Bozo's queries

[Lasercookie]

Thanks man your so helpful

No problem dude, I'm glad to try and help.

[iroflmfao]

Q 5 of  2007 VCAA of light and matter

2/3 < 1
So i wrote little or no diffraction
Is that the same as > There would be a diffraction pattern because the wavelength is of the same order of magnitude as the interatomic
spacing. 

[Lasercookie]

Q 5 of  2007 VCAA of light and matter

2/3 < 1
So i wrote little or no diffraction
Is that the same as > There would be a diffraction pattern because the wavelength is of the same order of magnitude as the interatomic
spacing. 

No it's not the same. You wrote 'no diffraction', while that states there is diffraction.

There is always diffraction (even if it is below or above 1), what varies is how significant/noticeable it is.
I would most definitely avoid the use of the term 'no diffraction will occur'.

That ratio wavelength/slit width is for the 'extent of diffraction'. If wavelength is larger than the slit width then you'd get a lot of diffraction. If the wavelength is smaller than the slit width and you get a low level of diffraction. But there is still diffraction. 

I'd tend to write something like that diffraction would occur because of the order of magnitude stuff, but due the wavelength being smaller than the slit width a weak pattern may be observed.

The Australian Institute of Physics gave two possible answers in their solutions:

Yes (1), the wavelength is comparable to the interatomic spacing (1). 
The answer ‘No’ (1)  could be argued for by saying the wavelength is smaller than the spacing and little diffraction will be noticed (1).

So you are correct going by the AIP. If the VCAA would accept the answer, I don't know.

edit: fixed wording

[HarveyD]

is there any difference between sound and light regarding that^

[Lasercookie]

is there any difference between sound and light regarding that^

I saw that on the report, I am not sure what they were talking about though.

The high and low frequency stuff for sound is just derived from that same ratio:
high frequency = smaller wavelength = less diffraction
low frequency = larger wavelength = more diffraction

Was there anything else particular about sound diffraction?

[HarveyD]

hmm not sure
i normally assume that when its above one theres more diffraction though

but if you were given two options
1) Almost 1
2) Greater than 1

With both sound and light, would you choose 2 for more significant diffraction?

[Lasercookie]

hmm not sure
i normally assume that when its above one theres more diffraction though

but if you were given two options
1) Almost 1
2) Greater than 1

With both sound and light, would you choose 2 for more significant diffraction?

Yeah I assume that as well, I would choose 2. Larger than 1 means that extent of diffraction is greater blah blah blah
Below 1 and it's less significant. But it's still there.

Reading what VCAA wrote again:

Many students confused the diffraction requirements from the Detailed study ‘Sound’ with what is necessary to form a useful diffraction pattern in this topic.

What are the definitions for useful diffraction pattern? Are they saying that the other AIP option is incorrect?

0.66 is still some level of diffraction I guess. Like obviously it wouldn't be a lot of diffraction, but you'd still notice it. Like in water waves, if the wavelength is smaller, you can still sort of see the circular bits at the edge. With light, you'd probably notice a blurred edge as well.



I don't believe that diffraction is vastly different for sound. I guess in sound we really only deal with where is the sound going and where will it be heard (e.g. you have to be in front of the source to hear high frequency sounds the best). Or it's role in loudspeakers etc. We don't consider the fact that it is still being slightly diffracted - really only given the two extremes. Perhaps this is what VCAA was referring to.

[iroflmfao]

Hmm so i should avoid using no diffraction, even though its there almost virtually nothing.

[Plan-B]

is there any difference between sound and light regarding that^

I believe that a lambda/slit width ratio of around 0.5 or greater is already significant for sound. Where as for light, it is only significant once it approaches or is greater than one. Don't take my exact word for it though, but I remember something like that, unless i'm wrong. :s

Another question:

http://images.tutorvista.com/content/feed/tvcs/series20parallel20resistor20circuits.JPG

Just found a simple parallel circuit. Would you say that current through R3, is from B to C, or C to B? Many of these practice exams have many inconsistencies, so I was wondering how you guys do it.
Although it's not so bad since VCAA makes interpretation explicit when they ask for direction.
Just wanted to clear it up just in case.

[Lasercookie]

Another question:

http://images.tutorvista.com/content/feed/tvcs/series20parallel20resistor20circuits.JPG

Just found a simple parallel circuit. Would you say that current through R3, is from B to C, or C to B? Many of these practice exams have many inconsistencies, so I was wondering how you guys do it.
Although it's not so bad since VCAA makes interpretation explicit when they ask for direction.
Just wanted to clear it up just in case.

We use conventional current, so it's the flow of positive charge. (so starts from positive end of the battery). The positive end of the battery is the long end.

In that picture, just use the arrows on the wire? So it'd be B to C. You can't really tell since the symbol of the battery isn't given.

http://images.tutorvista.com/content/electricity/parallel-resistors-energised-battery.jpeg
So here, current is A to B. (again, the arrows are there, but they give the battery symbol this time)

If it's in a generator/alternator, then current direction will be given by the right hand rules. With AC, the current will reverse according to the frequency e.g. 50Hz AC will change 50 times a second.

If you want more diagrams, I googled and found this website:
http://www.allaboutcircuits.com/vol_2/chpt_1/1.html
It seems to cover most of the circuit stuff we do in Unit 4.

[Plan-B]

Thanks, just wanted to make sure I was on the same page as you guys.

[HarveyD]

hey laserdd

heres a similar question I found similar to the magnetic field question
try your method and check if it works here

[Lasercookie]

hey laserdd

heres a similar question I found similar to the magnetic field question
try your method and check if it works here

Was this about this thread: TSFX '08 ?

That question is a slightly different situation actually. You have a current carrying wire generating the magnetic field. The field generated by this current will circle around it - I don't think the field in the TSFX 2008 question would curl around like that, as that was an external field.

Anyway, I would just use the regular right hand grip rule (fingers curled and all). The current is facing right, so your thumb points out right. Your fingers grip the wire.

Above the wire your fingers are pointing out of the page (so the little dots). If you extend that in your head, it would be point into the page (the crosses) below the wire.

This means it can be either A or D. The magnetic field in D is uniform. Since the field is due to the current-carrying wire, it'd get weaker as you move out from it.

So I'd say the answer is A.

Is that correct?

[HarveyD]

nah the loop one i posted before
but yeah that's correct

so above the wire, the direction will be the same as what's indicated by the right-hand grip rule, while below it, it will be the opposite?

[Lasercookie]

nah the loop one i posted before
but yeah that's correct

so above the wire, the direction will be the same as what's indicated by the right-hand grip rule, while below it, it will be the opposite?

Yeah, that was the case with this one. I'm not going to say that it'll always be opposite, since a situation might came up where it is isn't. It's easier for me to just think of it in terms of the RH Grip rule.

I tend to think of it as just imagining your fingers are longer. If your fingers are curled, and if they were longer they'd end up forming a full circle around the wire, right?

Here's another diagram (you can probably tell I like diagrams):


That other "method" (this post: Re: Bozo's queries) was just me screwing up my explanation, I wasn't thinking straight. My other two posts about that question is pretty much the same thing I did with this question.