6,262 reputation
1943
bio website jfitzsimons.org
location Singapore, Singapore
age 32
visits member for 4 years, 1 month
seen Nov 17 at 5:14

I have just moved to the Center for Quantum Technologies in Singapore, after spending the last 3 years as a Merton College JRF in Theoretical Physics and a Senior Research Fellow in Oxford University Department of Materials. My research focuses largely on theoretical aspects of quantum information processing. In particular I am interested in spin networks, measurement based computation, cryptography and computational complexity.


Jan
22
comment Why does a ballerina speed up when she pulls in her arms?
@Sklivvz: She needs to apply a constant force to keep the arms in the new position, yes, but this doesn't mean that pulling in harder will make her spin faster, her arms reach a new position and to keep them there there is only one value of F she can apply. The harder she pulls in initially the faster her arms reach that new position, but in the end the force has to be the same.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Sklivvz: No, its not. It's due to the fact that the Gibbs states for a gas in a gravitational potential are different to those for a gas free from gravity.
Jan
22
comment Why does a ballerina speed up when she pulls in her arms?
@Sklivvz: This isn't a personal attack, it simply that the line "This is due to the fact that the spin velocity is a function of how hard you pull your arms in." is false, and I was pointing that out. It's an unfortunate coincidence that I have problems with your other question in parallel. I've been trying to raise the quality of the answers by flagging errors.
Jan
22
comment Can gravitational potential energy be released in a fire?
To explain further, imagine there was no atmosphere at all, and you add gas either at sea level or at a much higher altitude. The gas has to settle into a pressure gradient in both cases, but in the latter this involves molecules falling on average, where as in the latter some will need to rise.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Sklivvz: No, it doesn't. The molecules literally fall (on average).
Jan
22
comment Why does a ballerina speed up when she pulls in her arms?
It has nothing to do with how hard she pulls in her arms (although it is related to the work done). If she pulls harder they come in faster, but her total change in angular velocity is the same.
Jan
22
comment Can gravitational potential energy be released in a fire?
The mass does not remain in the same place if there is gas produced. In that case some of the mass becomes gas, and the gas then falls into equilibrium.
Jan
22
revised Can gravitational potential energy be released in a fire?
added 58 characters in body
Jan
22
answered Can gravitational potential energy be released in a fire?
Jan
22
comment Can gravitational potential energy be released in a fire?
@Georg: Chemical potential is, as Noldorin says, a widely accepted term, and one you will find in many text books (although I suspect these will be at the more introductory level). While you are indeed correct that it is a form of potential energy, it is often useful to distinguish it from other types of potential energy (for example in this question the potential energy due to gravity). There is really no sense in having an argument over nomenclature, once everyone is clear as to what is being discussed.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Keenan: Your analysis is flawed. As this is now about your answer rather than Peter's, let's move this discussion to the comment thread on your answer.
Jan
22
comment Can gravitational potential energy be released in a fire?
Keenan, your additional analysis totally neglects the effects of gravity. Your model simply doesn't include it at all. If you really want to do this rigorously, write down the partition function for the gas in a gravitational field and do the full calculation.
Jan
22
comment Why does a ballerina speed up when she pulls in her arms?
@Noldorin: You could settle it once and for all by pausing it when she has the foot on the left hand side of the image elevated and pointing directly towards you, and measuring the minimum thickness of the ankle, and then doing the same thing once she has rotated 180 degrees. That would give you the definitive answer.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Keenan: The question is about whether gravitational potential energy can be released by a fire, and as Peter points out, in certain circumstances it can. Perhaps the easiest way to see this is to consider burning the fire inside a sealed vessel, and then releasing the resultant gas at different elevations.
Jan
22
comment Why does a ballerina speed up when she pulls in her arms?
As a side note regarding the new ballerina picture, it seems there is a right and a wrong answer, since the picture appears to use perspective rather than a parallel projection.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Peter: I think this is a good example of exactly the problem you highlighted on meta months ago.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Keenan: You need to take into effect the mass of the new gas molecules. There is an increase in the density of the gas due to the fire, which changes the potential energy, and leads to an increase in the average kinetic energy of the molecules in the atmosphere as it settles to equilibrium.
Jan
22
comment Can gravitational potential energy be released in a fire?
@Georg: Your objection is incorrect. As long as the same amount of fuel is burned, you are essentially dumping the same mass of gas into the atmosphere, whether at ground level, or at elevation. The effect of these new molecules falling on average, as they come into equilibrium with the atmosphere heats it.
Jan
22
comment Don't heavier objects actually fall faster because they exert their own gravity?
@Jerry: This isn't true in the earth's atmosphere, which is why people find it so counter intuitive in the case of a vacuum (a caveat which you should probably add).
Jan
22
comment Transfer function of an RLC circuit
This looks very like a homework problem, so I doubt you will find anyone to give you the answer here. For this reason I have also voted to close the question. However, I will point you in the right direction: What you are looking for is Kirchoff's laws (en.wikipedia.org/wiki/Kirchoff%27s_circuit_laws).