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# Thermodynamics

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What are the four laws of thermodynamics? I tried to Google them but all I got was senseless gibberish!

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The 2nd comes down to 'entropy rises' or 'everything moves from a more ordered state to less ordered state'

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Zeroth: You must play the game.

First: You can't win.

Second: You can't break even.

Third: You can't quit the game.

• 1

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The 2nd comes down to 'entropy rises' or 'everything moves from a more ordered state to less ordered state'

I understand entropy (thanks to Brian Cox) but I though Thermodynamics was about heat?

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I understand entropy (thanks to Brian Cox) but I though Thermodynamics was about heat?

You could try the BBC site...

I found this - might be of use.

http://www.bbc.co.uk/news/entertainment-arts-19703140

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zeroth says basically if one object is the same temperature as a 2nd, and the 2nd the same as a 3rd, the 1st and 3rd are also the same temperature. It was added later when it was found to be needed -so was sneaked in at 0.

1st says you can't create or destroy energy- only change its form.

2nd says basically that if you have a cold thing and a hot thing next to each other then cold thing will get hotter and hot thing colder, rather than the other way around

3rd says you can only get to a constant entropy as you get to absolute zero.

Edited by JulianO

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They sound right, but, my three laws of thermodynamics are:-

1st Law: If I am too cold, my body is stiff and I don't work too well

2nd Law: If I am too warm, I feel lethargic and don't work too well

3rd Law: Once I reach thermal equilibrium, I'm alert enough to avoid work altogether

• 8

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I knew 0,1 and 2 already! I just didn't realize that they were laws of thermodynamics!

The 3rd one makes a lot of sense.

Thank you very much! As always, you have succeeded where Wikipedia has failed!

So...

0. If A = B and B = C than A = C (If A,B and C are temperatures). That just seems like logic to me!

1. This raises quite an interesting question. If energy can't be created then how does it exist?

2. Heat always moves to a cooler area.

3. When something is at absolute Zero, it's entropy doesn't change. Does this mean that It is immortal?

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Might be useful ..

Odd things happen near absolute zero (super fluids etc) ...

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The first law of thermodynamics is... You do NOT talk about thermodynamics!

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Easiest one I can remember is, 'energy cannot be created or destroyed'

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I knew 0,1 and 2 already! I just didn't realize that they were laws of thermodynamics!

The 3rd one makes a lot of sense.

Thank you very much! As always, you have succeeded where Wikipedia has failed!

So...

0. If A = B and B = C than A = C (If A,B and C are temperatures). That just seems like logic to me!

1. This raises quite an interesting question. If energy can't be created then how does it exist?

2. Heat always moves to a cooler area.

3. When something is at absolute Zero, it's entropy doesn't change. Does this mean that It is immortal?

e=mc2

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All energy came about due to the big bang - that is, according to our current understanding of the univrrse.

Energy can condense into matter, matter can evaporate back into energy - one way to look at it.

Matter is just another form of energy. Apparently their is no such thing as a physical particle, a 'particle' is really a wave - according to recent CERN lecture I watched.

So we are not 'real' - as in the 'physical' sense, as all atoms are made of waves, we are all a collection of waves. It's gets stranger and stranger the more we learn! .. or so it seems

[babble mode off]

Edited by Cath

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2. Heat always moves to a cooler area.

Just a little thing, don't get heat and temperature mixed up. Temperature flows to a cooler area via the process of heat. Heat is the transfer of energy via temperature. If your thinking of doing physics at a higher level in school, don't let this catch you out .

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All energy came about due to the big bang - that is, according to our current understanding of the univrrse.

Energy can condense into matter, matter can evaporate back into energy - one way to look at it.

Matter is just another form of energy. Apparently their is no such thing as a physical particle, a 'particle' is really a wave - according to recent CERN lecture I watched.

So we are not 'real' - as in the 'physical' sense, as all atoms are made of waves, we are all a collection of waves. It's gets stranger and stranger the more we learn! .. or so it seems

[babble mode off]

Still doesn't explain why they can rearrange themselves into things that annoy me.

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Still doesn't explain why they can rearrange themselves into things that annoy me.

mmmmmmm , for our current understanding, none it is really explaining what the 'hidden' driving force really is that is causing atoms to ultimately arrange themselves into incredibly complex patterns that form 'life'.

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So...

0. If A = B and B = C than A = C (If A,B and C are temperatures). That just seems like logic to me!

Well maybe, certainly AxB is not necessarily equal to BxA, go learn ,matrix algebra and forces are described by matrices. I would guess that therefore there are instances whare A+B is not equal to B+A.

Best never to assume anything.

Apologies read A = B as A + B, same for the rest. Still wouldn't assume too much, unless you tie it down with conditions and/or presumptions.

Edited by ronin

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e=mc2

Or, in relativistic terms:

e=mc2/(1-(v/c)2)1/2

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Or, in relativistic terms:

e=mc2/(1-(v/c)2)1/2

Or for the full version:

E2=(mc2​)2+cp (I think that's the full version)

Edited by Ganymede12
• 1

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Or for the full version:

E2=(mc2​)2+cp (I think that's the full version)

Yes, if the last term is squared as well. Then, the above expression is equivalent to (exercise!)

Or, in relativistic terms:

e=mc2/(1-(v/c)2)1/2

The above expressions are for particles moving with speed v. E=mc2 is the special of these expressions when a particle is at rest, i.e., when v = 0.

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The above expressions are for particles moving with speed v. E=mc2 is the special of these expressions when a particle is at rest, i.e., when v = 0.

For a particle moving with speed V don't you need to add on the kinetic energy (1/2 mv^2) as well. I know this probably won't make a massive difference at low speeds but I thought I read it somewhere.

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For a particle moving with speed V don't you need to add on the kinetic energy (1/2 mv^2) as well. I know this probably won't make a massive difference at low speeds but I thought I read it somewhere.

This a non-relativistic approximation, and it is already included it in the relativistic expressions. For those interested, technical details follow. For a moving particle that has mass,

kinetic energy = particle's total energy - particle's rest energy.

The total energy is mc2/(1 - (v/c)2)1/2, and rest energy is mc2. If K is kinetic energy, and if (for notational simplicity) z is used for (v/c)2, then the above expression is

K = mc2(1 - z)-1/2 - mc2.

Now, (1 - z)-1/2 can be expanded in a series as (I think I took this in high school) as

(1 - z)-1/2 = 1 + (1/2) z + (3/8) z2 + ...

If the particle's speed v is small compared to the speed of light c, then z is very small and z2 is even smaller. For example, if v is 10% the speed of light, then v/c = 0.1, z =(v/c)2 = 0.01 and z2 = 0.0001. For speeds less than this, it is a good approximation to neglect all but the first two terms in the series, i.e. to approximate (1 - z)-1/2 as

(1 - z)-1/2 = 1 + (1/2) z.

Then, the expression for the kinetic energy K becomes

K = mc2(1 - z)-1/2 - mc2

= mc2(1 + (1/2) z) - mc2

= mc2(1 + (1/2)(v/c)2) - mc2

= (1/2) m v2

• 3

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This a non-relativistic approximation, and it is already included it in the relativistic expressions. For those interested, technical details follow. For a moving particle that has mass,

kinetic energy = particle's total energy - particle's rest energy.

The total energy is mc2/(1 - (v/c)2)1/2, and rest energy is mc2. If K is kinetic energy, and if (for notational simplicity) z is used for (v/c)2, then the above expression is

K = mc2(1 - z)-1/2 - mc2.

Now, (1 - z)-1/2 can be expanded in a series as (I think I took this in high school) as

(1 - z)-1/2 = 1 + (1/2) z + (3/8) z2 + ...

If the particle's speed v is small compared to the speed of light c, then z is very small and z2 is even smaller. For example, if v is 10% the speed of light, then v/c = 0.1, z =(v/c)2 = 0.01 and z2 = 0.0001. For speeds less than this, it is a good approximation to neglect all but the first two terms in the series, i.e. to approximate (1 - z)-1/2 as

(1 - z)-1/2 = 1 + (1/2) z.

Then, the expression for the kinetic energy K becomes

K = mc2(1 - z)-1/2 - mc2

= mc2(1 + (1/2) z) - mc2

= mc2(1 + (1/2)(v/c)2) - mc2

= (1/2) m v2

I love the way mathematics like this can be used to demonstrate that you can get back to "Galilean" relativity / Newtonian mechanics by plugging in "normal" speeds into the "Einsteinian" relativistic equations. The Lorentz transformations are only needed when you get to speeds close to c, which just happens to be pretty common when you're dealing with phenomena in cosmology... How else would we be able to resolve paradoxes such as the large number of muon decay products that reach our detectors on Earth!

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This a non-relativistic approximation, and it is already included it in the relativistic expressions. For those interested, technical details follow. For a moving particle that has mass,

kinetic energy = particle's total energy - particle's rest energy.

The total energy is mc2/(1 - (v/c)2)1/2, and rest energy is mc2. If K is kinetic energy, and if (for notational simplicity) z is used for (v/c)2, then the above expression is

K = mc2(1 - z)-1/2 - mc2.

Now, (1 - z)-1/2 can be expanded in a series as (I think I took this in high school) as

(1 - z)-1/2 = 1 + (1/2) z + (3/8) z2 + ...

If the particle's speed v is small compared to the speed of light c, then z is very small and z2 is even smaller. For example, if v is 10% the speed of light, then v/c = 0.1, z =(v/c)2 = 0.01 and z2 = 0.0001. For speeds less than this, it is a good approximation to neglect all but the first two terms in the series, i.e. to approximate (1 - z)-1/2 as

(1 - z)-1/2 = 1 + (1/2) z.

Then, the expression for the kinetic energy K becomes

K = mc2(1 - z)-1/2 - mc2

= mc2(1 + (1/2) z) - mc2

= mc2(1 + (1/2)(v/c)2) - mc2

= (1/2) m v2

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