Thermodynamics – Heat and Work

Thermodynamics is a branch of physics looking at how changes in energy, work and the flow of heat influence each other. It can explain the workings of an internal combustion engine, a refrigerator and the sun.

Laws of Thermodynamics

The heat gained or lost in a system is equal to the increase or decrease in the internal energy of the system plus the work done on or by the system.

The First Law

Mathematically speaking,

Total Heat = change in internal energy + work done Q = U + W (Eqn 11.13)

Where all three quantities are measured in joules.

In this equation, we have the following conventions:

1. Q is positive if you add heat to the system and negative if you remove heat from the system.

2. W is positive if the work is done by the system, and negative if work is done on the system.

3. U is arbitrary and can be expressed in terms of Q and W.

James Prescott Joule showed that heat, and the change in temperature it can cause, are forms of energy. By constructing an experiment to measure the temperature change in water due to work done on it by stirring, he showed that the loss of gravitational potential energy by dropping a weight was turned into various forms of energy.

It was converted into kinetic energy of the weight, which was converted to rotational energy of the paddles, which stirred the water and caused a increase in temperature of the water due to the increased internal kinetic energy of the water. So we can put hot materials to work for us, since the heat is a form of energy and energy is the ability to do work.

We most often use this idea to run the refrigerator. Using the first law, why is it cold?

Using equation 11.13, the components, in simple terms are the following

Work Done on the System (W)= from the electricity applied to power the refrigerator motor, which cools the refrigerator= negative since work is done on the system

Total Heat (Q)= heat needed to lower the temperature from room temperature to near freezing= negative since heat is removed from the refrigerator (cooling).

Change in Internal Energy (U)= negative since both Q and W are negative, therefore, the final state of (energy in) the refrigerator is less that its initial state.

The Second LawIt is impossible for heat to flow from a point of lower

temperature to a point of higher temperature without the application of energy from an external source.

The second law states that Figure 11.11a is possible if an external energy is applied to change the direction of heat flow. As explained in the first law, refrigerators have motors that do WORK.

And who says the flow of heat is from low to high temperature? Cooling devices make use of coolants like freon. This is the place where the heat in the refrigerators flow (so it’s still from hot to cold). Because of the coolants, there will be equilibrium in the refrigerator, cooling it. Then the wasted heat will be greater than the temperature at the room near it, exhausting outward (still hot to cold).

EntropyFrom Figure 11.11b, we saw the term “wasted heat.” What is it? Entropy is a measure of the amount of wasted heat in the system. It is a measure of the amount of disorder in the system.

Entropy change = heat added/Temperature (in K) (Eqn 11.14)

The entropy changes for reversible processes are bigger that those for non-reversible (real life) processes, which have fiction. Another way to express this law is that over time the amount of entropy in the universe increases.