Max Payne's Physics Questions Thread

[max payne]

I'm working through trials and I don't want to start a new thread for every question so I hope you all can help.

Is this a good explaination for why DC motors dont work without a commutator.
"The coil will rotate 90 degrees, oscillate and then stop.
The momentum of the coil will carry it past the 90 degree point but the reversal of the sides of the coil while the direction of the force on each side is still the same, will cause the net turning force to reverse and the coil will oscillate until it loses all its momentum."

oh and when a generator is rotated, is it correct to say that the induced current will create a 'magnetic field' that opposes the original field or the induced current will create a force that opposes the original force?

[FlorianK]

oh and when a generator is rotated, is it correct to say that the induced current will create a 'magnetic field' that opposes the original field or the induced current will create a force that opposes the original force?

Called back EMF

[max payne]

oh and when a generator is rotated, is it correct to say that the induced current will create a 'magnetic field' that opposes the original field or the induced current will create a force that opposes the original force?

Called back EMF

What's that?

[FlorianK]

oh and when a generator is rotated, is it correct to say that the induced current will create a 'magnetic field' that opposes the original field or the induced current will create a force that opposes the original force?

Called back EMF

What's that?

That's what you described.
The current creates a magnetic field that induces an EMF in the opposite direction

[max payne]

Oh ok.
I just did a trial that had questions about coherent and incoherent light in the light and matter section. I can't recall that being in the text book. Is that part of the old study design?

[Lasercookie]

Oh ok.
I just did a trial that had questions about coherent and incoherent light in the light and matter section. I can't recall that being in the text book. Is that part of the old study design?

From memory, yeah that was old study design content. I think it might be in one of the detailed studies now.

[max payne]

The book that I use has questions that require the emf at any given time, for example: e(t)=nBA(2*pi*f )*cos(2*pi*f*t)
I have never come across these type of questions in exams only ones that ask for the average emf. Is it even in the study design. Can they ask those types of questions?

[StumbleBum]

The book that I use has questions that require the emf at any given time, for example: e(t)=nBA(2*pi*f )*cos(2*pi*f*t)
I have never come across these type of questions in exams only ones that ask for the average emf. Is it even in the study design. Can they ask those types of questions?

I just searched (ctrl+F) the study design on the VCAA website and this was all that came up to do with emf was:

investigate and analyse the generation of emf, including AC voltage and calculations using induced
emf, ε = -nΔФ/Δt, in terms of
– the rate of change of magnetic flux (Faraday’s Law)
– the direction of the induced current (Lenz’s Law)
– number of loops through which the flux passes;

So without being 100% sure, i would assume that they can't ask the questions using that formulae from your book. I've personally never seen that formulae this year at all, so i'd really hope its not in the study design...

[max payne]

How would you go about answering this question:
"Calculate the magnitude of the average emf generated when the coil is rotated as shown:"
Basically the diagram is just a coil of area 0.0108m^2 with a magnetic field threading it of magnitude 0.5T rotating at 12Hz.

I assumed the average emf was the rms Voltage so I calculated the Vpeak with the formula " emf(max)=n*B*A*2*pi*f " then I divided it by sqrt(2).
What they did was calculate the average emf over one quarter of a revolution only by doing change in flux/one quarter of the period. That can't be right?

[meemz]

How would you go about answering this question:
"Calculate the magnitude of the average emf generated when the coil is rotated as shown:"
Basically the diagram is just a coil of area 0.0108m^2 with a magnetic field threading it of magnitude 0.5T rotating at 12Hz.

I assumed the average emf was the rms Voltage so I calculated the Vpeak with the formula " emf(max)=n*B*A*2*pi*f " then I divided it by sqrt(2).
What they did was calculate the average emf over one quarter of a revolution only by doing change in flux/one quarter of the period. That can't be right?

It is
for average emf in a motor, take a quarter of the period
i.e t= T/4