HSC Physics Marathon 2016 (4 Viewers)

[leehuan]

Yeah that would make sense that the main similarity is the eddy currents. I guess I went onto the more specifics, thinking how heat is produced in the induction cooktop and therefore somehow the teacher might want us to talk about heat and eddy currents with magnet/copper pipe.

The heat when produced from it falling down the copper pipe is just wasted energy. The heat energy from the induction cooktop is actually put to good use.

It was bloody 6 marks in the HSC 2011 lol

I wonder how I answered that question when I was studying then, cause that's just ridiculous. Can't see it any more than 4

 

[Nailgun]

Hey just a quick question about something I wrote in the exam:

Would it be correct if I wrote that when a magnet is dropped through a copper pipe, the eddy currents (... and here I state that its from Lenz's Law) will make the magnet fall slower and kinetic energy is converted into heat? The question was basically asking to compare the magnet falling in a copper pipe, to the induction cooktop. (So here it is somewhat similar that they both produce heat but in different ways).

Thanks

Not entirely sure here but from my understanding (Could be wrong lelele):
I see what you are trying to say ahah and you are sort of correct but the logical reasoning you have is a bit disjointed

When the magnet drops through the copper pipe, the pipe will experience a changing magnetic flux and hence according to Faraday's Law an emf proportional to the rate of change of magnetic flux will be induced into the copper pipe, generating eddy currents in the pipe. According to Lenz's law these eddy currents will produce a magnetic field which will directly counteract the original change in magnetic flux that produced them. So in this scenario, the magnet will fall slower due to the repulsive force of the induced magnetic field. Law of Conservation of energy says the net energy change of a system must be 0, and hence the kinetic energy loss must be a gain elsewhere. The loss of kinetic energy is equivalent to the gain in electrical energy in the conductor (which is proportional to the heat generated in the conductor). So while the kinetic energy is basically converted into heat in the end, it is not anywhere near a 1:1 conversion as your answer would imply. If I was going down the same path of reasoning as you (pipe and cooktop both generate heat and both use eddy currents) I would probably link it together using the electrical energy induced in the copper pipe.

 

[leehuan]

Not entirely sure here but from my understanding (Could be wrong lelele):
I see what you are trying to say ahah and you are sort of correct but the logical reasoning you have is a bit disjointed

When the magnet drops through the copper pipe, the pipe will experience a changing magnetic flux and hence according to Faraday's Law an emf proportional to the rate of change of magnetic flux will be induced into the copper pipe, generating eddy currents in the pipe. According to Lenz's law these eddy currents will produce a magnetic field which will directly counteract the original change in magnetic flux that produced them. So in this scenario, the magnet will fall slower due to the repulsive force of the induced magnetic field. Law of Conservation of energy says the net energy change of a system must be 0, and hence the kinetic energy loss must be a gain elsewhere. The loss of kinetic energy is equivalent to the gain in electrical energy in the conductor (which is proportional to the heat generated in the conductor). So while the kinetic energy is basically converted into heat in the end, it is not anywhere near a 1:1 conversion as your answer would imply. If I was going down the same path of reasoning as you (pipe and cooktop both generate heat and both use eddy currents) I would probably link it together using the electrical energy induced in the copper pipe.

Wait. Did we actually imply that all the Ek got converted to heat?

 

[Nailgun]

Not entirely sure here but from my understanding (Could be wrong lelele):
I see what you are trying to say ahah and you are sort of correct but the logical reasoning you have is a bit disjointed

When the magnet drops through the copper pipe, the pipe will experience a changing magnetic flux and hence according to Faraday's Law an emf proportional to the rate of change of magnetic flux will be induced into the copper pipe, generating eddy currents in the pipe. According to Lenz's law these eddy currents will produce a magnetic field which will directly counteract the original change in magnetic flux that produced them. So in this scenario, the magnet will fall slower due to the repulsive force of the induced magnetic field. Law of Conservation of energy says the net energy change of a system must be 0, and hence the kinetic energy loss must be a gain elsewhere. The loss of kinetic energy is equivalent to the gain in electrical energy in the conductor (which is proportional to the heat generated in the conductor). So while the kinetic energy is basically converted into heat in the end, it is not anywhere near a 1:1 conversion as your answer would imply. If I was going down the same path of reasoning as you (pipe and cooktop both generate heat and both use eddy currents) I would probably link it together using the electrical energy induced in the copper pipe.

Also if this was a compare question you really really don't need to go into this much detail, nor would I actually use this comparison

Similarities
- Both utilize eddy currents in order to do work, as there is a change in kinetic energy in the slowing down of the magnet, and there is a change in heat in the cooktop (Work equals change in energy)
- Both utilise the principle of Faraday's Law, where a change in magnetic flux is induces an emf - in the magnet this is a result of it falling, causing the pipe to experience a changing magnetic flux, whereas in the cooktop this is a result of the AC current in the solenoid causing the cooktop to experience a changing magnetic flux

Differences
- The application is different, as one slows the magnet down while the other is used to heat things
- One utilities a permanent magnet and a change in the relative motion between the conductor and the magnet to produce a changing magnetic flux, whereas the utilizes an electromagnet and AC to produce a changing magnetic flux.

 

[Nailgun]

Wait. Did we actually imply that all the Ek got converted to heat?

I was going off of this lelele

When a magnetic field moves through a conductor a current called an Eddy current is induced in the conductor due to the magnetic field’s movement. The flow of electrons in the conductor creates an opposing magnetic field to the magnet which results in damping of the magnet and causes heating inside of the conductor similar to heat buildup inside of power cords. The loss of energy used to heat up the conductor is equal to the loss of kinetic energy by the magnet.

 

[Jeff_]

Also if this was a compare question you really really don't need to go into this much detail, nor would I actually use this comparison

Similarities
- Both utilize eddy currents in order to do work, as there is a change in kinetic energy in the slowing down of the magnet, and there is a change in heat in the cooktop (Work equals change in energy)
- Both utilise the principle of Faraday's Law, where a change in magnetic flux is induces an emf - in the magnet this is a result of it falling, causing the pipe to experience a changing magnetic flux, whereas in the cooktop this is a result of the AC current in the solenoid causing the cooktop to experience a changing magnetic flux

Differences
- The application is different, as one slows the magnet down while the other is used to heat things
- One utilities a permanent magnet and a change in the relative motion between the conductor and the magnet to produce a changing magnetic flux, whereas the utilizes an electromagnet and AC to produce a changing magnetic flux.

Oh yeah, that would definitely have been a much better answer for the comparison question haha. Definitely will learn for next times

 

[Nailgun]

Oh yeah, that would definitely have been a much better answer for the comparison question haha. Definitely will learn for next times

lelele generally speaking if it wants you to compare legit just put the heading similarities and number them 1. 2. (and 3. depending on how many marks) and do the same with contrasts

easy marks tbh, they don't expect you to write more than two or three sentences for each one and they like it when they can just tick them off
(plus in this form even crappy similarities and differences get marks lele)

 

[Jeff_]

lelele generally speaking if it wants you to compare legit just put the heading similarities and number them 1. 2. (and 3. depending on how many marks) and do the same with contrasts

easy marks tbh, they don't expect you to write more than two or three sentences for each one and they like it when they can just tick them off
(plus in this form even crappy similarities and differences get marks lele)

Yeah damn should have definitely done that lol. Thanks

 

[astroman]

a Cylinder of aluminium has a diameter of 13 cm and the height that is twice the diameter. If the density of aluminium is 2.70 g/cm³, what is the mass of the cylinder in kg?

 

[Nailgun]

a Cylinder of aluminium has a diameter of 13 cm and the height that is twice the diameter. If the density of aluminium is 2.70 g/cm³, what is the mass of the cylinder in kg?

This is a maths question isn't it lol?

 

[astroman]

This is a maths question isn't it lol?

nah, its uni revision questions from physics, its basic formula, i think its 37.3kg.

 

[Nailgun]

nah, its uni revision questions from physics, its basic formula, i think its 37.3kg.

 

[Bestintheworld]

How fast must a pion be travelling to travel 12.0 m before it decays?
The average lifetime, at rest, of a pion is 2.6*10^-8 sec.

Can someone do this asap please?

 

[Nailgun]

How fast must a pion be travelling to travel 12.0 m before it decays?
The average lifetime, at rest, of a pion is 2.6*10^-8 sec.

Can someone do this asap please?

dayum nice question
gimme a sec

 

[Bestintheworld]

dayum nice question
gimme a sec

you seem to love Physics beyond any margin no question is a nice question

 

[Nailgun]

sorry latex took awhile + was checking over it lel

 

[Bestintheworld]

sorry latex took awhile + was checking over it lel

Just as I finished mine too, I got my answer correct as well although I did it on paper.

 

[leehuan]

Not sure what the next question is anymore so here's one for the people working ahead in physics:

a) Explain why there was massive debate over the nature of the cathode ray as a wave or a particle. (4)
b) A cathode ray particle is projected at a velocity of 5.0*10^6 ms^-1 into a magnetic field. Calculate the radius of orbit that the particle will undergo, if the magnetic flux density is 20T. (3)
c) Without a secondary calculation would happen if the magnetic flux density was reduced to 10T? (1) (hint: you can still use the formula)

 

[Jeff_]

Quick question:

There's a question that needs to use the Earth's orbital period.

I used the orbital period of 23h56mins (converted to seconds), however the sample answer used 24h.

Would I lose a mark? Because Earth's orbital period is actually 23h56mins?

 

[RachelGreen]

I like that you're exact, but just use 24 hours. It's so much easier to convert 24 hours into seconds compared to what you got there. 24 hours is fine, I don't think you will lose a mark