Calculating Work Energy Input for Compressing Air in a Piston-Cylinder Assembly

[freshbox]

A piston-cylinder assembly contains 20kg of air with a specific internal energy of 28K J/kg. It is compressed to a high pressure and the final specific internal energy is 65K J/kg. During the process there is a heat loss of 3.2K J. Determine the work energy input needed for the compressor.


Just want to ask how come I need to use the mass times the internal energy? What's the concept behind?



Thanks.

 

[BruceW]

You have been given the "specific internal energy". This is different to the "internal energy". And the question asks for a calculation of energy. So you need to calculate the internal energy by using the specific internal energy. I think you have already guessed how to do that.

 

[freshbox]

oops... why didn't i see that haha... anyway thank you Mr Brucew :)

 

[BruceW]

ha, no worries. It is easy to miss.

 

[Nickie]

The concept behind using the mass times the internal energy in this calculation is based on the conservation of energy principle. In this scenario, the initial internal energy of the air in the piston-cylinder assembly is 28K J/kg, and this energy is a result of the air's temperature and pressure. As the air is compressed, its temperature and pressure will increase, resulting in a change in its internal energy. The work energy input is the amount of energy needed to compress the air, and it is equal to the change in internal energy of the air. Therefore, we use the mass times the change in internal energy to calculate the work energy input needed for the compressor. Additionally, the heat loss of 3.2K J also needs to be taken into account as it is a form of energy that is lost during the compression process. I hope this explanation helps to clarify the concept behind using the mass times the internal energy in this calculation.