Video Transcript
The red line shows an alternating current. Which of the following best explains why the black dashed line does not show this current rectified? (A) The black dashed line represents a different current amplitude from the red solid line. (B) The black dashed line represents a current that reverses direction. (C) The black dashed line shows zero current at the instance that the red solid line indicates maximum current.
On our graph of current against time, we see our red solid line representing an alternating current. This current is alternating because it periodically reverses direction. Anytime the current has a positive value, that indicates travel in one direction around the circuit, while having a negative value indicates travel in the opposite direction. When a current is rectified, that means it’s modified so it always travels in the same direction. Conventionally, we say that a rectified current is always nonnegative. That is, its value is always positive or zero. We see that this is true of our black dashed line.
And because this black dashed line is never negative, that means that answer option (B) cannot be correct. This option after all claims that the black dashed line represents a current that reverses direction. That’s not the case. But if it were true, then this would be a good reason why, as our problem statement asks, the black dashed line does not represent the red solid line but rectified. Rather, a rectified current will have at every point in time a value equal to the magnitude of the current that was rectified. Practically then, we can imagine rectifying this red solid line, the alternating current, by inverting all the negative values about the horizontal axis. That will give us a curve that looks like this in orange.
Notice the difference between the orange rectified curve and the black dashed line. They both have the same shape but different amplitudes. Answer choice (A) describes this. It says that the black dashed line represents a different current amplitude from the red solid line. We see after all that the black dashed line has an amplitude of this magnitude, whereas the red solid line has an amplitude of this magnitude. It looks then that answer option (A) will be our choice.
Let’s consider answer option (C) just to make sure. This choice tells us that the black dashed line shows zero current at the instance that the red solid line indicates maximum current. If we look though at the instance in time where the red solid line has a maximum current magnitude, we see that actually at these moments the black dashed line also has a maximum current magnitude. The black dashed line does not have a value of zero at this instance. So answer option (C) won’t be our choice. We’ve confirmed then that the best explanation for why the black dashed line does not represent the rectified version of the red solid line is given by answer choice (A). The black dashed line represents a different current amplitude from the red solid line.
