Question Help please! (1 Viewer)

[aoniumz]

Can someone please explain this question?
http://prntscr.com/491cr4

 

[SquareHeartsAdrita]

C because

 

[aoniumz]

Answer says B however

 

[aoniumz]

Am I right if I say that...
Phi = B A

B is inversely proportional to A

Therefore as Area decreases B will increase?

 

[SquareHeartsAdrita]

iirc the magnetic flux density is the density of field lines. So in the first line it says "uniform magnetic field" so I though C)


and where did it say B)? this questions looks very familar

 

[aoniumz]

Its 2010 CSSA trial. The Marking guidelines says its B. But I seriously wonder if my answer is correct at all.

 

[SquareHeartsAdrita]

let me pull up the paper

 

[SquareHeartsAdrita]

well the answer key isn't very helpful, it just says B, H7 bands 4-5


but I am fairly confident with my answer

 

[aoniumz]

But why though? Im thinking as AB rolls down. The area of ABCD will decrease.

A is inversely proportional to B so as Area decrease B Increases

 

[SquareHeartsAdrita]

but that is the magnetic flux increasing. it is asking about the magnetic flux density

 

[aoniumz]

B is magnetic flux density? Isnt it?
My textbook definition
B definition: Amount of magnetic flux per unit area or magnetic flux density.
Phi definition " number of field lines passing through an imaginary area.

 

[Zeref]

Magnetic flux is the number of magnetic field lines passing through a given area and is the product of magnetic flux density and the perpendicular surface area. I think the magnetic flux is constant and since A decreases, density must increase. just a guess though.....

 

[SquareHeartsAdrita]

been so long since I've done HSC physics. Maybe/probs I am wrong

 

[aoniumz]

Well. Magnetic flux will decrease apparently as it is not perpendicular. But the rate it decreases may or may not equal to the area of decrease. Damm I'm so confused!

 

[someth1ng]

The correct answer is B.

When the bar rolls down, there's a change of flux in the closed loop so a current is induced (b to c through the conducting bar) - this current also produces magnetic flux in the same direction as the provided magnetic flux so flux density increases.

Am I right if I say that...
Phi = B A

B is inversely proportional to A

Therefore as Area decreases B will increase?

You are making the assumption that Phi is constant.

 

[photastic]

B because as it's falling at a faster rate due to gravity, hence according to Faraday's law, an emf will be induced. Remember, B=φ/A. φ is increasing as it slides down. Idk ceebs

 

[Fizzy_Cyst]

totes love this question.

Really need a good understanding of Lenz in order to understand this question

 

[anomalousdecay]

totes love this question.

Really need a good understanding of Lenz in order to understand this question

Sad thing would be that many texts and references would probably just refer to the formula and say that the area is decreasing so flux density must increase.

Not many texts or references would refer to this problem in the way someth1ng did, which is quite an elegant explanation of what does indeed happen.

 

[Fizzy_Cyst]

Sad thing would be that many texts and references would probably just refer to the formula and say that the area is increasing so flux density must increase.

Not many texts or references would refer to this problem in the way someth1ng did, which is quite an elegant explanation of what does indeed happen.

Have had a number of tutoring students whose teachers could not explain or explained similar to what you stated :'(

 

[anomalousdecay]

Have had a number of tutoring students whose teachers could not explain or explained similar to what you stated :'(

Oops just realised my typo area decreases lol.

It happens, and the way I would have seen it back from year 12 would have been the same. Worst thing here is that you can just get away with it and not realise that you have a misconception.