S-Cool Revision Summary

S-Cool Revision Summary

First law of thermodynamics

There are only 2 ways that you can change the internal energy, U of the gas. (without adding or removing any atoms).

  1. Heat it or cool it i.e. ΔQ or heat transferred.
  2. Compress it or expand it i.e. ΔW or work done.

This leads us to the first law of Thermodynamics but first let's just look at the two more closely.

Heat

If you heat up gas, you pass energy to the atoms. (It appears as Ek).

Heat flowing into the gas is positive.

Work

If gas expands it has to push back the stuff that was around it. It has to do work (use energy) to do this. This is related to its internal energy.

We define work done by the gas pushing back its surroundings as positive.

Equation for work done by a gas.

p-V or Indicator diagrams

We sketch graphs showing how pressure and volume vary when we do certain things to a gas.

Here are the three you need to understand:

Combinations of Resistors

Constant pressure (isobaric) process.

p-V or Indicator diagrams

Constant volume (isovolumetric) process.

p-V or Indicator diagrams

Constant temperature (isothermal) process.

Adiabatic process.

Note temperature increases, pressure increases and volume reduces, so none of these is unchanged. So why is this special?

Well in the processes that we've looked at so far we've had U = O and W = O but not Q = O. Well, this is it. In an adiabatic process →Q = O!

What does that mean? No heat can be transferred to or from the gas!!

Cycles

If you do a number of things to a sample of gas, one after the other you could end up with a graph like this:

Cycles

To deal with this and work out what's going on, consider each process separately.

For example:

Process 1 is constant pressure expansion (as its volume goes from small to large).

Process 2 is constant volume cooling (its temperature drops - see constant temperature lines).

Process 3 is constant pressure contraction (volume decrease).