Bazza's 3/4 Physics question thread

[Lasercookie]

It looks to me that the area part is the dodgy part, like what's up with that 15cm arrow. They've only taken a segment of that magnetic field and used that to calculate the EMF?

[Aurelian]

i've never seen a question like it before D:

yeah answer is 0.68; answers do something dodgy
(which i do not understand)
E=faraydaylenzlaw = -3x (30x10^-3 x 0.15 x 0.30)/(6x10^-3) = 0.68 V

no idea whats going on

thanks for the help

Yeah okay so it's basically just a really badly worded question. My train of thought was this:

1) The set-up is designed to light up a light bulb, so the light bulb has to be in the 'circuit' somewhere
2) The only probable places for it to exist are across the big metal railings, either on the left or the right
3) Since you're given the length for the distance from the LHS of the set up to the 3 metal rods, so I figured they must mean for the light bulb to be on the left.
4) Now we have a close loop through which we can determine a flux. The area of this loop is (0.15m)(0.30m) = 0.045m^2. So the initial flux is given by (30x10^-3T)(0.15m)(0.30m) = 0.00135Wb. Since the flux decreases to zero, this also represents the change in flux. The change in time is 6*10^-3s, so the induced emf is given by (0.00135Wb)/(0.006s) = 0.225V.

But there are effectively "three" loops, so the overall induced emf is given by (3)(0.225V) = 0.675V = 0.68V.

I presume that's their reasoning.

As an aside, I'm not entirely sure the "3 loops" fact has the effect that they seem to thing it does. If the rods are metal and conducting, and are bundled together (i.e. they are touching), and the rest of the railing is furthermore shared between the loops, then we're actually really just dealing with one loop.

All in all, a pretty poor question.

[WhoTookMyUsername]

Thanks for the help

Yeah i thought there must have been something dodgy lol xD

...
I'm utterly confused about whether electrons can only absorb "discrete" amounts of energy (ie. only specific wavelengths of light)
Some places seem to say can only absorb a photon if the photon's energy is exactly equal to the difference between electron energy levels (shells)
but other places say doesn't have to be exact, the extra energy can be given off as heat etc. or absorbed as increased kinetic energy

Not sure which is correct! please help
thanks!

[WhoTookMyUsername]

In other words:

If the difference  between shell one and shell two is 11 joules
And 12 joules are supplied, can the electron jump from shell one to shell two?
If this energy is supplied in the form of a photon, can 1 joule be dispelled as heat?

Thanks!

[Aurelian]

Electrons can only absorb discrete amounts of energy if they are in a bound system, such as an atom. You can view this as being because there are boundary restrictions on what (de Broglie) wavelengths will form standing waves. This corresponds to discrete, quantized energy levels.

A free electron, on the other hand, can have any value of energy - i.e., a continuous distribution.

As for your specific question in the second post, the answer is no. If the difference between two energy levels required 11J of energy, and you provided 12J, the transition would not occur. Photons can only be absorbed all-at-once; you could not absorb 11J worth of the photon's energy and dispel 1J as 'heat'. (It's worth considering what 'heat' would even mean in this instance; invariably it would have to be in the form electromagnetic radiation - that is another photon. You can see in this way that you really would have 'split' the photon).

Hope that helps!

EDIT: Actually, something else which will probably help your understanding. Consider the above situation once again, but replace the photon being absorbed with a free electron colliding with a bound electron. In this situation, because free electrons do not have to impart all their energy at once, you could indeed cause a transition to occur with a 12J electron hitting the bound electron. In this case, the 1J left over would just be retained in the free electron's kinetic energy.

So I guess the fuller answer to your question is "it depends on how the excitation occurs".

[WhoTookMyUsername]

What is more important for a 'best' frequency response curve, linearity or "flatness"

(See attached question, given answer is B, i put D)

Thanks!

[paulsterio]

I honestly think it's where the curve is highest