Physics Help Forum

Pressure Variation Formula

strgrl — Sat, 04 Jul 2009 03:02:17 GMT

Write an expression that describes the pressure variation as a function of position and time for a sinusoidal sound wave in air, taking wavelength=0.1m and delta Pmax= 0.2 N/m^2.

So this problem seems really straight forward, just solve for k and omega and plug them into the formula P=delatPmax sin(kx - wt + phase constant). Using wavelength, I found k=62.8 1/m. However, with the information given I do not know how to solve for omega or the phase constant. Without velocity, I can't think of any formulas that might relate the given information to the needed values. Any suggestions?

Heat questions

Luigi — Sat, 04 Jul 2009 02:56:50 GMT

1) In solar heating applications, heat energy is stored in some medium until it is needed. Should this medium have a high or a low specific heat? Suggest a substance that would be appropriate for use as a heat-storage medium, and explain its advantages.

2) Explain why specific heat is specific and how it gives a relative indication of molecular configuration and bonding.

Momentum in quantum mechanics

Pmb — Sat, 04 Jul 2009 02:32:05 GMT

Hi everyone. I'm doing an experiment. In another forum I got into a heated argument regarding the definition of mass. I.e. when physicists use the term mass in relativity it means one of two things, either proper mass or relativistic mass. The objection is that if you have more than one meaning to a term mass then all hell breaks loose. (Bigsmile)

I pointed out that this is not unique to mass since the meaning of the term momentum depends on the context it's used. In Newtonian mechanics it refers to linear mechanical momentum p= mv. In relativity it can mean 4-momentum and in quantum mechanics it means conjugate momentum, aka canonical momentum. So with your kind patience and indulgence I'd like to try an experiment here.

You are all aware that the uncertainty principle relates uncertainty in momentum to the uncertaintly in position. But what you don't learn until advanced courses on quantum mechanics is that the momentum this refers to is actually what is called "cannonical momentum" (aka conjugate momentum). Its different than the momentum p = mv that you learn about in basic physics. Suppose a charged particle is moving in a magnetic field. The cannonical momentum p has the value

(B-45) p = mv + qA

where A is known as the magnetic vector potential. A graduate text in quantum mechanics by Cohen Tannoudji explains this on page 225) as follows

Quote:

Care must be taken not to confuse p (the momentum of a particle, also called conjugate momentum of r) with mv (the mechanical momentum of the particle): the difference between these two quantities appears clearly in (B-45). ...However it is the conjugate momentum p and not the mechanical momentum mv which becomes in quantum mechanics the operator p which satisfies the canonical commutation relations.

Since canonical momentum is a rather advanced topic and probably not known to most visitors here I'm curious as to whether such a fact is of interest to anyone, i.e. would you want to know that p is a function of the magnetic field through the quantity A?

I read an article on this subject. It appeared in the American Journal of Physics. If there is no magnetic field then p = mv and which implies . When there is a uniform magnetic field present then this becomes
e\hbar/2m^2c ||" title="\Delta v_x \Delta v_y > e\hbar/2m^2c ||" style="border: 0px; vertical-align: middle;" />

This would not be apparent if one didn't know how p is actually defined. So my question is this - Do you feel that lower level quantum mechanics texts are not preparing the reader well for this fact if they don't clarify what the momentum that quantum mechanics really refers to?

Thank you all for your help.

Pete

Can someone explain the flaw in this free energy scheme

Ariel — Fri, 03 Jul 2009 23:58:25 GMT

Ive seen demonstrations that a temporary super conductor cooled by liquid nitrogen becoming super conductive will repell a permanent magnet. Consider an apparatus consisting of a temporary super conductor attached by a lever to a generator, a vat of liquid nitrogen, and a powerfull permanent magnet below the vat. The temporary superconductor decends into the vat becoming superconductive. It is than repelled by the magnet upwards out of the vat. The work done is collected by the generator. It is than warmed by air at room temprture lossing its superconductivity and decends back into the vat and the cycle is repeated. If the magnet is strong enough the energy generated will exceed the energy required to maintain a stock of liquid nitrogen. Thus a free energy device is obtained.

optics

abhishek arora — Fri, 03 Jul 2009 02:24:51 GMT

first of all from the question i was unable to understand what is meant by the term they both have same no of waves and how do we apply the concept over here
(note the red one cube is made of material A and green one of material B and blue of material c)

Attached Images

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Recommend me Math Cources for Physics Major

rubrix — Fri, 03 Jul 2009 01:08:29 GMT

My Univ requires at least:

Calc I
Calc II
Calc III
and Intro to Differential Equations (which is ODE)

I was recommended to take Applied Linear Algebra but I took Linear Algebra as it counts for my Math major too. My univ also offers Linear Algebra II however, I don't want to take it unless there is really a good reason to do so.

They also offers Applied Differential Equations, which is mainly covers Numerical Methods, Introduction to PDE, and Green's function from what I can gather. I'm thinking of taking this class, regardless of whether it will help with Physics or not but I ought to know, does it?

Another class that i'm looking into is Advanced Calculus I. I really liked Calc I, Calc II, and Intro to DFQ too (not much into Calc III, cough triple integrals in polar coordinates) so i guess this one will be fun but on the downside they don't offer Advanced Calculus II if i wish to continue on. Nevertheless, I'm curious if anyone has taken it and how can it help with Physics?

Other class that I hear folks talk about it Real Analysis and Complex Analysis. Unlike Advanced Calculus, they offer part I and II for this course but I have very little idea of this course so i could use some help here with someone who has taken this course in particular. I don't want to walk into the class thinking i'll like it and get out hating it...i don't want to waste semester on this course if this isn't going to help. On contrary, if it will, i'm down with talking both Real and Complex, part I and part II.

If you recommend any other courses, post them too please.

Thanks in advance!

-rubrix.

average acceleration

dogdrum — Fri, 03 Jul 2009 00:48:01 GMT

Hello.....My first post on the forum. I will be taking Physics 101 next month and need all the help I can get!

Basic problem which I can't seem to figure..

Car traveling 85 km/hr strikes a tree. The front end of the car compresses
and the driver comes to rest after traveling .80 m. What was the average acceleration of the driver during the collision? Express the answer in terms of "g's", where 1.00 g = 9.8 m/s2.

I start by 85 * 100 / 3600 = 23.61m/s
average acceleration = change in distance , divided by change in time. Now is the change in time equal to .8 of second? Surely I'm missing a step or two.. Thanks in advance..I'm working with a text that provides the answer as -35 g's.

Greg

Question on optical pipes and fibers

arze — Thu, 02 Jul 2009 08:34:59 GMT

I have attached a picture of the question. Number 4, the last part about showing that internal reflection occurs when the angle of incidence meets the required criteria.
I'm quite lost on this, would really appreciate if you could provide a good explanation.
Thanks!

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100_4642.jpg (600.0 KB)

prism

abhishek arora — Thu, 02 Jul 2009 08:13:01 GMT

acc to me as we increase the refractive index r1 ie first angle of refraction will decrese so 30-r1 will increase i am not able to solve furthur plese help me

Attached Images

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Basic terms for Physics

RonyLee — Wed, 01 Jul 2009 09:26:34 GMT

Hello friends,Today I want to tell you something interesting about Physics. Physics is the general analysis of nature, conducted in order to understand how the world and universe behave. physicshandbook.com is an online exclusive reference and handbook for Physics. This website has technical description in details, question answers on several topics in Physics. Additionally Charts, Laws, Inventions, Experiments Online, Technical Definitions, Videos on advance topics and Quiz is also here for further knowledge.

Specific Heat of Metals

SaraE — Wed, 01 Jul 2009 03:46:45 GMT

Any help would be great:

1) Why is specific heat of water equal to unity, ie., 1.0 cal/g-Celcius or 1.0 kcal/kg-Celcius?

2) Given that the specific heat of one material is twice that of another, compare the relative amounts of heat required to raise the temperature of equal masses of each material by 1Celcius?

3) Say the same amount of heat was added to samples of the materials in Question 2, and each sample had the same increase in temperature. Compare the relative masses of the samples.

Friction (check my answer)

mj.alawami — Tue, 30 Jun 2009 14:58:18 GMT

An 80N wooden block is placed on top of a wooden table.If a force of 28N is necessary to keep the block moving at a constant velocity.
a)What is the co-efficient of sliding friction for the wooded block?
b)if a wieght of 40N is placed on top of the wooden block what force will be required at a constant velocity across the table?

Attempt:
a)
f=28 , N=80

b)80*28=120*x

Am I correct?

Laws of motion (check my answer)

mj.alawami — Tue, 30 Jun 2009 14:56:57 GMT

Question:
An empty truck with mass of 2500kg has an engine that will accelerate at a rate of .What will be the acceleration when the truvk has an additional load of 1500kg?

Attempt:
2500*1.5=4000*x

Am I correct ?
Thank you

Converting total heat in mechanical work.

george — Tue, 30 Jun 2009 14:08:24 GMT

Converting total heat in mechanical work, is currently regarded as a utopia.

I have a theory that demonstrates that this is possible.

Believe someone? george,Romania.

Find 'Mean Square Quantization Error'....

jeeee — Tue, 30 Jun 2009 13:29:46 GMT

A signal given by time function s(t)=2sin 2πt. The signal is to be transmitted as 4 bit PCM.. Find?

Find 'Mean Square Quantization Error'

thanks !!