# Internal energy in piston

1. Oct 23, 2012

### Smileyxx

If the temperature is kept constant,what happens to internal energy if the pressure is increased in piston?

2. Oct 23, 2012

### SHISHKABOB

I believe that in this situation you'd make use of the ideal gas law.

3. Oct 23, 2012

### Staff: Mentor

For an ideal gas, the internal energy is a function only of temperature. If the temperature is kept constant, the internal energy is constant. From the first law, this means that the heat added is equal to the work done on the surroundings. If the pressure increases, that means that the volume is decreasing, and the surroundings are doing work on the system. This means that an equal amount of heat must be removed from the cylinder (in order to maintain constant temperature).

4. Oct 24, 2012

### Smileyxx

So am i suppose to mean that the internal energy decreases because of transfer of the heat to surrounding?

5. Oct 24, 2012

### sophiecentaur

It is being kept at a constant temperature - by your definition in the OP. Increasing the pressure involves doing work but the temperature (average KE) is being kept constant so that must mean that the added energy must be taken away by the surroundings.
Is this counter-intuitive?

I'm only repeating what's in the earlier post but you seemed to be questioning it.

6. Oct 24, 2012

### Smileyxx

Ya thats what my question is. Is it correct?

7. Oct 24, 2012

### sophiecentaur

Two people have told you the same thing. Have you a good reason not to believe it? Show us your reasoning.

8. Oct 24, 2012

### Smileyxx

I was just confused between ideal and normal gas.So for ideal gas with constant temperature ,pressure falls or rise doesnt make difference but if its not ideal gas ,pressure fall may expand volume so their is greater internal energy and vice versa? Is their any mistake in this statement?

9. Oct 24, 2012

### torquil

An ideal gas will also expand if the pressure is decreased while keeping the temperature constant. But the internal energy is constant, since temperature is constant, by your assumption.

10. Oct 24, 2012

### Smileyxx

Yeah i get it now.thanks