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Question Video: Determining Whether a Hot Wire Ammeter Can Measure an Alternating Current Physics • Third Year of Secondary School

Can a hot-wire ammeter measure an alternating current?


Video Transcript

Can a hot-wire ammeter measure an alternating current?

To find out, let’s look at a circuit with a hot-wire ammeter in it and has an alternating current. The most basic circuit we can make with this involves having an alternating-current source, the hot-wire ammeter, and a resistor to make sure that there is a current. Now in order for the hot-wire ammeter to be able to properly measure current, the hot wire within it must be able to get hot. To see why, let’s open up this ammeter and look at the individual components.

There is a lot going on inside. When there is a current across the ammeter, it splits along a parallel path, with one path leading to a shunt resistor and the other path leading to some platinum–iridium wires, which are the hot wires in the hot-wire ammeter. Now, before we look at the other stuff, let’s look at these two circles here. These are indicating that everything to the right of them is a non-circuit diagram. These diagrams are showing objects that you would typically not find in a circuit, like a spring, a silk string, and a pulley. All of these unusual components working together allow us to measure the current of this circuit.

The full measurement process is based on already-known ratios between all of the components. But we don’t actually need to know these ratios; only the manufacturer of the ammeter does. To see how these ratios work together, we’re going to go through the measurement process, which will give us some insight as to whether the hot-wire ammeter can actually measure an alternating current or just a direct current. Either way, there must be current because the first step is making sure that there is current on the hot wires.

These platinum–iridium wires are not the wires that we have in circuit diagrams, which are assumed to have no resistance. Instead, we assume that these hot wires do have some resistance, which means they are subject to an effect called resistive dissipation, which is any time you have a current passing through a medium with some resistance, a portion of the electrical energy is converted to thermal energy, causing it to heat up. So a simpler way of saying this is that the current on the hot wires causes them to heat up. And when metal heats up, it slightly expands.

So, so far when there is a current across the hot wire, it causes it to heat up, which causes it to expand. This expansion causes this silk string, which is attached to the wire, to be pulled over towards the spring, which is holding it under tension, so this string is pulled towards the spring. And as it does so, since it is attached to this pulley, it causes the pulley to rotate, which in turn causes the pointer for the dial to also move, which gives a reading of current, the current across the hot wires.

So these are the general steps that occur inside of a hot-wire ammeter in order to measure the current across the ammeter. When looking at direct-versus-alternating current, we have to consider how these steps might change. Starting with the first with the current on the hot wires, a direct current will always have the current traveling in one direction. And if nothing else changes in the circuit, it is assumed to be constant. As the electrical charge travels through the wires, which have a resistance, they begin to heat up, with a higher direct current corresponding to a higher temperature, which is the second step in this process, which means the third and fourth will occur just fine. So, that’s direct current. We know it works; the wire heats up just fine. So, let’s look now at alternating current.

At first, alternating current is just like direct current, it having one direction. But then after some time, the current direction flips. And that’s not all; the magnitude of the current changes with time as well. Let’s see this by clearing some space. Here is a graph of a typical alternating current. We see that it starts in one direction with its magnitude peaking at a certain point in time before dropping its magnitude and eventually hitting zero, meaning that at some points there is no current in the wire before flipping directions, attaining the highest magnitude again, but in the opposite direction, and so on, the current in the wire constantly changing direction.

So, how does this constant switching and occasional points where there is no current affect the heating of the wire? The good news is that the direction of current in the wire doesn’t matter for heating it. The effect that causes the hot wires to heat up, resistive dissipation, depends on the total charge passing through the resistive medium, not the direction, which makes sense because a wire wouldn’t cool down if you just reverse the direction of current.

But the magnitude still varies, doesn’t it? Wouldn’t the dial that’s measuring current be flip-flopping all over the place? But this is not a problem either, since the hot wires take time to change their temperature, both increasing it and decreasing it, meaning that the constantly changing magnitude of current will eventually even out in regards to how it heats up the hot wires. So after it heats up, a hot-wire ammeter will be able to measure a constant value of current even with an alternating current.

So, the answer to whether a hot-wire ammeter can measure an alternating current is yes, it can measure an alternating current.

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