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 different current frequency from the red solid line. (C) The black dashed line
represents a current that reverses direction.
This question asks us to recognize
what makes a current an alternating current and what makes a current a rectified
current. If we think back to the mechanism
that generates a current, we can get a clear picture of what we’re seeing in the
graph. Clearing some space on screen,
recall that an electrical generator is a device that is made up of two basic
components: a magnetic field generator, depicted by the magnet here, and a coil that
rotates through the magnetic field.
Though the magnitude of the
magnetic field is constant, the total magnetic field passing through the coil
changes as it rotates. As the magnetic field strength in
the coil changes, an electric potential is induced in the coil. From this potential difference, a
current is generated, which is then transferred to an external circuit.
If we imagine the motion of the
rotating coil, we can start to see where the red line on the graph comes from. As the coil moves through the
magnetic field, the current in the coil changes direction. The current changes direction every
time the coil completes a half rotation. In one instant in time, the current
may move in the positive direction; at another instant, it may move in the negative
direction. This is part of why we call it an
alternating current.
A rectified current, on the other
hand, is a current that has been changed so it always has the same direction. We call this a direct current. For an alternating current, as the
current changes direction, its magnitude changes. As the current reaches a value of
zero, the line passes through the time axis then reaches the maximum amplitude in
the opposite direction. That’s why we see crests, as
circled in red, and troughs, as circled in blue. A rectified current, on the other
hand, won’t have these troughs, but the amplitude will be the same as for the
alternating current.
For the currents in this question,
the red and black lines have the same amplitude, or maximum value, for both
directions of the current. Therefore, answer choice (A) is
incorrect.
Answer choice (B) is incorrect
because both the black and red lines rise and fall with the same frequency, or
regularity. The currents depicted by both lines
change at roughly the same rate. A rectified current would have the
same frequency as an alternating current, anyway. The only difference, remember, is
that the rectified, or direct, current doesn’t change directions.
The only answer choice remaining is
(C). The black dashed line can’t depict
a rectified current because it depicts a current that changes direction.
