistors in this circuit. So we’ll write that down as our answer.

And finally, we want to know how many voltmeters are here in this circuit. Recalling the symbol for a voltmeter, that’s a circle with a 𝑉 in it. Our circuit has a circle with an 𝐴 in it. But that’s an ammeter, a device for measuring current, rather than a voltmeter. We don’t see any of those in the circuit shown. So once again, we write down an answer of zero.

In this electrical circuit then, there are two light-emitting diodes, zero thermistors, and zero voltmeters.