Post by: vamsiaus on October 02, 2009, 11:13:25 pm
1. Magnetic flux is the product of the field and the area of the loop right? The answer has it one decimal off to my answer.
change in flux = BA = .5 x .02 = .01 wb. This is my answer.
The solutions have it as .1 wb.
2. I got it Wrong as a result of question 1. But this is my process:
Emf= change in flux / change in time = .01/.05 = .2 volts.
The answer is 2 volts with a negative. When do i put this negative, and when do i leave. Is it wrong if I don't have a negative.
3. Alright this is what I don't get at all.
1. Do all DC graphs not go below the x-axis. Why?
2. I got the right graph but just not sure behind the theory. Initially the coil is parallel to the magnetic field and thus according to the equation (change in flux = BA costheta) since theta = 0, the magnetic flux should be at a max to start off with. Since flux is at a max, so should EMF. I am just not sure about whether or not this theory is right though.
4. I have come across some of these questions where in the process of finding out the magnetic flux, the solutions deliberately leave out the number of turns in a coil. I was just not able to pick up the pattern on this. When do I use the 'n' in the formula(flux = nBA), and when do I leave it out.
These maybe noob questions, but due to our stupid school any explanations regarding these questions are never made in class. So lacking a bit of theory knowledge.
Thanks in advance to anyone who can help.
Post by: Edmund on October 02, 2009, 11:27:59 pm
Post by: vamsiaus on October 02, 2009, 11:31:50 pm
Post by: kamil9876 on October 03, 2009, 12:15:23 am
Quote
Since flux is at a max, so should EMF. I am just not sure about whether or not this theory is right though.
In fact the opposite is true. When flux is max or min the emf is 0 and when flux is 0 the emf is at it's max or min. In order to explain why I could use calculus(maths methods will suffice) but I don't know if you do the subject so I'll try not to. Say flux is at a max when
Using the fact that
Play around with the formula
As for the signs, it's all about the direction given by lenz's law. In this case the current is in one direction since every half turn the ends of the coil swap with each other and hence are touching the different part of the commutator, and this cancels out the natural change in direction that occurs every half turn in the coil(you can check this by using the
Feel free to ask for clarifications.
Post by: vamsiaus on October 03, 2009, 12:31:46 am
Post by: kurrymuncher on October 03, 2009, 12:43:25 am
Say for the situation in the exam. Since the coil is in parallel with the magnetic field, the flux would be max according to the rule right? Then since Emf is a derivative function of the mag flux, shouldn't the emf start at 0 then. (The solutions have it starting with a max as in a cos graph not sin.
No, the flux is not maximum when the coil is parallel to the magnetic field. The cos(thetha) in the flux formula is the angle the flux makes with the normal of the coil plane. therefore, when the coil is parallel, the angle between the flux and the normal of the plane is 90 degrees, therefore flux=BAcos(90). Cos(90)=0, hence the flux is zero when the coil is parallel to the magnetic field. Remember that angle is between the flux and the normal of the coil plane.
So, as the magnetic flux is zero, when the coil begins to rotate, the flux vs time graph will look like a sine graph. To get the EMF vs time graph, you have to take the NEGATIVE derivative of the flux graph, therefore, the EMF vs time graph will be a cos wave.
Post by: vamsiaus on October 03, 2009, 12:58:08 am
Post by: kurrymuncher on October 03, 2009, 01:12:53 am
notice how the angle between the plane of the coil and the flux is 90 degrees, therefore cos (90) is zero, so the flux is zero.
sketch isn't that great, but I tried lol :laugh:
flux= green lines
Post by: kamil9876 on October 03, 2009, 01:24:04 am
Say for the situation in the exam. Since the coil is in parallel with the magnetic field, the flux would be max according to the rule right? Then since Emf is a derivative function of the mag flux, shouldn't the emf start at 0 then. (The solutions have it starting with a max as in a cos graph not sin.
The flux would initially be zero as kurrymuncher pointed out. The emf(Voltage) will initially be as a max, in agreement with what i said earlier and the solutions (cos graph).
Post by: vamsiaus on October 03, 2009, 09:27:37 am
The question asks for the direction of the north pole and the compasses are showing.
This is what I thought. Since the current is travelling up the wire, using the right hand grip rule the direction of the field is clockwise(when viewed from the top). Then I thought that field always goes from North to South, so the direction of the North in that field will be in the opposite side of the direction of the field (as field is travelling from North to south, and I want North). Thus I chose C, but the answer is B. Not Sure why though.
Thanks
Post by: vamsiaus on October 03, 2009, 11:52:41 am
Post by: sachinmachin on October 03, 2009, 12:08:29 pm
aww thanks for the help guys really appreciate it. One more question, attached is q3 from Lisachem 2007 exam.
The question asks for the direction of the north pole and the compasses are showing.
This is what I thought. Since the current is travelling up the wire, using the right hand grip rule the direction of the field is clockwise(when viewed from the top). Then I thought that field always goes from North to South, so the direction of the North in that field will be in the opposite side of the direction of the field (as field is travelling from North to south, and I want North). Thus I chose C, but the answer is B. Not Sure why though.
Thanks
yeh the answer would be B. im guessing you've used the right hand grip rule, but have managed to get the direction of the magnetic field wrong. because the current is travelling upwards, your thumb should point up. thus the magnetic field would be anticlockwise as you wrap your fingers around the wire which should curl around in an anticlockwise direction. dont worry, i used to do the same mistake as you, as i constantly muddled up the right hand grip rule.
Post by: vamsiaus on October 03, 2009, 12:11:08 pm
Post by: sachinmachin on October 03, 2009, 12:18:19 pm
yea, i know how to use the grip rule lol. But doesn't field travel from north to south, so the direction of field would be in the direction opposite to the side shown by the compass (which points north).
remember for current carrying wires, you cannot assume the direction of magnetic field unless there are two magnets (different polarity) on either side of the wire. especially when you use the right hand grip rule, you must be careful as to how you see the direction of which your fingers curl around. i suggest maybe looking at checkpoints, and do about 20 questions of this type (other sources as well) until you feel you've got some understanding of how it works.
Post by: vamsiaus on October 03, 2009, 12:40:08 pm
Post by: sachinmachin on October 03, 2009, 12:44:20 pm
So, for these sort of questions, would u just assume that the direction of the magnetic field is North and that the compass would point in the direction of the magnetic field(north).
well for this particular question, it specifies that the arrows represent north as the direction of the magnetic field, as they have referred to it as a compass. however if they had not mentioned it, it wouldnt have made much difference as we can still find the direction of the magnetic field using the right hand grip rule. never assume though for these type of questions.