Question Video: Identifying Rectified Current on a Graph Physics

The red line shows an alternating current. Which of the lines shows this current but rectified? [A] The green solid line [B] The black dashed line [C] The purple dotted line [D] None of these lines


Video Transcript

The red line shows an alternating current. Which of the lines shows this current but rectified? (A) The green solid line, (B) the black dashed line, (C) the purple dotted line, or (D) none of these lines.

In our diagram, we see a plot of current against time. The red line shows an alternating current. And we see that indeed sometimes this current is positive, sometimes it’s negative, and it alternates between them. When this current changes from positive to negative or negative to positive, that means it changes direction in its circuit. We want to know which, if any, of the other lines shown demonstrate this current being rectified.

A current is rectified when it always points in the same direction. Typically, this is demonstrated by all of the negative values of a given current — say, the negative values of our red curve — being inverted, flipped, about the horizontal axis. Say, for example, that we had a current-versus-time graph like this. If this current were to be rectified, changed, so that it always points in the same direction, then the rectified current would look like this. That is, we’ve taken the negative current values at this second half of the wave cycle and we flipped them about the horizontal axis.

Considering the red line on our original diagram, we can see that sections of this graph are already positive. However, there’s this section of that graph which is negative. And so if this current was rectified, this section would be flipped about the time axis. We see that one line showing this is the black dashed line. Everywhere the red line is positive, the black dashed line is positive as well. More than that, in these regions, the black dashed line lies exactly on top of the red line. This is in contrast to both the purple dotted line and the green solid line. The only time the black dashed line and the red line diverge is when the red line goes into negative current territory. Over this time period, the black dashed line represents the magnitudes of each negative value on the red line.

For our answer then, we choose option (B). It’s the black dashed line that shows the red line current but rectified.

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