An error or maybe not?

[methodsboy]

hey every1 this is from the 07 Lisachem Unit 4 exam:

[IMG]http://img42.imageshack.us/img42/2419/coilg.jpg[/img] [IMG]http://img42.imageshack.us/img42/coilg.jpg/1/w436.png[/img]

Question - Looking along the axis of rotation from end X in fig 8, the coil is rotated clockwise. The induced current will flow in direction:
- from R to Q
- Q to R
i chose R to Q but it says its Q to R..... can anyone please explain this?

[VxBlitzxN]

It's R to Q, from the RHS rule, u can see the magnetic fields go from N to S and the clockwise turning moment would be due to the force being upwards on SP end and downwards on the RQ.
Thus the current travels  RQPS. Maybe the answer is funny.

[TrueTears]

Current is definitely flowing from R to Q, just simply use Right Hand Slap rule on SP or OR.

[Damo17]

Current runs from R to Q for it to rotate clockwise, but it is asking for induced current, so from Q to R.

[IntoTheNewWorld]

I also get Q to R...using the right hand induction rule or whatever its called.

EDIT: (Considering the left side of the thingy)
        As in Thumb = the positive charge moving up as it rotates clockwise
               Fingers = Magnetic field from N to S
                  Palm = The positive charge is forced from S to P (Q to R)

[methodsboy]

Current runs from R to Q for it to rotate clockwise, but it is asking for induced current, so from Q to R.

so induced current is always in the opposite direction as the real current direction (the thumb)?

[kurrymuncher]

Look fagboy,

As the coil is in that position, it has zero flux. Once you start to rotate it clockwise, there will be an increase in flux through the coil. Using Lenzzzz'zzs law, the induced current will be in a direction such that the flux it creates will oppose the change in flux that produced it. Therefore imagine the coil rotating clockwise, the induced flux lines will be going from South to North, then by using the right hand grip rule you will see that the current is from Q to R.

Note that they are asking for the Induced current, NOT the current that allows it to rotate in a clockwise direction.

[methodsboy]

ok thanks, fagmuncher

[homghomg1]

hey every1 this is from the 07 Lisachem Unit 4 exam:

[IMG]http://img42.imageshack.us/img42/2419/coilg.jpg[/img] [IMG]http://img42.imageshack.us/img42/coilg.jpg/1/w436.png[/img]

Question - Looking along the axis of rotation from end X in fig 8, the coil is rotated clockwise. The induced current will flow in direction:
- from R to Q
- Q to R
i chose R to Q but it says its Q to R..... can anyone please explain this?

Using RHG rule when current is being induced will get you the wrong answer
If you want you can always use RHG rule for these questions and give the direction of current as the opposite, but you are often required to give an explanation as part of the question so you might as well do it correctly

There are two ways of looking at it. One is from a flux point of view, like kurrymuncher explained.

The other way is to consider a negative charge on the coil side QR. As the coil rotates clockwise from a horizontal position, the negative charge moves downwards, and this therefore means that current is upwards. NOW you can use RHG rule. Current up, magnetic field to the right, force on NEGATIVE CHARGE is from R to Q. Therefore electrons are being pushed from R to Q, which indicates current is in the opposite direction. Answer is Q to R.

You can consider a positive charge as well, just don't forget to you would have to apply the force to the positive, not negative particle. This means current downwards, magnetic field to right, force on POSITIVE CHARGE is Q to R, and therefore current is Q to R. Considering a positive charge is actually easier, but for some reason I prefer considering a negative charge.

Anyway, don't hesitate to challenge the suggested solutions given by exams, I find that they're wrong pretty often, specially for physics exams.

[methodsboy]

thanks guys i get it now