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Question Video: Explaining How an Alternating Current Is Rectified Physics

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.

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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.

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