Video: Understanding Types of Electric Current

The graph shows the potential difference against time for three current sources. Which source provides rectified alternating current? Which source provides direct current?


Video Transcript

The graph shows the potential difference against time for three current sources.

And as we can see in the graph, we’ve got source a, source b, and source c. Now there are two parts to this question. The first part asks us which source provides rectified alternating current.

So we need to determine which one of a, b, or c is rectified alternating current. Now, the first thing to notice is that the question is talking about current, whereas the diagram shows us the potential difference of each source. And this is a point to be careful about. However, luckily for us, the law known as Ohm’s law tells us that the voltage or potential difference across a component in a circuit is equal to the current through that component multiplied by the resistance of that component.

So for the current source, the voltage is basically directly proportional to the current. And the proportionality constant is the resistance of the source. In other words, when we plot potential difference against time, then that graph will have the same shape as a graph that shows the current against time. The only thing that will be different will be the numbers along the vertical axis. In other words, this is not something that we need to worry about. The shape of the current against time graph will be the same as the shape of the potential difference against time graph.

So now that we’ve got that sorted out, let’s work out which one of the three sources is the answer to our question. Firstly, let’s look at the phrase “alternating current”. An alternating current is one which keeps switching direction over and over again at regular intervals. In other words, as time progresses, the current goes from positive to negative to positive to negative at regular intervals. It basically looks like a sine curve or a cos curve, depending on where you start. But that’s what an alternating current source looks like. And remember we said earlier that the current is directly proportional to the potential difference. So we could easily replace the label on the vertical axis with the potential difference 𝑉. And so, source c is our alternating current.

But that’s not the answer to our question because there’s another word in the question which is the word “rectified”. We are looking for a rectified alternating current, not just an alternating current. Now, an alternating current is rectified by what’s known as a rectifier. Makes sense, doesn’t it? A rectifier basically takes in current which is constantly changing direction and modifies it so that it flows in one direction only. In other words, it takes in a current like the one we’ve just drawn on the left. It takes in an alternating current. And it converts all of the negative parts of the alternating current into a positive current. In other words, it does this. All of the positive parts stay the same. And then as soon as we get to the part which becomes negative, the rectifier switches it. And so, this is the pattern that we see.

So the source which provides the rectified alternating current looks like our orange graph here. And that of course is source b. And so, we can say that the answer to our question is that source b is the source which provides the rectified alternating current. So now that we’ve answered that, let’s move on to the next part of the question.

The second part asks us which source provides direct current.

Well, a direct current is one which flows in one direction only. And usually, direct current also means a current which is constant as time progresses. In other words, we’re looking for a flat line, kind of like the one from source a, because this current is constant. Well, of course, technically, the graph shows us that the voltage is constant. But remember, we said that the current was proportional to the voltage. Hence, the current is also constant. And it flows in one direction only because it doesn’t switch sides from positive to negative. And so, source a provides the direct current. And so, that is the answer to the second part of our question, which brings us to the end of our question.

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