MCAT Flashcards: Voltmeters

What are voltmeters? Ken Tao is an MCAT expert and explains that a voltmeter is a device used to measure voltage. To better understand how voltmeters work, enjoy this video.

Full Transcription

In this flashcard video, we’re going to go over voltmeters. A voltmeter is a device used to measure the voltage. To see how voltmeters work, let’s take a look at this whiteboard drawing.

So over here, we have a circuit with a battery, a resistor and a voltmeter. If we want to measure the voltage across this resistor, I’m going to need to connect a voltmeter and parallel with it as shown in this drawing.

The reason why it’s important to connect the voltmeter in parallel with the resistor is because voltage is the same and parallel. So that means this voltmeter is going to be able to measure the same voltage that is across this resistor.

OK, now just as with ammeters, voltmeters are circuit elements and have an internal resistance. So we need to address the question of: Do we want our voltmeter to have a high or low internal resistance? And the key thing is the voltmeter is designed to measure the voltage across a circuit element and it should not impact the circuit. So, one thing you want to keep in mind is how resistance works in parallel. And you remember you get these fractions.

So one over the total resistance is equal to one over the resistance of the resistor, plus one over the resistance of the voltmeter. So what resistance of the voltmeter is going to have the smallest impact on the circuit? And as it turns out, we want a very high internal resistance, because you can see here, if the internal resistance of the voltmeter is a very, very, very large number, then one divided by a very large number is essentially zero. So anything plus 0 is just itself. So that’s why if you’re voltmeter has a high internal resistance, it’s going to have a minimal impact on the circuit and allow you to more accurately measure the voltage across that circuit element.

So that’s how voltmeters work. They are devices used to measure the voltage. You need to connect them in parallel and you also want them to have a high internal resistance.