TrueTears question thread

[TrueTears]

LOL that is a shit question. Its almost like they are asking you to state the range and domain of that graph. lemme guess....Lisachem??

it's a fucking joke... god... why is lisaCHEM doing trial exams for physics?

[TrueTears]

The wire is coming out of the page.

Now since AC is used the wire will move up the page and then move down the page and oscillate like this. But the problem is determining the period. Is the period of the wire where it moves up to its furthest from the original position and then come back to original? Or is the period when the wire moves up to the furthest position, return to original and then move downwards to its furthest position and then return to normal?

[NE2000]

As the current is increasing in positive direction (out of the page), the wire is being forced up. The magnitude of this force increases with the magnitude of the current. Then the current will be decreasing but will still be positive. So the force is still up. So the wire reaches its maximum displacement when the current is at T/2 in its AC sinusoidal waveform. Then the current will be negative and the force will reverse, the magnitude of the force will increase and decrease but the wire will continue travelling down. Hence after one period of the AC current, the wire has returned to its original position. It will not go below this position, but will quickly start going up again.

Hence the period of the wire's motion is the same as the period of the current.

[TrueTears]

As the current is increasing in positive direction (out of the page), the wire is being forced up. The magnitude of this force increases with the magnitude of the current. Then the current will be decreasing but will still be positive. So the force is still up. So the wire reaches its maximum displacement when the current is at T/2 in its AC sinusoidal waveform. Then the current will be negative and the force will reverse, the magnitude of the force will increase and decrease but the wire will continue travelling down. Hence after one period of the AC current, the wire has returned to its original position. It will not go below this position, but will quickly start going up again.

Hence the period of the wire's motion is the same as the period of the current.

Yeap thanks for that, but the answer says 20Hz which gives a period of 0.05 s but since it's meant to be the same with the current, shouldn't it be 0.1s?

[/0]

I think I can predict what will happen next... the answers are wrong again

[TrueTears]

I think I can predict what will happen next... the answers are wrong again

:knuppel2: grrrrrr

[kamil9876]

(Bieng a bit naughty with the upper terminal but in my defence this is only physics)

assume





The big coefficient at the front is a constant, while the term in parenthesis is ALWAYS negative. Therefore the sign of the velocity should never change and so it shouldn't oscillate me thinks. I had my doubts about this question from just a simple intuitive argument but I guess I included this since the three ppl invovled all do spec. The intuitive argument is that even though force reverses, velocity does not neccesarily, in fact the force(when reversed) does most of it's work slowing the object down to 0 and then once at zero the force reverses back to positive, hence speeding the object back up. 

This all gives me doubts about this question, only possible resolution is that B isn't constant (weaker as the thing moves up) or the current is NOT sinusoidal. 

[TrueTears]

(Bieng a bit naughty with the upper terminal but in my defence this is only physics)

assume





The big coefficient at the front is a constant, while the term in parenthesis is ALWAYS negative. Therefore the sign of the velocity should never change and so it shouldn't oscillate me thinks. I had my doubts about this question from just a simple intuitive argument but I guess I included this since the three ppl invovled all do spec. The intuitive argument is that even though force reverses, velocity does not neccesarily, in fact the force(when reversed) does most of it's work slowing the object down to 0 and then once at zero the force reverses back to positive, hence speeding the object back up. 

This all gives me doubts about this question, only possible resolution is that B isn't constant (weaker as the thing moves up) or the current is NOT sinusoidal. 

Wow thanks there, so what's the period?

[kamil9876]

Whatever the answers say of course.

Should be the same as the current frequency, since if you antidiffirentiate the speed once again you get a term of the form. Hence just like the current.

Actually, I think the problem I adressed wouldn't be a problem unless the initial speed WAS NOT zero, but the maximum speed such that the -1 term cancels.

[/0]

Didn't you say it wouldn't oscillate?

[kamil9876]

Actually, I think the problem I adressed wouldn't be a problem unless the initial speed WAS NOT zero, but the maximum speed such that the -1 term cancels.

otherwise it wouldn't osscilate. Just realised that. So there is a chance it is osccilating and in the case it does, the term is the same hence same frequency as current.

[NE2000]

(Bieng a bit naughty with the upper terminal but in my defence this is only physics)

assume





The big coefficient at the front is a constant, while the term in parenthesis is ALWAYS negative. Therefore the sign of the velocity should never change and so it shouldn't oscillate me thinks. I had my doubts about this question from just a simple intuitive argument but I guess I included this since the three ppl invovled all do spec. The intuitive argument is that even though force reverses, velocity does not neccesarily, in fact the force(when reversed) does most of it's work slowing the object down to 0 and then once at zero the force reverses back to positive, hence speeding the object back up. 

This all gives me doubts about this question, only possible resolution is that B isn't constant (weaker as the thing moves up) or the current is NOT sinusoidal. 

Aah, didn't think of that...forgetting Unit 3 motion (concept of force, acceleration, velocity etc.) amidst all this magnetism lol. Thanks.

Practically, if the current is initially running out of the page then the wire will move up and continue moving up until it is out of the significant field of the magnet , but if we assume that the magnet is infinite width then the period ought to be the time for the wire to come to rest again after moving upwards right? And that should be the period of the current in the wire.

But that's not really oscillation is it, what am I missing here?

[kamil9876]

I did my integral under the assumption that the initial velocity was 0. If instead it was then it would be:



and notice that iff  then the -1 term cancels out and so displacement in this case would just be some sinusoidal function. Wheras if the -1 term didn't cancel then the function would be of the form   and the linear part shows that as gets big the thing just flies away from the vicinity

[TrueTears]

I did my integral under the assumption that the initial velocity was 0. If instead it was then it would be:



and notice that iff  then the -1 term cancels out and so displacement in this case would just be some sinusoidal function. Wheras if the -1 term didn't cancel then the function would be of the form   and the linear part shows that as gets big the thing just flies away from the vicinity

Right, and your point is? It doesn't oscillate or does it?

[TrueTears]

So what's the difference between a incandescent light source (such as a globe) and a sodium lamp.

I said that incandescent light source emits light due to thermal vibration and thus has a continuous spectrum, so it emits light of different wavelengths. However a sodium lamp emits light of discrete wavelengths since electrons can only jump certain shells and thus emitting light with the difference in energy between 2 shells.

However what I don't understand is why doesn't a globe emit discrete wavelength of light? I know it emits a spectrum of wavelengths but doesn't each light that make up the spectrum have a discrete wavelength?

Thanks!