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About Me
- MacEachern
- Position: Associate Professor, Department of Electronics, Carleton University. Teaching: Graduate level courses on VLSI chip design and analog signal processing; Undergraduate level courses on electromagnetic theory, analog circuit design, and fundamental circuit analysis. Research: analog and mixed-signal ICs, optoelectronic devices and circuit interfaces, and sensors and sensor processing.
24 comments:
Hello Professor MacEachern,
I am looking at lecture slide 636, which is showing the derivation of the equation for magnetic force on a circular ring of current I due to a solenoid of finite length. I am confused about the final expression for the force,
Fdown = 2*pi*r*Bradial.
I don't understand why it is the radial B field, since it appears that the radial B field components cancel in this case.
To me it seems like it should be Bz.
If you could please clarify this I would greatly appreciate it.
Thanks,
Lydia Downing
(Posted by Len based on an email from Lydia).
Hi Lydia,
I'm going to create a blog posting using your question and my answer. Others may want to add questions and answers as well.
Anyway to answer your question:
dF = Idl X B
So don't look at the B's canceling in some way. Look at the direction of the forces. They all add -- they don't cancel. All the forces due to the B's going around in a circle point down. The forces (due to the B field) aren't radially directed, so they don't cancel. Hope that helps.
Here is a good link with lots of magnetics problem in section 30. there are lots of problems and solutions for each problem so its a good review for the exam.
http://books.google.ca/books?id=7TdIliBdAmwC&pg=RA1-PA535&lpg=RA1-PA535&dq=%5B10+points%5D+Now+assume+that+the+magnetic+field+changes+with+time+as+the+loop+is+rotating,+according&source=web&ots=t5N7VcC3Bv&sig=CHzWj17bSAf0DQ_Bkp_cI1T8Z0U&hl=en&sa=X&oi=book_result&resnum=8&ct=result#PRA1-PA530,M1
By the looks of it, the final is on magnetostatic right?
seems like it should be mostly magnetostatics, maybe a few "general knowledge" or "reasoning" questions on electrostatics. thats my view at least (don't take my word for it)
testing testing
Hey prof, will you be in your office on Thursday after 5pm?
Does the Hall effect work in reverse?
If you put a conductor with a current through it between charged plates will it create a magnetic field perpendicular to B and I?
Would an applied E and B field induce a current?
typo...
"If you put a conductor with a current through it between charged plates will it create a magnetic field perpendicular to E and I?"
Is what I meant
Professor MacEachern,
I seem to remember you saying something along the lines of "you don't need to study transformers", but I just wanted to verify that.
As well, do we need to study mutual inductance and motors?
has anybody worked through the multiple choice for last years final. I am really not confident on a lot of my answers
Hello Professor MacEachern,
Can you please hold office hours tomorrow ( wed ), ( please not between 2-5 because I have an exam) .
Thanks
Professor MacEachern
What is the lecture 17 slides? I am trying to do the Review Questions for Final's no 4, the one with a square loop wire, and I am having trouble coming up with and expression for the sides of the square. Also, A couple of us are trying do the tilted helmholtz coil question but having difficulties coming up with an experssion for the magnetic field. maybe we can use the angle of the tilt, so if the B of a coil when it is straight is found, then a tilted coil's B at a point is B/cos(angle)?
@Anonymous 4:16pm: Thanks!
@Anonymous 4:37pm: six questions. Two electrostatics, recycled from the sixteen given in the course as sample problems (given by moi). Four magnetostatics, similar to those given recently.
@Anonymous 8:10pm: Can't do. My little girl needs her dad at home. Volunteers for sitters could solve that!
@Anonymous 8:58pm: Uhhhh...
Lydia: No mutual inductance. No transformers. There is Faraday's Law and Lenz's Law. There is a "motor", but it's along the lines of the example question given on Saturday. I would not study the slides on motors for this exam.
LLL: Drop by tomorrow and I can walk you through these.
any one knows the answer
I want to confirm ,
does any one know
The Example Question 4 the answer is 0 because all forces cancel eachother
Question number 2 from final ,
part (b)
what does it mean sketch B? does it mean lines poit to posetive x direction
part (d) yes coil #1 has net resultant but it depends on the R2 if both coils are paralel and they have same R they will attract each other ,or coil#1 points to x direction
thanks
Farshad: When he asks to sketch B in Q2.b he means to take the function you got from Q2.a and draw the graph of the function from x = (-inifinity, infinity). basically he wants a plot of the function. if you look through the questions the prof posted for the last PA (he put a file online with questions scanned from a book) there is this exact question in that file. that should help you with the solution and what he means by "sketch B"
Farshad: here is the link to what i mentioned above
http://dl.getdropbox.com/u/198089/3105/PA_Sessions_Questions/PA04_Questions.pdf
check out page 428-429 (Problem 8-24). it is basically the same as Q2 just rotated.
does anyone knows the answer part (d) question (4) final exam(Farday's law):
I got integral of
B.cos(Wbt).cos(Wt) with respect to time.. there is no such an integral in formula shit. and also part (e) cross product of By X Vy is going to be zero . It means nothing changes
i think for Q4.e that the velocity v induces a current in the loop in the opposite direction as the current found in Q4.d. Knowing this we can just add the two currents to find the net current in the loop. Having the loop moving away from the origin with velocity v means that the flux is decreasing so therefore an induced current must be present in the loop to account for a decrease in flux. I think all you need to do is find the relationship for induced current due to a decrease in flux caused by a velocity. (probably already solved for in the notes somewhere)
adding to what i wrote above, since the loop is rotating there will be points in time when the induced current is present (same as for Q4.a when there were instances when the flux was 0 and instances when the flux was BWL, depending on the angle). Looking at slide 857 of the notes shows an equation for net flux due to a velocity. I think if you use that and take note of the rotation of the loop, you get that net flux is the equation show on slide 857 multiplied by either a sine or cose to account for the rotational aspect of the problem. All this might help or might be way off on a tangent but its worth a shot.
anyone have any ideas as to which electrostatics questions might be asked (prof, if you are reading this, a reply from you regarding this issue would be of great help lol). I am thinking the laplace question deriving the equation for a capacitor from laplaces equation would be a good one for the final as we didn't see that one on the midterm. Anyone else have an ideas of what could be asked.
I thought that for Q.4 (e) (Faraday's Law question) that you use the equation for motional emf (Lenz's Law) which is emf = ∫ (v x B)dot dl-∫(dB/dt dot da) (I found this equation in the lecture slides, page 200 I believe).
v (velocity) is in the same direction as B in this case (y hat direction), so v x B = 0. Then you are just left with
emf = ∫ (dB/dt dot da) which is just dB/dt ∫ da = dB/dt(WL).
To find I (current) I just divided my answer by R. ( I = V/R ).
After looking back on that, however, I keep thinking to myself that all the question was asking for was the direction of the current now that the loop is moving away from the B field.
In this case, the answer would just be -I (in the opposite direction) that it was initally flowing in.
I'm not really sure whether we're supposed to get a new answer completely for I, or just say that I is in the opposite direction, for this question.
Any help with this would be much appreciated!!
@Anonymous 3:31pm: You take the prize for best guess of the year!
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