Video Transcript
Which of the following best
explains how alternating current is rectified? (A) The direction of rotation of a
conducting loop in a generator is periodically reversed. (B) The connection directions of a
conducting loop in a generator to an external circuit are periodically reversed. (C) The direction of the magnetic
field around a conducting loop in a generator is periodically reversed.
In this question, we are asked to
think about an alternating current being rectified and figure out which of the given
options best explains this process. Before we can figure this out,
let′s remember some information about generators.
A generator is a device that
converts mechanical energy into electrical energy. This is done by applying a
mechanical force to rotate a loop of wire within a uniform magnetic field. When the loop of wire spins through
the magnetic field, a current is induced in the wire. The current, when induced this way,
is an alternating current. This means that the current
generated changes directions at uniform intervals, and in the case for this
generator, it will change direction every time the loop completes half of a
rotation. This can be an issue if we need a
current that is always in the same direction, that is to say, if we want to rectify
it.
Rectifying a current means turning
an alternating current into a direct current, with only one direction. One way this is accomplished in a
generator is through the use of a commutator. A commutator looks like a metal
cylinder that has been split into two. The two halves are connected to the
outer circuit through brushes that keep contact with the halves as they rotate. Remember, the current that was
being generated in the wire loop is an alternating current that changes direction
every time the loop completes half of a rotation.
Adding this commutator in, the half
in contact with each brush swaps every time the loop makes a half-rotation. This in turn changes the direction
of the current into the external circuit every half-rotation, meaning the current
into the external current will always be in the same direction. This graph now shows what the
current being generated looks like when a commutator is used.
Now, let′s look through our answer
options and figure out which one is right. The first option states that the
rotation of the conducting loop in the generator is reversed and that rectifies the
current. However, the wire loop in a
generator spins only in one direction, so this would not explain how the current is
rectified.
The next option states that the
direction of the connection of the conducting loop to the external circuit is
reversed. This is what we found in our
examination of how generators rectify current, so this is correct.
The third option states that the
current is rectified when the direction of the magnetic field is reversed
periodically. The magnets in a generator are
stationary so the magnetic field cannot be reversed within them. So, this is also incorrect.
Therefore, the second option, the
connection directions of a conducting loop in a generator to an external circuit are
periodically reversed, is the correct answer.
