Energy seems to disappear when gas is compressed?

  • B
  • Thread starter Wazza
  • Start date
In summary, the conversation discusses a home project involving compressing gas and retrieving the stored energy at a later date. The individual is trying to model the idealized situation assuming zero losses but is having trouble with the equations. They are then given advice on how to correctly calculate the energy and work done during adiabatic compression. The conversation also addresses the difference between isothermal and adiabatic processes and the use of the first law of thermodynamics.
  • #1
Wazza
2
1
TL;DR Summary
Trying to find the right equations which described the energy transfer when compressing a gas.
Hi there,

I am working on a home project which involves compressing a gas and then retrieving the stored energy at a later date. Obviously in a real world scenario the recovered energy will be less than the energy it takes to compress the gas, due to system losses. However as a starting point I am trying to model the idealised situation, assuming zero losses, and am having trouble making the equations stack up. I was initially misled by isothermal equations and have now learned the difference between adiabatic and isothermal. Still, when I perform the seemingly trivial calculations, energy seems to be disappearing.

It may be simplest if I describe my question using an example.

My starting point is a belief in the first law of thermodynamics : energy cannot be created nor destroyed, in a closed system it is simply transferred from one state to another.

Suppose I have chamber which is perfectly insulated and fully sealed except for an opening for a piston which seals perfectly and is frictionless.
The chamber is full of air which starts off at atmospheric pressure.
Work is done to push the piston down which compresses the air inside the chamber.
As far as I know; Work = (the transfer of) Energy.
So the increase in energy in the gas in the chamber should equal the work done to compress it. Correct me if I'm wrong.

Let's say

Chamber is 2000 litres = 2 m^3
Piston plate is 1 m^2
Piston plate moves by 500 mm = 0.5m (down, i.e. compression)
Air inside is air pressure to start with = 101325 kPa
Temperature is 25C
Air can be thought of as an ideal diatomic gas with gamma = 1.4Energy in the chamber (to start with) is pressure x volume
= 101,325 Pa x 2 m^3
= 202,650 Joules constant = pressure x volume^gamma
= 101,325 x 2^1.4
= 267,398.3Final volume
= initial volume - (piston plate area x piston plate movement)
= 1.5 m^3 Final pressure = constant / volume_end^gamma
= 267,398.3 / (1.5^1.4)
= 151,576.14Energy in the chamber (to end with) is pressure x volume
= 151,576.14 Pa x 1.5 m^3
= 227,364.2 Joules Energy increase in the gas =
227,364 - 202,650 = 24,714work done pushing the piston = average pressure x change in volume
= 1/2(P2 + P1)(V2 - V1)

= 1/2 (151,576.14 + 101,325) (2.0 - 1.5)
= 126,450.6 x 0.5
= 63,225.3 Joules

So, the work done (63,225 Joules) is much greater than the energy increase in the gas (24,714 Joules).

Where has the energy gone ??

I note that gamma of 1.4 is an empirical value for a diatomic gas. If I arbitrarily fiddle this value to be 2.044 then I can make the energy equation balance, but arbitrarily fiddling values is not good physics. What am I missing ?

Please note that this question is not about what happens in the the real world. I am trying to start by modeling an idealised situation so that I can understand the basic physics. Once I have that, I can then start introducing losses and real world considerations.

Can anybody please tell which of my calculations are incorrect and what the right calculations should be ?
 
Science news on Phys.org
  • #2
Wazza said:
Energy in the chamber (to start with) is pressure x volume

No it's not. It's nCvT (for ideal gas). Also, regarding the work - average pressure need not mean arithmetic average. It is only if pressure changed linearly.
 
  • #3
Wazza said:
Energy in the chamber (to start with) is pressure x volume
= 101,325 Pa x 2 m^3
= 202,650 Joules
You are considering a diatomic ideal gas, so the internal energy is
$$
U = \frac{5}{2} PV
$$

Wazza said:
work done pushing the piston = average pressure x change in volume
= 1/2(P2 + P1)(V2 - V1)

= 1/2 (151,576.14 + 101,325) (2.0 - 1.5)
= 126,450.6 x 0.5
= 63,225.3 Joules
This equation works if the pressure changes linearly, which is not true for adiabatic compression. You have to calculate
$$
W = - \int_{V_i}^{V_f} P \, dV
$$
In order to save oneself the trouble of calculating this integral, one usually actually uses the first law to calculate work along an adiabat as ##W = - \Delta U##.
 
  • #4
Thank you, thank you, thank you. That has put me on the right track now and I can make the equations balance !
 
  • Like
Likes berkeman

FAQ: Energy seems to disappear when gas is compressed?

Why does it seem like energy disappears when gas is compressed?

It may seem like energy disappears during gas compression because the energy is being converted into other forms, primarily heat. When gas is compressed, work is done on the gas, increasing its internal energy, which often manifests as an increase in temperature. This heat can then be transferred to the surroundings, making it seem like energy has disappeared.

What happens to the internal energy of a gas when it is compressed?

The internal energy of a gas increases when it is compressed. This is because work is done on the gas to decrease its volume, which increases the kinetic energy of the gas molecules, thereby raising the gas's internal energy and often its temperature.

Is energy conserved when gas is compressed?

Yes, energy is conserved when gas is compressed. According to the first law of thermodynamics, the energy added to the system as work is equal to the increase in internal energy plus any heat transferred to the surroundings. No energy is lost; it is merely transformed from one form to another.

How does heat transfer affect the perception of energy disappearance during gas compression?

Heat transfer can make it seem like energy is disappearing during gas compression. When a gas is compressed and its temperature rises, heat may be transferred from the gas to its surroundings. If this heat transfer is not accounted for, it can appear as though energy has disappeared, when in fact it has merely been redistributed.

Can the energy change be measured during gas compression?

Yes, the energy change during gas compression can be measured. By monitoring the work done on the gas, the change in internal energy, and the heat exchanged with the surroundings, scientists can quantify the energy transformations involved. Tools like pressure gauges, thermometers, and calorimeters are often used in such measurements.

Back
Top