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June 2011 - MCQ 6: Induced current

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I know this question concerns Lenz's Law and Faraday's Law. The way I see it, when the switch is closed, a current flows through Y and a magnetic field is induced. According to Fleming's Right Hand Rule, a North Pole is induced to the left of Y and a South Pole to the right. If X is moved closer to Y, according to Lenz's Law, the direction of the current is such that to oppose this change, thus a North Pole is induced on the side facing Y and the current in X is anti-clockwise. Vice versa if X is moved away from Y, thus the directions of current will be the same in both X and Y if X is moved away from Y.

As you can see, that's not any of the options, and I don't know how else to work it out. :erm:
To induce a current in the same direction in X as the one in Y is now flowing, you have to arrange it so that X "sees" the field in Y decrease.
Original post by Stonebridge
To induce a current in the same direction in X as the one in Y is now flowing, you have to arrange it so that X "sees" the field in Y decrease.


Okay, but how does that work?

If we turn the switch off, since it's a DC circuit, there is a change in magnetic flux linkage in X. This induces a current. But how do we figure out the direction of the current to know whether it's in the same or different direction to Y?
Original post by You-know-who
Okay, but how does that work?

If we turn the switch off, since it's a DC circuit, there is a change in magnetic flux linkage in X. This induces a current. But how do we figure out the direction of the current to know whether it's in the same or different direction to Y?


Lenz's Law states that the current in coil X will oppose the change in Y.
Opening the switch will reduce the field in Y to nothing.
This is the same (as far as X and Lenz is concerned) as suddenly moving the coil Y away from X. The field disappears. (A north pole pointing from Y to X disappears.)

X opposes this and produces a south pole pointing towards Y to oppose the removal of the N pole from X that is going away. This means the brief induced current in X must be in the same direction as Y.
(edited 9 years ago)
Original post by Stonebridge
Lenz's Law states that the current in coil X will oppose the change in Y.
Opening the switch will reduce the field in Y to nothing.
This is the same (as far as X and Lenz is concerned) as suddenly moving the coil Y away from X. The field disappears. (A north pole pointing from Y to X disappears.)

X opposes this and produces a south pole pointing towards Y to oppose the removal of the N pole from X that is going away. This means the brief induced current in X must be in the same direction as Y.


Thank you! :smile:

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