Can we use temperature differences for power generation?

[tomcps]

TL;DR

I have some weird idea about energy, and they are very likely violating many basic laws of physics. Sorry if I shouldn't post them here.

I remember that the definition of temperature is the average kinetic energy. Then how about if I keep shaking a box, wouldn't the kinetic energy of the box increase and thus the temperature of it increase? (neglecting the thermal energy gain from friction)

Also, is it possible to use air-conditioner or refrigerator to gather heat, and use the heat to generate power? If we can then wouldn't global warming be solved?

 

[russ_watters]

Then how about if I keep shaking a box, wouldn't the kinetic energy of the box increase and thus the temperature of it increase? (neglecting the thermal energy gain from friction)

Yes.

Also, is it possible to use air-conditioner or refrigerator to gather heat, and use the heat to generate power?

Yes.

If we can then wouldn't global warming be solved?

No. You won't generate as much as the air conditioner consumes.

 

[Drakkith]

I remember that the definition of temperature is the average kinetic energy.

Not quite. You also have to account for the energy in the vibration and rotation of molecules, not just their translation velocities.

Then how about if I keep shaking a box, wouldn't the kinetic energy of the box increase and thus the temperature of it increase? (neglecting the thermal energy gain from friction)

Yes, but only because of friction and the way that the energy used to accelerate the particles in the box is transferred. The uniform direction of the acceleration of the molecules near the edge of the box ends up becoming randomized as the collide and scatter off of other molecules. If every particle in the box is accelerated in the same direction at the same time there would be no increase in temperature. This is why shaking a brick won't heat it up. The molecules are rigidly bound together and can't move about relative to each other.

Also, is it possible to use air-conditioner or refrigerator to gather heat, and use the heat to generate power? If we can then wouldn't global warming be solved?

You will only capture a portion of the energy used to run the AC thanks to inherent inefficiencies in power generators. So no, it can't solve global warming.

 

[tech99]

You will only capture a portion of the energy used to run the AC thanks to inherent inefficiencies in power generators. So no, it can't solve global warming.
[/QUOTE]
But you can warm a house using a heat pump, taking heat from the ground. This is a tool which the UK are planning for all new houses to help combat climate change.

 

[Orodruin]

But you can warm a house using a heat pump, taking heat from the ground. This is a tool which the UK are planning for all new houses to help combat climate change.

This still requires an input of energy. The point is that it is more efficient than using the same energy directly to generate heat (eg to run an electric radiator) and so you spend less energy to heat the houses. This does not remove the need for an energy input.

 

[Vanadium 50]

neglecting the thermal energy gain from friction

It's the friction that turns work into heat.

 

[Dale]

I remember that the definition of temperature is the average kinetic energy.

This is not the definition of temperature. In fact, the relationship is only true for ideal monoatomic gasses. For all other materials energy is also found in other internal degrees of freedom.

 

[tomcps]

Wouldn't it be wonderful if we can "collect" the thermal energy and turn it into electricity
Such a shame that we can't make this feasible (yet?)

 

[Dale]

Wouldn't it be wonderful if we can "collect" the thermal energy and turn it into electricity
Such a shame that we can't make this feasible (yet?)

We can. That is precisely what any heat engine does. Not only is it feasible, it is the basis of most electrical power produced today.

 

[tomcps]

This is not the definition of temperature. In fact, the relationship is only true for ideal monoatomic gasses. For all other materials energy is also found in other internal degrees of freedom.

Oh now i get it!

 

[tomcps]

We can. That is precisely what any heat engine does. Not only is it feasible, it is the basis of most electrical power produced today.

What I meant is collecting energy from air like
10 unit of 25°c air
-> 9 unit of 20°c air + 1 unit of air which is hot enough to run the generator

 

[Dale]

air which is hot enough to run the generator

You don’t need air at a certain temperature to run a generator. All you need is a temperature difference. If you have a reservoir of 25 deg C air and a reservoir of 20 deg C air then you can design a generator to run off that. The bigger the difference the more efficient the generator.

 

[tomcps]

You don’t need air at a certain temperature to run a generator. All you need is a temperature difference. If you have a reservoir of 25 deg C air and a reservoir of 20 deg C air then you can design a generator to run off that. The bigger the difference the more efficient the generator.

Oh now I see..
Sorry for wasting your time for answering my dumb question