MC Q14 and Q20 (1 Viewer)

[justing943]

Heya!

I'm wondering about these 2 questions.

In Q14, it says a the induced emf is 'measured.' That would mean that the 'circuit' had to be closed for a measuring device to be inserted. However, as the diagram itself stands, there is no complete circuit and hence no emf around the ring (assuming it's a ring that's like an open coil, not a thick ring), and hence no induced emf, and yet they say 'measure.' What did people respond on this one? :O I personally chose A.

For question 20, did people interpret half as simply mathematically halving the negative potential energy value, hence obtaining a less negative and hence larger value, or interpret half as a decrease in magnitude and hence doubling the negative value? (Not sure about the latter, heard someone discussing it). I took it as a mathematical halving of the value and got D.

Thanks,
jt

 

[taeyang]

The answer was A, eddy currents are still produced in the ring, they are just small ones as a complete one does not work... so... due to lenz's law the current would result in A.

 

[justing943]

Okay, thanks. =)

 

[OmmU]

Q20: If grav Ep is halved, then radius is doubled... So according to F = Gm1m2/d^2 , Grav force is quartered... Hence answer is D .... From this post: http://community.boredofstudies.org/showthread.php?t=270565&page=2


See this thread for full discussion on MC: http://community.boredofstudies.org/showthread.php?t=270565&page=1

 

[socialist]

I do remeber though Q.14, I was aware of the tiny eddy currents that would form however the question said something along the lines of "what would be measured between X and Y" i.e that gap. Wouldn't in that case mean no EMF is measured (D)

And Q.20 I took it as a decrease in magnitude as in closer to the earth, so i got 4x.

"Half" though is such a subjective word, if we take halved as in 'weaker by half' then that means radius decrease and the answer is 4x, but if you mathematically halve a negative you have increased GPE thus 1/4 x. But by doing it mathematically you have just increased the valued by 50% rather decreasing it by 50%. Ambiguous wording is crappy for the non rigour types out there.

 

[taeyang]

I do remeber though Q.14, I was aware of the tiny eddy currents that would form however the question said something along the lines of "what would be measured between X and Y" i.e that gap. Wouldn't in that case mean no EMF is measured (D)

And Q.20 I took it as a decrease in magnitude as in closer to the earth, so i got 4x.

"Half" though is such a subjective word, if we take halved as in 'weaker by half' then that means radius decrease and the answer is 4x, but if you mathematically halve a negative you have increased GPE thus 1/4 x. But by doing it mathematically you have just increased the valued by 50% rather decreasing it by 50%. Ambiguous wording is crappy for the non rigour types out there.

I agree completely, the first approach is what I took.. I think a little bit of order is necessary

 

[khorne]

What are you talking about? The definition of Gpe is work done in moving a object from a point to infinity, so Gpe = -Gm1m2/d
halving gpe is actually increasing it, from the formula, as it is less negative. Thus, if Gpe is increased, then force of attraction decreases. It's really simple.

 

[JamieP]

Question 14 is badly worded and drawn. Question 20 is badly worded.

The word "measure" makes or breaks things in the world of quandum physics, it actually leads to the collapse of the wave function! The word has a profound meaning in question 14 as well - it implies a closed circuit is formed! - therefore an induced emf, not Eddy current, is formed!!!

I consider these two questions big stuffups! Does BOS check their questions at all? Ambiguity in exam papers should not be tolerated!

 

[khorne]

Question 14 is badly worded and drawn. Question 20 is badly worded.

The word "measure" makes or breaks things in the world of quandum physics, it actually leads to the collapse of the wave function! The word has a profound meaning in question 14 as well - it implies a closed circuit is formed! - therefore an induced emf, not Eddy current, is formed!!!

I consider these two questions big stuffups! Does BOS check their questions at all? Ambiguity in exam papers should not be tolerated!

you guys are actually all idiots...

When the U shape goes through the field, an induced EMF flows UNTIL it reaches the circuit end points. Then it stops. There is induced EMF as the electrons in the middle are able to flow due to the force. Thats why the graph is only a spike, then drops to 0, then spikes the other way.

 

[JamieP]

To khorne: read my post CAREFULLY, we are talking PHYSICS here, how much physics do you know?

You are not even wrong!

 

[BadMeetsEvil]

To khorne: read my post CAREFULLY, we are talking PHYSICS here, how much physics do you know?

You are not even wrong!

dude youre contradicting yourself.

 

[JamieP]

Study some real physics and you will know what I mean.

 

[khfreakau]

khorne is right, if you didn't read the question, it specifically said the emf is measured BETWEEN X AND Y. between these two points, there is a buildup of charge, which is effectively a potential difference, which for the purposes of the hsc, is induced emf. hence, as khorne described, the answer is a.

 

[justing943]

If you're measuring a current, unless you're magical and don't need a device to do so, some sort of meter has to be attached to the ring to actually measure the current, and once that's done, the current will be closed. The question was too unclear, and I think they should be accepting multiple answers. A for the people who are looking at the potential difference between X and Y, and another answer (I forget which, I don't have the paper on hand) for the people the realised nothing can be 'measured' without first closing the circuit.

 

[megabakedpotato]

you guys are actually all idiots...

When the U shape goes through the field, an induced EMF flows UNTIL it reaches the circuit end points. Then it stops. There is induced EMF as the electrons in the middle are able to flow due to the force. Thats why the graph is only a spike, then drops to 0, then spikes the other way.

+1

 

[khorne]

If you're measuring a current, unless you're magical and don't need a device to do so, some sort of meter has to be attached to the ring to actually measure the current, and once that's done, the current will be closed. The question was too unclear, and I think they should be accepting multiple answers. A for the people who are looking at the potential difference between X and Y, and another answer (I forget which, I don't have the paper on hand) for the people the realised nothing can be 'measured' without first closing the circuit.

you're being too literal. This is a proof of concept thing. The ring passes through, a potetial difference is set up to oppose the original change in flux, but no current flows and the potential difference is destroy, as it is no complete. As the ring leaves, it sets up an opposite potential difference to oppose the change in flux (this time it was to stay in the field), but again, no path for the current to flow, so it doesn't.

 

[JamieP]

If you haven't come across the concept of "collapse of a wave function"(beyond HSC), you may have trouble understanding the word "measure" - which leads to a closed circuit, hence induced emf, not Eddy current. Of course if you don't "measure", you only get Eddy current.

Conclusion: ill considered question.

For those who don't understand why you are not even wrong, the HSC Q2Q should give you enough hints.

 

[socialist]

If you haven't come across the concept of "collapse of a wave function"(beyond HSC), you may have trouble understanding the word "measure" - which leads to a closed circuit, hence induced emf, not Eddy current. Of course if you don't "measure", you only get Eddy current.

Conclusion: ill considered question.

For those who don't understand why you are not even wrong, the HSC Q2Q should give you enough hints.

+1 bro you are right, because the voltmeter would complete the circuit. Hence putting a voltmeter with the ring would be the only way to measure it but doing that that mean a full ring is made instead of the split ring shown.

I'll reiterate what you said in solidarity to khorne atleast who is thick. Measure is the crux of the problem, to measure you need to close the circuit with the voltmeter, think about it khorne how will you measure a broken circuit where would you attach the voltmeter?

 

[khorne]

+1 bro you are right, because the voltmeter would complete the circuit. Hence putting a voltmeter with the ring would be the only way to measure it but doing that that mean a full ring is made instead of the split ring shown.

I'll reiterate what you said in solidarity to khorne atleast who is thick. Measure is the crux of the problem, to measure you need to close the circuit with the voltmeter, think about it khorne how will you measure a broken circuit where would you attach the voltmeter?

you guys dont know how stupid you sound. Firstly, this is not an applicable event for quantum mechanics, there is not collapse of the wave function. Next, a volt-meter measures voltage potential between two points in parallel. It does not complete the circuit, but merely measures as if it were parallel to the two points. Retards like you shouldn't be allowed to post tbh, but what can you do? Not everyone is smart enough to state rank at least.

 

[BadMeetsEvil]

you guys dont know how stupid you sound. Firstly, this is not an applicable event for quantum mechanics, there is not collapse of the wave function. Next, a volt-meter measures voltage potential between two points in parallel. It does not complete the circuit, but merely measures as if it were parallel to the two points. Retards like you shouldn't be allowed to post tbh, but what can you do? Not everyone is smart enough to state rank at least.

gg