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# OCR A2 Physics exam question - Magnetic fields, need some help. watch

1. I'm doing the 2010 June past paper, and there is a question that I don't fully understand and I would really appreciate some input from anyone here to explain what is going on.

Past paper:
http://www.ocr.org.uk/Images/58049-q...of-physics.pdf

Mark Scheme:
http://www.ocr.org.uk/Images/63477-m...ysics-june.pdf

Thank you
2. I do AQA but I should be able to help. I'm going to assume you're talking about question 4.

To understand the answer, think of Magnetic flux as parallel field lines going from Top to bottom.

C)i
The coil at its current position in fig 4.1 perpendicular to the field lines right? To see it Practically, think of this as being the same as placing your hand out flat in front of you. Imagine lines going from your body to an object a distance in front of you. When you rotate your hand it are initially cutting through a lot of the lines. But the closer you hand gets to the normal, the less lines you're cutting through. Once your hand is perpendicular, you aren't cutting through any more lines because your hand is now parallel to the lines and so thats why the magnetic flux is 0.

Now just imagine it was a coil instead of your hand. At perpendicular to the field line (when your hand was flat) you cut through the most lines and at parallel you're cutting through the least. You can see then that the number of 'lines' you're cutting through depends on the angle from the normal the coil is at. Using trig this angle will be cos theta. As you're rotating, the magnetic flux(number of field lines cut through) changes which is why you get the wave curve. You get a negative wb value because after you pass the normal, the current going through the wire is going in the opposite direction, using right hand rule you should be able to see that force is in the opposite direction too.

C)iii

Unsure how to explain this. Gap in my knowledge means I don't have full confidence what I'm saying is right.

C)V

If you're cutting through twice the number of field lines in the same time, then the rate of flux change must be double right?
Lenzs law says that induced emf is such that it induces a current whose magnetic field opposes the rate of flux change. So if the rate of flux change has doubled, the emf must be double too to produce the current to oppose that change. Does that slightly make sense?
3. Thank you for your reply, you explained it very well, thanks again

and sorry, I totally forgot to mention which question I was talking about.
4. No problem. Hopefully someone helps with the second question.
5. Hi , can you please help me on question 5 (c) (ii) on the June 2011 paper?

Here is the link to the paper : http://www.ocr.org.uk/Images/62267-q...of-physics.pdf

Mark scheme : http://www.ocr.org.uk/Images/58522-m...ysics-june.pdf

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