ugh physics

[fredrick]

Got a SAC on monday n forgot my text book at school l0l. Heres a few questions i need answered please explain how you got the answer aswell cheers!

Question 1)
[IMG]http://img67.imageshack.us/img67/4475/q1qt0.th.jpg[/img]

Question 2)
[IMG]http://img147.imageshack.us/img147/2179/q2or0.th.jpg[/img]

Question 3)
[IMG]http://img59.imageshack.us/img59/6300/q3ds2.th.png[/img]

[ed_saifa]

I think for question 1, the change in flux is going to the right. Therefore, it means the "reaction flux" is going the left. Use the right hand grip rule with the magnetic field to the left, same as the "reaction flux". So i guess the current is from x-y. I dunno if I'm right though. I dunno how to do the other ones.

[Mao]

I think for question 1, the change in flux is going to the right. Therefore, it means the "reaction flux" is going the left. Use the right hand grip rule with the magnetic field to the left, same as the "reaction flux". So i guess the current is from x-y. I dunno if I'm right though. I dunno how to do the other ones.

the current through the resistor is indeed x-y, your reasoning is quite correct.

question 2
since the graph did not give any values on the V axis, I'm going to assume by "average" they don't mean RMS, hence the average would obviously be 0

question 3
by the right hand grip rule, the direction of flux INSIDE the coil would be to the left, and hence magnetic field lines would flow OUT of the left, i.e. this is the north, and the other side would be south. you'd just draw the field lines line a normal bar magnet.
another way to think about this is, at the south end, the current flow clockwise (in the direction of the two ends of the "S")

[bigtick]

Q2

[Umesh]

Q2

how did u find the area of the coil to find the change in flux?

[cara.mel]

Doing some digging, this question comes from the 2005 exam

In order to complete this question, you need a piece of information given earlier:
"Question 2
With the coil vertical as shown in Figure 2, the flux through the coil is 8 × 10^-–6 Wb. What is the area of the
coil?"

[chid]

Hi guys I'm a little confused about something:

If you move the north pole of a magnet into a coil aren't the field lines going from north to south (eg: In question 1 the 'intial flux' is to the left and the 'reaction flux is to the right)

Also in the right hand slap rule the fingers give the direction of the field, but does that mean the fingers point towards the South pole or the north?


Thanks.

[cara.mel]

Hi guys I'm a little confused about something:

If you move the north pole of a magnet into a coil aren't the field lines going from north to south (eg: In question 1 the 'intial flux' is to the left and the 'reaction flux is to the right)

Also in the right hand slap rule the fingers give the direction of the field, but does that mean the fingers point towards the South pole or the north?


Thanks.

EDIT: This post is wrong because I am an idiot.

I went to do the question and got it flows from y-x. This made me retreat to my room and look through questions and try to work out how I was wrong but I feel I am right:
- To oppose the change in flux, the coil induces a magnetic field with the north pole on the left (imagine it repelling the magnet)
- Therefore using right hand grip rule, you draw little down arrows along the coil and hence it goes from y -> x

It means your fingers point towards the south. The magnetic field lines run in the direction from north to south

If there is something horribly wrong with my post, please tell me, I don't feel confident at all...

[chid]

Hi guys I'm a little confused about something:

If you move the north pole of a magnet into a coil aren't the field lines going from north to south (eg: In question 1 the 'intial flux' is to the left and the 'reaction flux is to the right)

Also in the right hand slap rule the fingers give the direction of the field, but does that mean the fingers point towards the South pole or the north?


Thanks.

I went to do the question and got it flows from y-x. This made me retreat to my room and look through questions and try to work out how I was wrong but I feel I am right:
- To oppose the change in flux, the coil induces a magnetic field with the north pole on the left (imagine it repelling the magnet)
- Therefore using right hand grip rule, you draw little down arrows along the coil and hence it goes from y -> x

It means your fingers point towards the south. The magnetic field lines run in the direction from north to south

If there is something horribly wrong with my post, please tell me, I don't feel confident at all...

Well that's why I was confused! But I feel more confident now that someone thinks the same way as me! Can anyone confirm that y to x is actually the correct answer?

[dcc]

I did this question and got the same answer as Mao, that is, the current flows from X to Y through the resistor.

• The change in flux is to the right (as the magnet is increasing the magnetic field strength around the coil).
• By Lenz Law, the induced current in the coil must induce a flux which opposes this change in flux (i.e. any induced current must induce a flux to the right)
• For this coil to create a magnetic flux to the left, we require that the current flows through the coil such that the coil has a resultant magnetic field to the left.  A simple application of the right-hand grip rule says that current flows from X to Y through the resistor

[chid]

Hmmm...

When the magnet comes in, won't the coil set up a north pole on the left (Lenz's law) and then the field through the coil is to the right. And after applying the right hand grip rule you get y to x as the direction of current flow.

Or the other way to think about is that the magnet inducing the current has its north pole 'coming in' so the field lines are initially directed towards the left. This gives the same resultas above.


Could someone explain where this reasoning is wrong?

Thanks.

[dcc]

When the magnet comes in, won't the coil set up a north pole on the left (Lenz's law) and then the field through the coil is to the right. And after applying the right hand grip rule you get y to x as the direction of current flow.

This is not the expected behaviour of the magnetic field set up by a magnet.  Consider a normal bar magnet, with two ends, labelled N and S.  When we draw the magnetic field lines, we draw a line starting at N then moving AROUND the bar magnet, before entering the magnet at S. The magnetic field lines are NOT drawn starting at the N and then moving through the bar magnet to the S pole. 

Consider the following picture of the magnetic field set up by a current passing through a coil. 


In this example, the left of the coil is the north pole and the right is the south pole, and the magnetic field lines are mainly towards the left.  This is exactly the same as the situation in Question 1, because we require that the magnetic flux to be to the left (to oppose the change of flux to the right by the moving magnet).  Note that as expected, to set up this magnetic field we would require current to flow through the resistor from X to Y.

[pinchies]

Thanks, your q's & ans's helped me too!

[chid]

Thanks dcc.