VCE Physics Question Thread!

[P0ppinfr3sh]

This question has popped up in the textbook a couple of times an I can't seem to get the right answer:
How would you calculate the maximum induced voltage? I know that the maximum rate of change of magnetic flux occurs when the plane of the coil is parallel to the magnetic field lines but  I can't seem to translate this into Faraday's Law.

Thanks

P.S. (I've got a SAC on this tomorrow and this is the only thing I'm unsure about so please answer quickly  ).

[lolipopper]

This question has popped up in the textbook a couple of times an I can't seem to get the right answer:
How would you calculate the maximum induced voltage? I know that the maximum rate of change of magnetic flux occurs when the plane of the coil is parallel to the magnetic field lines but  I can't seem to translate this into Faraday's Law.

Thanks

P.S. (I've got a SAC on this tomorrow and this is the only thing I'm unsure about so please answer quickly  ).

can you tell me which question it is? is it from hinemann?

[SocialRhubarb]

Find the average RMS voltage and multiply by to get the peak voltage.

[P0ppinfr3sh]

can you tell me which question it is? is it from hinemann?

Well question 5 and questions 8 of 10.4 involve peak induced voltage and magnetic flux; and yes, it's from Heinemann.

Find the average RMS voltage and multiply by to get the peak voltage.

How would you go about finding the average RMS voltage of a generator? I can find the average induced voltage but I don't think this is equivalent to the average RMS voltage.

Also, I might as well post the related question(s) from the textbook:
5. 100 turns, area=20cm^2, B=5.0mT, coil rotates at a rate of 15degrees per millisecond. What is the peak value of the induced EMF for this coil?.
ANS: 0.263V.

8. Peak voltage=8.0kV, N=1000 turns, each coil has a radius of 10cm, magnetic field strength=B, frequency=50Hz. Calculate the strength of the magnetic field required to produce a peak voltage of 8.0kV.
ANS: B=0.81T.

Thanks.

[09Ti08]

This question has popped up in the textbook a couple of times an I can't seem to get the right answer:
How would you calculate the maximum induced voltage? I know that the maximum rate of change of magnetic flux occurs when the plane of the coil is parallel to the magnetic field lines but  I can't seem to translate this into Faraday's Law.

Thanks

P.S. (I've got a SAC on this tomorrow and this is the only thing I'm unsure about so please answer quickly  ).

Faraday's law: emf=-d(phi)/dt
If you draw the graph, it should look like a cosine graph (fyi flux is the dot product of area and magnetic field, that's why we have the cos function). After drawing the graph, i think it should be clear that the gradient of the graph is greatest/lowest when magnetic flux=0 (if you know calculus, this is easy to prove). Greatest/lowest gradient (which is basically the biggest/smallest value of -d(phi)/dt) gives you the maximum value for the magnitude of emf.

[P0ppinfr3sh]

Faraday's law: emf=-d(phi)/dt
If you draw the graph, it should look like a cosine graph (fyi flux is the dot product of area and magnetic field, that's why we have the cos function). After drawing the graph, i think it should be clear that the gradient of the graph is greatest/lowest when magnetic flux=0 (if you know calculus, this is easy to prove). Greatest/lowest gradient (which is basically the biggest/smallest value of -d(phi)/dt) gives you the maximum value for the magnitude of emf.

Yeah, that seems to make sense, but I don't really know how to apply it to the questions I've mentioned above. 

[lzxnl]

This question has popped up in the textbook a couple of times an I can't seem to get the right answer:
How would you calculate the maximum induced voltage? I know that the maximum rate of change of magnetic flux occurs when the plane of the coil is parallel to the magnetic field lines but  I can't seem to translate this into Faraday's Law.

Thanks

P.S. (I've got a SAC on this tomorrow and this is the only thing I'm unsure about so please answer quickly  ).

Find the average RMS voltage and multiply by to get the peak voltage.

Erm, RMS voltage and multiply by square root 2 only lets you find the RMS voltage of a sinusoidally voltage. Something whacky like a square wave in which the voltage is equal to V for half a period and -V for the other half would have an RMS value of V. Just an example.

Besides RMS has nothing to do with this question.

Faraday's law: emf=-d(phi)/dt
If you draw the graph, it should look like a cosine graph (fyi flux is the dot product of area and magnetic field, that's why we have the cos function). After drawing the graph, i think it should be clear that the gradient of the graph is greatest/lowest when magnetic flux=0 (if you know calculus, this is easy to prove). Greatest/lowest gradient (which is basically the biggest/smallest value of -d(phi)/dt) gives you the maximum value for the magnitude of emf.

Indeed. Another instance of where VCE physics just fails. You have flux = BA*cos(theta)
but as theta is changing at a constant rate (constant rotation), theta = wt
so flux = BA*cos(wt)
differentiate: emf = BAw*sin(wt) with the sign depending on how the voltage is measured

So peak emf is BAw

[Alwin]

Yeah, that seems to make sense, but I don't really know how to apply it to the questions I've mentioned above. 

How would you go about finding the average RMS voltage of a generator? I can find the average induced voltage but I don't think this is equivalent to the average RMS voltage.

Also, I might as well post the related question(s) from the textbook:
5. 100 turns, area=20cm^2, B=5.0mT, coil rotates at a rate of 15degrees per millisecond. What is the peak value of the induced EMF for this coil?.
ANS: 0.263V.

8. Peak voltage=8.0kV, N=1000 turns, each coil has a radius of 10cm, magnetic field strength=B, frequency=50Hz. Calculate the strength of the magnetic field required to produce a peak voltage of 8.0kV.
ANS: B=0.81T.

5. This is what happens when you TRY to calculate peak voltage with RMS...

It rotates 15 degrees per millisecond. So, in 6 milliseconds it turns 90 degrees. Say if it start parallel to the field, then after 6 milliseconds it had turned to perpendicular to the field.

^ close but no cigar. You should note that from the textbook, a rate of 15° per millisecond (i.e. a frequency of 42 Hz) is not actually correct since not accurate enough.. it is actually 15.12 degrees per millisecond.. but that's still a bit off the correct answer

What you should actually do is use the formula:


8. Pretty much same thing,
The maximum EMF is equal to NBA2πf, so:


EDIT: didn't see nliu's post since was latexing. If you're wondering w is angular velocity, ω=2 pi f

Hope it helps / makes sense

[P0ppinfr3sh]

Thanks alot guys for your help. 

You're all awesome!

(I can now sleep peacefully)

[SocialRhubarb]

I guess sometimes you just have to use calculus, huh.

[lzxnl]

I guess sometimes you just have to use calculus, huh.

But that's too difficult for the delicate minds of VCE physics students.

[BasicAcid]

But that's too difficult for the delicate minds of VCE physics students.

Well you seem to be one of those guys who has no respect for the VCE design (for all subjects) even though you'll be one of the people who simply ace VCE.
May I ask why?

Also is it wrong that I never fully understood motion in physics (which we was the first thing we did in term 1) until I began kinematics in spesh a few weeks ago?
And do you know if we're allowed to use I and J systems in physics?

[lzxnl]

Well you seem to be one of those guys who has no respect for the VCE design (for all subjects) even though you'll be one of the people who simply ace VCE.
May I ask why?

Also is it wrong that I never fully understood motion in physics (which we was the first thing we did in term 1) until I began kinematics in spesh a few weeks ago?
And do you know if we're allowed to use I and J systems in physics?

Why? VCE physics is an absolute joke. As you've seen yourself, maths teaches physics better than physics does. I'm not surprised; I actually learnt forces, energy and kinematics firstly from spesh in year ten before doing physics, and it worked perfectly fine.

Why is physics a joke?
1. They cut down the maths. In a subject which is built on maths. It's like teaching a guy to build a house but you only give him sand. He needs to find the bricks somewhere, but if you're giving them to him...

2. You have frigging notes for the exam. Come on. You can literally summarise EVERYTHING you need for the exam in four sides of paper as long as you write small. I mean come on. That's really lazy. I can understand giving the formulas, but letting us bringing in a cheat sheet? It's a wonder 90% is still an A+ on the exams.

3. Why the frick are we repeating motion after doing it in year 11? Why the frick are we going over basic circuits when that should have been covered in year 8, at most year 10?

4. Why the bloody heck are we cutting a course down when we're increasing the study time for the course by removing the mid-year exam study period and making it available for class time? Hint. English and Maths all have three hours worth of exams in total, but they didn't cut anything down.

5. Actually test us on physical knowledge instead of boring subbing into equations. I don't know how many I used E = hc/wavelength for revision for my last SAC.

That rant enough for you?

If you use vectors in physics, it'll work, but there's no need to use something so advanced for VCE physics. I'm not kidding.

[lzxnl]

nliu1995, I am very intrigued by you at the moment.

Never seen anyone who seems to actually be capable of virtually destroying VCE with minimal effort.

But what? Spesh in year 10? In year 10 I was still learning what a parabola was lol...

I'm just a more outspoken member of the "dissatisfied with VCE" group. There are heaps of people like me. They just don't want to be so vocal for fear of offending people.

You should read pi's VCE Physics 3/4 review that's stickied. It's even better.

lol well, I spent too much on maths in earlier life. Parabolas was year five for me. I still remember fumbling around to understand what this f(-b/2a) meant in primary school.

[lzxnl]

I am simply speechless...
Man what the hell, although deep down know you're telling the truth, I simply cannot believe what you are saying.

Year 5... Pokemon Emerald/FireRed dominated my life throughout that year?

I only learnt what that f(-b/2a) thing meant last year lol.

You see, I am where I am now because I sacrificed gaming time for study. I looked like a dork then. I was even bashed one day for having a year eleven maths book in year six. But now, I'm laughing at those who laughed at me. It's well worth it
I think my decision to pre-learn (at least read upon) all the uni courses that I'll be doing next year in the long break makes sense now.