Can you see?

[kappage]

just recently we started doing transformers in physics, i asked for the previous years test and this is question five from their.
Can you see whats wrong?
Read this first though, this is out of my physics text book.

A changing magnetic field will produce an EMF in a nearby coil

and here is the question

Question five.jpg (40.85 KiB) Viewed 3996 times

This question was in a previous years TEE question (TEE is the higher subjects what we need to pass to get into university)

Post what you think it is.

[Alathald]

I see nothing wrong with it, it's a classic transformer. It's also a trick question as well though I'll leave it below incase someone else wants a shot at it

Highlight: Because it's DC, there is no inductance...part c is still tricky though because it is a changing current so there will be inductance.

[aloverof1]

what do you mean you c nothing wrong.

[Alathald]

a.) assuming I remember my right hand rule correctly (and I may not), the magnetic field created due to the primary coil would be pointing to the bottomright corner of the page at P

b.) It's a DC current so no inductance, so zero current in the secondary coil

c.) since the change in magnetic flux with respect to time is equal to the negative of the electromotive force (ΔΦ/Δt=-emf) then the average emf would equal -0.006Wb/0.03s = -0.2V

Now somebody owes me a cookie for working all of that out...

[DMB2000uk]

Sorry:



Dan

[JuneBug]

c.) since the change in magnetic flux with respect to time is equal to the negative of the electromotive force (ΔΦ/Δt=-emf) then the average emf would equal -0.006Wb/0.03s = -0.2V

So whats Wb stand for and why is it negative 0.006?

[stev]

c.) since the change in magnetic flux with respect to time is equal to the negative of the electromotive force (ΔΦ/Δt=-emf) then the average emf would equal -0.006Wb/0.03s = -0.2V

So whats Wb stand for and why is it negative 0.006?

See article "C" in the test question.

[Alathald]

Wb stands for Weber, which is the SI unit for magnetic flux. Wikipedia has more info on magnetic flux, probably more than you ever wanted to know...[link]

The only reason I can think of for kappage et al seeing something wrong with this problem is that they got electromagnetic field and electromotive force mixed up, which is understandable because while they can both be written as emf, the usual shorthand for electromotive force is a scripted E () while the usual shorthand for the electromagnetic field is emf. Very common for a student to get those mixed up but I'm surprised that the teacher got it wrong...the book definition kappage gave and the way the problem is worded clearly points to electromotive force...

BTW, I hope you enjoyed my cookie Dan!

[Alathald]

He annoys me

Who annoys you? Me? I was just trying to help out a fellow forum member...*sniff* *goes to sit in corner and have a good cry*