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# Electromagnetic Braking, unit 4. watch

1. Electromagnetic Braking question..docx

Or if the above link does not work try

Hi, I cannot understand the question attached below. It involves a horizontally spinning aluminium disc with a fixed iron core with a coil round it.

This question is based on electromagnetism, so i think the topic is under magnetic fields in unit 4.

I don't know what to draw for the diagram... According to the mark scheme the "two currents meet up."

For part i) I have no idea.

For part ii) I'm thinking Len'z law, but what else am I missing?

last part of question: No idea what formula to use.

Thanks!

An error occurred while reading the document.

3. (Original post by krisshP)

An error occurred while reading the document.

Hmm, works for me. Can anyone else confirm this?

4. That's what I get.
5. (Original post by krisshP)

That's what I get.
Hmm, longer route but the question is from the old past paper database, the links you shared. It is question number 43 in "electric and magnetic Q" folder. I'll try and upload the file again in the meantime.

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6. Post pictures of the questions. Try the snipping tool or print screen and paint.

Simple.
7. (Original post by krisshP)
Post pictures of the questions. Try the snipping tool or print screen and paint.

Simple.
Edited, should see the question now
8. (Original post by Jaydude)
Edited, should see the question now
Continue the straight current lines as they are just showing you a small region of the disc I suppose.

The iron core is an electromagnet as the current in the coil around forms a magnetic field around it. The direction of the current in the coil surrounding the iron core is given. Thus you can determine the magnetic field direction using Fleming's right hand grip rule as in the attachment. See these two images if you are unsure about this rule

http://4.bp.blogspot.com/-8e_cyTZXHw...rip-Rules2.png

As a small region of the disc enters the region around the iron core electromagnet, there's an increase in the magnetic flux through it. According to Faraday's law of EM induction, this magnetic flux change through the small region in the disc causes an induced emf in the region. By V=IR there's a current.

Lenz's law tells us that the induced emf in that small region of the disc acts in a direction such as to oppose the cause of the magnetic flux change that caused it. I said above that there's an INCREASE in the magnetic flux through the disc region entering the disc, clearly due to the disc rotating. So the induced emf will act in a direction such that it prevents the magnetic flux increase by slowing the disc down. The disc's rotation is the cause of the induced emf anyway.

http://www.s-cool.co.uk/a-level/phys...e-it/lenzs-law

Sorry if I poorly explained :/

9. The braking effect is due to the induced emf which in turn is due to the braking current. But I still am not sure how the braking effect is proportional to the square if the braking current.
10. Thanks! To clear things up...

The discs motion and the magnetic field is perpendicular to the rotation of the disc, so there is a change of magnetic flux linkage, so an emf is induced such that to oppose the motion creating it. The Emf then leads to the force opposing the discs motion due to the Flemmings right hand rule.

Kind of hard to imagine, but how exactly is the disc cutting this magnetic field in the coil of wire? Is it in that small region only?
11. (Original post by Jaydude)
Thanks! To clear things up...

The discs motion and the magnetic field is perpendicular to the rotation of the disc, so there is a change of magnetic flux linkage , so an emf is induced such that to oppose the motion creating it. The Emf then leads to the force opposing the discs motion due to the Flemmings right hand rule.

Kind of hard to imagine, but how exactly is the disc cutting this magnetic field in the coil of wire? Is it in that small region only?
No.

Think of it like this. Initially there are 0 magnetic field lines passing through the small region on the disc. But as the disc rotates and when this region is close to the iron core electromagnet, the number of field lines passing through the region increases. Magnetic flux is essentially just a measure of the number of field lines passing through perpendicular to something and magnetic flux density is like the number of field lines passing through something perpendicular to it per unit area.

Lenz's law states that the induced emf acts in a direction such as to oppose the magnetic flux change that caused it. However Fleming's right hand GRIP rule can tell you the CONVENTIONAL (+-->-) current direction if you already know the magnetic field direction (N-->S). Alternatively it can tell you the magnetic field direction if you already know the current direction.

12. The initial position of the small region in the disc I was talking about is the pink dot on the disc there (left). The final position of the same small region in the disc after some rotation is the red dot on the disc (under the iron core). See

http://macao.communications.museum/i..._2_4_2_eng.png

So at the red dot, just under the iron core, there's high magnetic flux through it, but at the pink dot there's little magnetic flux through it. Thus as the region moves from pink to red, Theresa magnetic flux change.
13. (Original post by krisshP)
No.

Think of it like this. Initially there are 0 magnetic field lines passing through the small region on the disc. But as the disc rotates and when this region is close to the iron core electromagnet, the number of field lines passing through the region increases. Magnetic flux is essentially just a measure of the number of field lines passing through perpendicular to something and magnetic flux density is like the number of field lines passing through something perpendicular to it per unit area.

Lenz's law states that the induced emf acts in a direction such as to oppose the magnetic flux change that caused it. However Fleming's right hand GRIP rule can tell you the CONVENTIONAL (+-->-) current direction if you already know the magnetic field direction (N-->S). Alternatively it can tell you the magnetic field direction if you already know the current direction.
But your also saying that magnetic flux is changing too,

magnetic flux = Φ. And magnetic flux linkage = Δ(NΦ)

So if Φ is changing, surely the flux linkage is too? So by faradays law, an emf is induced in this way?

Where am i going wrong?
14. (Original post by krisshP)

The initial position of the small region in the disc I was talking about is the pink dot on the disc there (left). The final position of the same small region in the disc after some rotation is the red dot on the disc (under the iron core). See

http://macao.communications.museum/i..._2_4_2_eng.png

So at the red dot, just under the iron core, there's high magnetic flux through it, but at the pink dot there's little magnetic flux through it. Thus as the region moves from pink to red, Theresa magnetic flux change.
I dont quite understand, is the region supposed to be where the flux lines cut through the disc? If so, how did you work that out? Doesnt the field lines cut through anywhere on the disc? As you can probably tell, i'm very lost...
15. (Original post by Jaydude)
But your also saying that magnetic flux is changing too,

magnetic flux = Φ. And magnetic flux linkage = Δ(NΦ)

So if Φ is changing, surely the flux linkage is too? So by faradays law, an emf is induced in this way?

Where am i going wrong?
Yes, you are right here.

16. (Original post by Jaydude)
I dont quite understand, is the region supposed to be where the flux lines cut through the disc? If so, how did you work that out? Doesnt the field lines cut through anywhere on the disc? As you can probably tell, i'm very lost...
The region is just a general region I chose on the disc to illustrate my point of changing magnetic flux.

Also, magnetic flux linkage I tend to use when talking about a coil of wire of the magnetic flux linking it.
17. http://www.s-cool.co.uk/a-level/phys...nduce-an-elect

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18. (Original post by krisshP)
The region is just a general region I chose on the disc to illustrate my point of changing magnetic flux.

Also, magnetic flux linkage I tend to use when talking about a coil of wire of the magnetic flux linking it.
Thanks for clearing that up. Ill try and be careful when using such key terms in questions. (Those links are very helpful!)
19. (Original post by Jaydude)
Thanks for clearing that up. Ill try and be careful when using such key terms in questions. (Those links are very helpful!)
Yeah, I know how it feels, VERY hard at first to grasp, but eventually can sink in. Try the question on this thread and see my explanation for the answer below

http://www.thestudentroom.co.uk/show...php?p=45797638

I feel that it may help give you a better grasp of Lenz's law.

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