OCR B Physics G495 June 19th 2014 Unit 5

And also in the pre-release material could someone please explain what is meant by doping?

Could someone give me some brief details about how EMF induction is affected by current please? For example on Q10c on January 2012;

http://www.ocr.org.uk/Images/79604-question-paper-unit-g495-field-and-particle-pictures.pdf

It confuses me so much. I'm sure it's hardly mentioned in the revision guide and it feel's like it keeps popping up. I understand that a current carring wire has a magnetic field and an alternating current induces magnetic flux etc but I'm more on about that question specifically. Thank you!

In response to the above post, @DanielCook95 @ RATKING

I agree with most of what you say apart from one major point.

"The rotor coil has an input direct current. This gives it a constant magnetic field." - I agree with this.

"It spins around and so it experiences a change in magnetic field as it turns past the stator which has a constant magnetic field too due to a direct current coil around it." - this is not correct.

If a DC current was used in the stator coil to magnetise the stator core, the rotor would NOT turn, but be locked in position. The inclusion of a commutator would be required somewhere in the motor design to induce rotation in the rotor to 'simulate' an alternating magnetic field. This is a far more simple motor.
There is in fact an alternating current present in the stator coil which provides an alternating magnetic field in the stator core. This alternating magnetic field then interacts with the constant magnetic field of the rotor and induces motion in the rotor.
I understand that by referring to the motor as a 'd.c. motor' in the exam paper it could be somewhat misleading. But the design shown would simply not work if there was no a.c. current in the stator coil. For that very reason I don't like referring to them as a.c. motors and d.c. motors, but rather asynchronous motors and synchronous motors respectively. Worth a wiki if you're not too stressed out already!

Back to the original question as to how the emf in the rotor coil is being affected. As stated above:
"Due to the rotor coil experiencing a changing magnetic field this induces an EMF in the coil and thus a current..."
I agree with the point you make. To add to this - as the motor turns, flux of the magnetic circuit passes through the rotor and is linked with the rotor coil. So now you have a rate of change of flux linkage, aka emf in the rotor coil.
As the motor spools up to its operating RPM, the rotation speed increases. Now you have an even greater rate of changing flux, and so an even greater emf/current is induced in the rotor coil as the rotor spins faster.

This emf being induced in the rotor coil is, as once again stated above, opposing the applied emf of the d.c. current in the stator coil as a result of Lenz's Law.
Lenz's Law goes something along the lines of:

"An induced electromotive force (emf) always gives rise to a current that opposes the current that gave rise to it."

It's a result of the conservation of energy - you can't get something for nothing! I.e. If you're going to produce a current, you must have first taken it away from the original current, and so the original (applied) current gets smaller. Hence why we see the emf in the rotor coil get smaller the faster it spins.

Hope this helps in some way sorry I couldn't quote posts - not sure if you can as a non-member.

edit: turns out I had to sign up anyway to post :L

Yes I wasn't too sure and your explanation is brilliant, covers practically all the fundamentals you need to know for electromagnetism in general. Thank you!!

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RATKING, 188GTS and Karoia thank you very much! Helped massively. How's everyone feeling for the exam then?

x

Could someone explain to me why flux is proportional to current? The question I'm referring to is 10a on June 10 (http://www.ocr.org.uk/Images/66749-question-paper-unit-g495-field-and-particle-pictures.pdf)

Thanks

Alternating voltage is supplied to the coil, so an alternating magnetic field is created, so there's a change of flux. The electrons in the coil experience a force due to the change of flux, so they move. Movement of electrons is a flow of charge.

At least, that's what I think. Don't quote me on anything

Quoted!

Thank you, that kind of makes sense!

Also under that it says that the radiation raises the energy level of the electrons where they become trapped? How/why are they trapped at that energy level?

I would also like to know this, could anyone help out?

Thanks for the wonderful explanation

Just an extra question, does this also apply for a generator? Or is it any different..

This emf being induced in the rotor coil is, as once again stated above, opposing the applied emf of the d.c. current in the stator coil as a result of Lenz's Law.
Lenz's Law goes something along the lines of:

"An induced electromotive force (emf) always gives rise to a current that opposes the current that gave rise to it."