Bozo's queries

[HarveyD]

ah yeah i guess that make sense
just have to judge from the perspective they give i guess

[HarveyD]

Any ideas on how to do these?

[Lasercookie]

Any ideas on how to do these?

For the first one, it's simply right hand grip rule. I'm guessing that the answers will be either out of the page, or into the page (judging from the fact we are asked about inside and outside the loop). Take your hand (with your fingers in the correct position, thumb starts pointing right and your fingers are curled as if it's gripping the wire), and follow the current direction.

A - Out of the page
B - Out of the page
C - Into the page
D - Into the page



For the second one, you have a wave moving into an area of normal air pressure.
Straight off, remember that sound is a longitudinal wave. It can only be left or right.
The first part of the wave to reach the particle is a rarefaction.
A compression pulls particles, and a rarefaction pushes particles.
So if the wave is moving to the right, a compression would pull the particle with it - the particle would move to the right.
However, a rarefaction is the opposite, so it'd push the particle left.

A - Left.

That was an interesting sound question.

[HarveyD]

with 2, just was confused as to why A and D are different
since the current is going in the same direction?

Edit: All your answers are correct btw, thanks!

[Lasercookie]

with 2, just was confused as to why A and D are different
since the current is going in the same direction?

Edit: All your answers are correct btw, thanks!

So the field starts out of the page. It gets to the loop. Once you're out of the loop, the field has effectively been reversededit: this does not make any sense, see my post below.

Like I said, try to follow the wire with your hand. It's kind of like a rollercoaster going round a loop.

Maybe some badly drawn scribble over the diagram will help:

[HarveyD]

ohh okay
never knew that :S
thanks ^^

[Lasercookie]

Wait, my badly drawn diagram explanation doesn't help at all. The field is effectively reversed (bad use of terminology I guess) because A and D are located at different points.

The reason why A and D are different: The field curls around the wire.
A is above the wire. The field is poking out of the page.
D is below the wire. The field at this point is into the page.

Where the 'I' is, the field would be out of the page - the same as A. If you had a point below the wire (directly below A), it would be into the page.

Perhaps thinking about it in three-dimensions will help. Picture an actual wire with a field around it.

[cranberry]

weird questions, must be 2011 trials ay ?

[HarveyD]

nah before 08

[Lasercookie]

weird questions, must be 2011 trials ay ?

Looks like an Insight exam to me.

I didn't think the field question was too odd, just right hand grip rule. My explanations aren't that good, it's much easier to explain right hand grip rule stuff in real life lol.

For the sound stuff:
http://www.physicsclassroom.com/class/sound/
Lesson 1 explains the rarefaction/compression stuff a bit.

I just googled that website, but it looks like it pretty much covers the most of the VCE Physics sound stuff (minus the stuff about microphones and speakers). I might read through all that actually.

[iroflmfao]

Probs a stupid question how you know its a rarefaction

[Lasercookie]

Probs a stupid question how you know its a rarefaction

Well here's the picture of the wave (with my annotations):


Here it's not represented with lines, it's represented with dots.

I assumed the evenly spaced out dots were representing normal air pressure. I think this is reasonable, it does say that Particle X is originally at normal air pressure.

A compression is a point where there's a lot of pressure (compressed pressure if you will?), right? So when the dots are close together, that point is a compression.

When they're spaced out a bit, that's a rarefaction

I liked this diagram:

You might be used to those graphs that look a bit like a sine wave. Those a graphs of change in air pressure vs time. The line at 0 represents normal air pressure usually. As you would know, a compression is a crest (point of higher air pressure), a trough is a rarefaction (point of less air pressure).

If the wave was moving to the right, then the rarefaction dots would be the dots to reach the particle first right?

[iroflmfao]

What if x was on a compression point
what happens then ?

Nothing ?

[Lasercookie]

What if x was on a compression point
what happens then ?

Nothing ?

You mean like this: (sorry about the rough editing )


What's actually happening is interference (since you have waves interacting). If a trough and crest meet, then they destructively interfere and nothing would happen.

What we were getting before was basically "nothing" and a trough - leaving just a trough (point of low pressure) - would you call that interference at all? (if anything, would it be considered constructive interference?).

If you meant that if X was originally at normal pressure and a compression came along, then it should move to the right initially?

Of course, if you had a rarefaction and rarefaction meet up, you'd get constructive interference and up with a point that has an even lower pressure than before.

[iroflmfao]

Yer destructive interference.
Thanks man your so helpful