TSFX '08

[BakaNeko]

HELLLOOO

Has anyone done TSFX '08? 

How do you do AOS1, Q4?
It has a plane and the field is going through the plane from the bottom. And then it wants to know which wing is positive.

Uh yeah, help please

[Lasercookie]

Ooh, another year 11 that's doing physics

Well, let's pretend the wing is a wire. We have a field and we have a current induced.
We're talking about directions. That should ring alarm bells to use the right hand rule.

Conventional current is the flow of positive charge. So, if our thumb represents the direction of current, that means it the current starts off from the positive end. You have your fingers pointing up. This leaves your thumb pointing to the left.

I think I need a diagram to ensure I explain this well enough.



The start of the arrow is the start of the positive charge's journey, so that's the positive end.

[Lasercookie]

Oh wait... I neglected something very important: Lenz's law. That should mean that the flux induced would be opposite. What I do next, I'm not sure is entirely correct (but it is in line with a similar question from STAV 2010).

However, note that since the field is pointing directly up, we end up using basically what is the RH Slap rule, but ignoring the palm. Due to Lenz's law, the field points down. This leaves current still pointing left, the right wing stays positive and all is well.

(if we were talking forces, then that would have been reversed - that's something we ignore though, it has nothing to do with it)

[BakaNeko]

Yep, cool physics kids are cool

Wait, but to use the right hand slap rule, you need the palm (so you need the direction of the force). If the force isn't out of the page/screen then your current will be in the opposite direction.

Unless you don't need the force? O__O"

But you are correct XD The answer is the right wing but if you assumed the other direction for the force, you would have it the wrong way around...

[Lasercookie]

Yep, cool physics kids are cool

Wait, but to use the right hand slap rule, you need the palm (so you need the direction of the force). If the force isn't out of the page/screen then your current will be in the opposite direction.

Unless you don't need the force? O__O"

But you are correct XD The answer is the right wing but if you assumed the other direction for the force, you would have it the wrong way around...

I didn't assume the other direction for the force (I ignored it). I assumed the other direction for the field. Be sure not to confuse the two.

Lenz's Law must be accounted for. We have a field inducing a current (Faraday's Law). To ensure that conservation of energy is obeyed, we use Lenz's law.

My reasoning was that you can use the right hand grip rule, where you have your fingers curled. However, the field is external - it wouldn't curl around. It'd just stay pointing directly up.  So your fingers are pointed flat, directly up - just like it is in the right hand slap rule. I really just mentioned the RH Slap just to give a better idea of what I was trying convey.

You might have noticed the current direction doesn't actually change (but the force does). Since this is the case, the answer stays the same - right wing (field points down, thumb still points left and palm is in opp. direction). Only this time, we accounted for Lenz's law.

Though do remember that a force actually does occur on the electrons. 10.2, page 363, second paragraph of the Heinemann book explains it nicely. Take note that the qvb force is the magnetic force. There's more to it, but it's way beyond our VCE physics level.