# A level physics help

1. Jan 1, 2012

### aashish2025

I am an A Level Physics students and I have tonnes of questions to ask. So, below are some of them. Hope you will answer them. Please answer the ones you have any idea about even if just a singe question. Hope I will get the answers!

• The only difference between Kelvin and Celsius is the y-intercept(Since T = θ + 273). Then, how simply using Kelvin instead of Celsius fix the problem of different types of thermometer disagreeing with each other even at the same temperature.(The reason my teacher gave for why do they disagree was, because the property whose change we measure does not vary linearly(and in different ways in different thermometers) with temperature but after determining the fixed points, we assume that they vary like that.) So, if the reason my teacher gave me is correct, what difference would the use of Celsius or Kelvin make on that?(Hope you got what I meant!)

• Will a matter have zero internal energy at zero Kelvin(let's assume that it can be attained)? My answer is no because internal energy is not only the energy due to the temperature(kinetic energy). Internal energy is the sum of the kinetic and potential energy. So, at zero Kelvin, though the kinetic energy is zero, potential energy isn't. So, the internal energy must not be zero! Am I correct?

• Why does specific heat capacity vary from one material to the other and why does it depend on the temperature?

• Dulong and Petit's law states that "all solids have nearly 25 Jmol-1Kmol-1 specific heat capacity." This implies that specific capacity depends on the number of atoms or molecules. But, number of atoms/molecules per unit volume is closely related to density. So, does that mean that specific capacity of a substance depends on the density of the substance? Further, the number of atoms/molecules depends on the mass as n = (M/Mr). That implies a relation between specific heat capacity and mass but there is none. Please explain.

• Why do moving charge particles get affected by magnetic filed? Because they themselves have a magnetic field, right? Then, how does a moving charge generate magnetic field?

• Any body follows Simple Harmonic Motion if a $\alpha$ -x. Well, in a recent exam, we had a question in which there was a sinusoidal graph given and the question that was asked underneath was something like, "What feature of the above graph tells you that the object is following Simple Harmonic Motion?" I wrote "sinusoidal nature of the graph" and scored zero. My teacher reported that the answer is, "constant time period" But, I believe that, Simple Harmonic Motion implies constant time period but the converse is not always true. From the equilibrium position, I can have a body move super-fast to a certain point(before the extreme end), then stop for a while and then slowly move to the extreme end and then return back to the equilibrium in another unpredictable way. We can have all that and still have a constant time period but that won't be a simple harmonic motion, will it? And, shouldn't "sinusoidal nature of the graph" be the correct answer because the solution of the differential equation d2x/dt2 = -kx will be a sine function(or cosine)?

And, the last but not the least, can someone give me a good link to study operational amplifier. All the books I have start with too technical touch and I want someone gentle and something like written for a layman. Can I find something like that? It would be a great relief if I did.

Hope you understood my questions and hope someone out there will help! And, sorry for my not so good English!

2. Jan 1, 2012

### Amok

1- I don't get your question. To me if two termometers disagree with each other, one of them must be broken, I've never heard of this problem.

2- I'm pretty sure that even at 0 K kinetic energy will not be zero, translational KE will. Potential energy won't be 0 either. So the answer is no.

3- Get back to you on that one.

4- Why do you say the heat capacity of a solid depends on the number of atoms or molecules? One kilo of iron will have the same heat capacity as 500g of iron. The fact that you have a "per mol" in the units means that it will take 25 joules to increase the temp of 1 mole of the solid by 1 K.

5- If a magnetic field is present, the Lorentz force will act on a moving charged particle. If the particle has 0 velocity, then this force will be 0 (assuming no electric field). I can't really give a better explanation than this, I think this is pretty much axiomatic, it's like asking why things get accelerated when a force acts on them. The same goes for how a moving charge generates a magnetic field (Maxwell's laws). Maybe someone else can help out.

6- Why do you say that the situation you described has constant time period? I don't think that's true at all. How would even define a time period for that kind of motion?

Last edited: Jan 1, 2012
3. Jan 1, 2012

### technician

Temperature scales can be defined using any physical property that varies with temperature (resistance, length of liqid in a tube etc) but because different physical properties vary with temperature in different ways 2 defined temp scales may not agree, except at the fixed points.
The Kinetic theory of gases reveals that temperature (absolute temperature) is a direct measure of the average kinetic energy of molecules and this means that PRESSURE and VOLUME of an IDEAL gas provide an excellent physical property to define a temperature scale.
Most gases behave like an ideal gas and therefore measurement of pressure or volume can be used to define this temperature scale.
The main problem is that 'gas thermometers' are bulky and not convenient for practical use but they can be used to check or CALIBRATE thermometers using other physical properties.
Be sure you know the difference between DEFINING a temperature scale and CALIBRATING a thermometer scale.
hope this helps in a tricky topic

4. Jan 1, 2012

### TheShrike

I'm not sure whether you're correct or not, but I should point out that zero Kelvin cannot be attained even in principle (see Third Law of Thermodynamics). In quantum mechanics the lowest energy states available to a particle are non-zero, thus we cannot have zero Kelvin, which would indicate zero kinetic energy for the particle.

Moving charged particles experience a force from a magnetic field because they are charged, not because they themselves generate a magnetic field. Have you come across the Lorentz force law? It states:

$\textbf{F}=q\textbf{E}+q(\textbf{v}\times\textbf{B})$

...where E& B are electric and magnetic fields respectively, and v is the velocity of a particle with charge q. You can see that there is no force due to a magnetic field if the particle is stationary.