Harvey's Question Thread

[HarveyD]

Any help is appreciated.
First Question:
Could someone explain to me how they would fill out the table at the bottom

[Lasercookie]

I would look at what happens at the start and then after the 90 degree turn. You don't really need to bother with magnitudes, just look at the directions and use that to find if there is a force or not (and when it is at a maximum or minimum). e.g. when it's horizontal, the field+coil is perpendicular, so there is a force; when it's vertical, it's parallel so therefore no force etc..

The question states that there is a force at AB when it's horizontal, so that makes for an easy starting point.

My answer didn't seem to make sense (one of the sides should be zero throughout shouldn't it?). I won't post it in order to avoid confusion.

[HarveyD]

ah okay, ill try it again

in stav 2008, there's a question where it gives both the energy and momentum of a photon
would it matter which we used to determine the wavelength?

[Lasercookie]

ah okay, ill try it again

in stav 2008, there's a question where it gives both the energy and momentum of a photon
would it matter which we used to determine the wavelength?

I don't think it would matter.
or
Just be careful with Planck. With that momentum one, you must use 6.63 *10^-34 (same with de Broglie).

If you try both methods, and the answers are a bit different (I'd be surprised if they were off by a lot), I would say that in that case it's likely you've been given a rounded off value. I came across a question like this before, I'm really not sure which is the better answer. I'll look for a VCAA question where they do this, and see what the assessor's report says (I'm thinking possibly the x-ray vs electron diffraction question from last years exam might be a similar sort of case).

[HarveyD]

yeah they were quite close, so i guess it wouldnt matter

have you done chemology 2008?
theres a question where they give you the speed and ask you to find the emf
im getting different answers to when i use the V =  B x L x speed
and V = change in flux/change in time
Does it matter which one you use?

Edit: I figured out why mine was different
how come they used the 9cm length in the formula rather than the 12cm if it was moving with a speed of 1.2 m/s to the right
do we always use the vertical side or something

[Lasercookie]

yeah they were quite close, so i guess it wouldnt matter

have you done chemology 2008?
theres a question where they give you the speed and ask you to find the emf
im getting different answers to when i use the V =  B x L x speed
and V = change in flux/change in time
Does it matter which one you use?

ill upload the question if you havent seen it

Nah I haven't done it yet. I have the exam on my computer though, which question are you talking about.

edit: seen your post edit, looking at it now.

[Lasercookie]

Question 12 right?

It looks like they used the 12cm.
They used: t=d/v=0.12/1.2=0.1 seconds
And then substituted that into the rate of change formula.

I'm not sure why the EMF=vbl doesn't work when you sub in 0.12
Is it to do with how it's derived (it comes from f=qvb, which is substituted into the work equation)?
and (where the rate of change formula originates from).
The difference may be in the theta, they are representing different things. I would imagine that is the reason you would opt to use the 9cm when applying EMF=vbl. 

To be honest, I think I've never used EMF=vbl. I always opt to use rate of change of flux.

[HarveyD]

yeah
they used 9cm for the EMF = vbl and end up getting the answer, but im not sure why its 9 and not 12
anyway i guess i should use the other formula to be safe

[Lasercookie]

yeah
they used 9cm for the EMF = vbl and end up getting the answer, but im not sure why its 9 and not 12
anyway i guess i should use the other formula to be safe

I'm pretty sure the difference is to do with the theta, how one refers to the plane of the coil and one is from the normal to plane of the coil. I'll try and see whether this is true or not later, when I have a bit more time to look into it.

[Lasercookie]

I found a question today where I did have to use , I couldn't figure out how to do it with the rate of change formula.

It was from STAV 2011. It gave you a car that had a long antenna. The car was moving at through a field of (some value like that).



You can't really find the flux, since you weren't given an area - just the length of the wire.

[HarveyD]

ah yeah i think it's probably be best to do it when thats the only info we have and then convert it to period for every other time

anyway how would you do this?
cause my method seems to be wrong...

[Lasercookie]

I think the graph is of:


You're given the stopping voltage of 1.1 V

It looks like the graph starts at y= 1.1 V (I'm not too sure)? If it does then you can probably just look at the x-axis.

You could extrapolate the line and find the y-intercept (which will give you the Work function). Once you know that, you can just solve for 'f'.

Which exam is this from? I want to try this question on paper.

[HarveyD]

2009 Insight
the answers say to use E = hf
thats wrong yeah?
because the voltage indicates Ek max not just E...

[HarveyD]

oh if you're doing that exam, tell me your opinion on this question (below)
I would have thought that A best describes it?

[Lasercookie]

Isn't it C?

Amy is at the centre.

The low frequencies diffract a lot, but diffraction patterns are strongest at the centre, so she'd be able to hear the low frequencies.
Higher frequencies don't diffract that much compared to lower frequencies, so she'd be able to hear those as well.

This is something I was going to look into, one of the VCAA assessor reports said that students often confuse how diffraction behaves in light and matter as opposed to how it behaves in sound. I'm not too sure what they meant by that, but I'm pretty sure that the central maximum thing would still hold.