/0's physics phread

[TrueTears]

Why must it be alternating? I thought the diagram could represent DC motor or AC/DC generator [since it doesn't show slip rings or split ring commutator]. But DC motor uses DC current and DC generator is pulsating DC current, so it may not be alternating? (even though the commutator reverses the current in a DC motor, the +ve and -ve terminals don't switch.)

The current in the coil must always be alternating (every half turn) to maintain the torque in the same direction. This is the purpose of a split-ring commutator, to alternate the current inside the coil.

Ahh yeap, thanks that's what I meant

[/0]

thx

also in VCAA 2003, AOS 2 Question 10  http://www.vcaa.vic.edu.au/vce/studies/physics/pastexams/physics12003.pdf or  Checkpoints Q108

Why isn't C a possible option alongside B?
Designating N to S as positive shouldn't really make a difference should it? After all, isn't that designation independent of the wire's position? I thought starting with the flux as positive is pretty arbitrary.

[TrueTears]

Ahhh I remember this tricky fellow

Now, we know the current-time graph has same frequency and looks the same as the emf-time graph cept just with different amplitude.

Now, the graph is a bit tricky in checkpoints but the exam paper makes it much clearer.

The "bottom" wire is connected to the last slip ring and the "top" wire is connected to the first slip ring. (the one "closer" to X)

Now lets use Lenz Law on the coil.

As the coil rotates clockwise, flux gets smaller, so change in flux is negative hence its from S to N direction, so induced mag field must be from N to S, use RHG rule and we see current goes from X to W to V to U.

BUT! the question wants current from U to V. So normally you would have A as your answer right [because that represents V to U NOT U to V]? But this time we want the - (-derivative) of the flux-time graph. Hence we just need the "derivative" graph, which is clearly D.

[/0]

err thansk but that's 109

[TrueTears]

Oh shit, soz I thought you said Q 109 LOL.

Anyways for Q 108, C is clearly wrong because it says N to S is positive.

Yes direction for change in flux is arbitrary, just plain flux direction is also arbitrary, but clearly if it says N to S is positive it means the first "face" that the mag field hits yields a POSITIVE flux reading.

[Hard to explain just by typing but imagine the square coil with 2 "faces" the diagram depicts the mag field first hitting the "left face" and then passing through to the "right face", clearly if N to S is positive, the flux would be positive for the "left face" because that what the mag field hits first and as a result the "right face" is negative]

C would depict the flux-time graph if N to S was NEGATIVE. Ie, hitting the "right face" first.

[/0]

ah ok thx TT, thought that was a bit weird

[TrueTears]

ah ok thx TT, thought that was a bit weird

Weird about what?

[kamil9876]

[Hard to explain just by typing but imagine the square coil with 2 "faces" the diagram depicts the mag field first hitting the "left face" and then passing through to the "right face", clearly if N to S is positive, the flux would be positive for the "left face" because that what the mag field hits first and as a result the "right face" is negative]

That is correct.

I found that concept far-fetched back in my day but the two faces makes sense if say you pick some rotating reference frame where the coil is stationary and the magnets rotate. That way you know why it is a useful convention to "negativize"(is that even a word?) flux when going through the other face. It's because then the magnets swap(due to rotation) and the flux changes direction and so does current and so the flux to emf relationship holds and you don't have to worry about direction.

Stick to the "two face" concept since it's less confusing in problem solving. But i found the above reasoning helped in adding rigour, being convinced and appreciating the usefulness of the idea.

edit: poor expression and english skills

[TrueTears]

[Hard to explain just by typing but imagine the square coil with 2 "faces" the diagram depicts the mag field first hitting the "left face" and then passing through to the "right face", clearly if N to S is positive, the flux would be positive for the "left face" because that what the mag field hits first and as a result the "right face" is negative]

That is correct.

I found that concept far-fetched back in my day but the two faces makes sense if say you pick some rotating reference frame where the coil is stationary and the magnets rotate. That way you know why it is a useful convention to "negativize"(is that even a word?) flux when going through the other face. It's because then the magnets swap(due to rotation) and the flux changes direction and so does current and so the flux to emf relationship holds and you don't have to worry about direction.

Stick to the "two face" concept since it's less confusing in problem solving. But i found the above reasoning helped in adding rigour, being convinced and appreciating the usefulness of the idea.

edit: poor expression and english skills

Asif poor expression, you couldn't have said it any better

[kamil9876]

after editing

I was embarrased by spelling "over" instead of "other". Also I thought the idea may be confusing.

[/0]

I understand the two-face concept of the coil, that you can assign 'positive' and 'negative' sides to it. However, say you have a stationary coil with rotating magnets as you described. The stationary coil is lying horizontal and initially the north pole of the magnet is directly above it.
As the magnet rotates around the coil, the sign of the flux when the N pole is above the coil will be different to the sign in flux when the N pole is under the coil. However, without defining the 'sign' of the faces of the coil, we can't decide in which configuration the flux will be positive or negative, right?
Even if we say "let N to S be positive", that doesn't help us because what matters are the 'signs' of the actual face.

... at least, that's what I reckon

[TrueTears]

I understand the two-face concept of the coil, that you can assign 'positive' and 'negative' sides to it. However, say you have a stationary coil with rotating magnets as you described. The stationary coil is lying horizontal and initially the north pole of the magnet is directly above it.
As the magnet rotates around the coil, the sign of the flux when the N pole is above the coil will be different to the sign in flux when the N pole is under the coil. However, without defining the 'sign' of the faces of the coil, we can't decide in which configuration the flux will be positive or negative, right?
Even if we say "let N to S be positive", that doesn't help us because what matters are the 'signs' of the actual face.

... at least, that's what I reckon

It does define what's positive or not.

Think about it, if N to S is positive then the "left face" to "right face" is positive direction and vice versa is negative.

[kamil9876]

by saying N to S always positive it still makes calculating emf unambigous. u get the same answer (ie: ABCD or DCBA). And i think this is a cool way of thinking about it because it marries what you learnt previously (the fundamentals) without having to introduce some new "face law". (I used it to justify the "face law")

Say it's how u described it, N on top. N to S is positive, hence the flux is down. So once magnet turns 180degrees the flux is up and so the other face is getting hit first, which is the same as the physical situation. We don't have to even look at which face is getting hit first in order to work out emf using this.

Refer back to ussual questions such as, "coil removed from field" and notice the parralels. In fact in such questions you don't worry about faces and it still works.

edit: bahhaa I accidentally talked about keeping magnet stationary  :idiot2:

[TrueTears]

Say it's how u described it, N on top. N to S is positive, hence the flux is a constant down. So once the coil spins 180degrees the other face is getting hit first, analogous to the conventional, physical situation.

That's right hence it is the opposite sign to what you have defined.

[kamil9876]

ok so i thought about it a bit:

So N is on top initially and we spin the N magnet anticlockwise(assume constant one radian per second). It can be shown that emf at time is equal in magnitidue but opposite to direction to emf at time . This can be done without any sign conventions, just the plain use of lenze's law(just like coil removed from field problems). However introducing the sign convention that say, flux going down is positive, we see that flux changes sign and using this definition of it can be shown that . And so you see it is analogous to what we found out that really happens using just lenze's law without any sign convention. Hence the sign conventions "positive flux is one whose vertial component is down, negative flux is one whose vertical component is up" and "negative emf means in the oppositie direction" coupled with the formula relating flux and emf, yields the correct results that we know are correct from lenze's law.

Interestingly, Once we choose which direction is positive flux and abide the formula we automatically choose which direction in the coil is positive emf. If we change our convention of what is negative flux (i.e: ) then we change our direction of what we consider as positive emf. However what is important is the sign changes every . Hence these sign conventions and use of formula are consistent with what we know is really true(what we derived without thinking about sign conventions but just ordinary hand rules)