Changing temp at constant internal energy??

[Metabolics]

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1) The examkrackers book states that "We can change the temperature of a substance at constant internal energy by changing only the volume".

I was wondering how this is true? Internal energy= q + w. I thought internal energy is proportional to temperature, and that was the only way to raise temperature?

 

[chiddler]

i'm wrong. ignore.

 

[EnginrTheFuture]

Wow wow wow....

In the context of the MCAT, unless you are specifically told, you cannot change the internal energy without changing temperature... they are synonymous. Internal energy is a state function dependent on the kinetic and potential energy within a system. The potential energy is defined as static electric energy of atoms within molecules or crystals, and the static energy of chemical bonds. Unless you go through a phase change or something the mcat doesn't have us consider ever, this PE should be relatively constant.... The kinetic energy of the system is directly proportional to temperature... since this is all we can change in the context of the mcat, any change in internal energy = change in kinetic energy = change in temp.

You can change the volume without changing internal energy (isothermal expansion of a gas), you can change the internal energy without changing volume (isochoric addition of heat)... but you cannot change the temperature without changing internal energy of a system.

That would be like saying you are changing the temperature of the system without changing the temperature of the system.

 

[EnginrTheFuture]

when you reduce pressure, stronger intermolecular bonds form between gas particles lowering their energy state. thus, with their new found low energy state, they release energy just as an exothermic reaction does.

also, work = -PΔV

or consider this. energy is conserved. Internal energy is a measure of kinetic energy of gases. What happens to kinetic energy of gases when volume is reduced? They are closer so they exert more attractive forces on each other so they slow down. Kinetic energy is reduced. But it can't disappear. Therefore, it takes the form of heat.

I think you are mixing up the idea of heat and internal energy (temperature). I believe what you are describing is an isothermal expansion or compression of a gas. Here the change in work equals the change in heat and so the change in U is 0. But just because heat (q) was positive to cancel out the work doesn't mean anything about the temperature. Temperature is a state function, U is a state function... q is a unit of change in those state functions. A positive q means that the temperature was changed by a certain amount, but a - w means that the temperature was changed negatively. The change in temp depends on the sum of these individual changes not just q. In other words, the change in temp depends only on the change in U because U is proportional to T.

Khan explains this every well.... check it out.

 

[chiddler]

damn it

...bleh.

i will thanks.

 

[MedPR]

1) The examkrackers book states that "We can change the temperature of a substance at constant internal energy by changing only the volume".

I was wondering how this is true? Internal energy= q + w. I thought internal energy is proportional to temperature, and that was the only way to raise temperature?

Constant internal energy means no temperature change. Perhaps they meant heat and not temperature.

 

[EnginrTheFuture]

Constant internal energy means no temperature change. Perhaps they meant heat and not temperature.

That's definitely what they meant 👍