Multiple Choice (2 Viewers)

[weirdguy99]

How is 14 A when the pendulum only swings across the magnetic field once ???....

When the broken ring passes the left edge of the magnetic field, it experiences a change in flux -> induced emf. When it passes out of the magnetic field moments later, it also experiences a change in flux -> induced emf. While it is inside the B field, there is no change in flux, so induced emf is zero. So it's either A or B, but it's A

 

[ryan1894]

a

 

[JamieP]

Two of the multiple choice questions are problematic!

Question 14. The split ring is drawn too thick if Eddy current is not to be considered, if (D) is the intended answer.

Another big problem is The word "measured" - this implies a close circuit must exit!!! Otherwise how can you "measure" ???

Tthe only way you can "measure" the induced emf between X and Y is to form a closed circuit, once a closed circuit is formed, (A) is the answer to the question. If you don't "measure" it, no induce emf will occur, then (D) is the answer. In conclusion, the Board of Studies may have to award a mark to both (A) and (D).

At best, question 14 is a badly worded question.

Question 20: the concept of "halving" a negative potential energy is very ambiguous at best, if not wrong.

Half usually means "less", but if you have -10000 KJ of potential energy to start with, when you half it, do you get -5000 or -20000? Unless you take the meaning of HALF by multiplying 0.5 - another badly worded question - two correct answers ?!?!

What a stuffup!

 

[lovingphysics]

yes, that is true, an emf is induced across the ends of the pendulum ( as the charges in the pendulum are forced by the motor effect to move to opposite ends) ...so the answer is C, and not A as you stated earlier.

Of course if it was a complete loop moving through the constant field then the emf's would sum up to zero.

 

[lovingphysics]

measurement of the emf will of course require an ext circuit with a voltmeter etc............ one way of looking at why the correct answer is C is that in such a circuit the area of the loop containing the pendulum, the connecting wires, voltmeter etc is changing....so there is a change in flux while the pendulum moves through the constant magnetic field. All other options A, B and D all require the emf to be zero during this time to be zero, which cannot be the case.

Change in flux is a pain sometimes.....better to think about charge separation, the motor effect in these cases. So, that is why for example, a closed loop, any shape, will always have a zero net emf induced as the emf's oppose each other on each side of coil.

 

[lovingphysics]

q14 is C - see my other replies

remember - a charge moving in a magnetic field experiences a force - so when it is in the constant field the charges in the pendulum still experience a force - so there is still some charge separation ie an emf.

 

[Fizzy_Cyst]

Nice to see that Q14 was A, not C

I'm just lucky I got all the MC correct, haha.