Homework Helpers: Physics

9 Heat and Thermodynamics


Lesson 9–5: The Laws of Thermodynamics

Thermodynamics is the study of the conversion of energy between heat and other forms of energy. It is the interrelation between heat, work, and internal energy of a system.

The First Law of Thermodynamics

The first law of thermodynamics is a specific statement of the law of conservation of energy. It tells us that when heat is transferred to a system, it can increase the internal energy of a system and/or it can be used by the system to do work.

The First Law of Thermodynamics

Q = ΔU + W

where Q = net heat gained by the system,
ΔU = change in internal energy, and W = net work done by the system.

A money analogy is often used to describe the first law of thermodynamics. If you suddenly came into some unexpected extra money (like extra heat energy), you could save it (increase internal energy) and/or spend it (use it to do work). Of course, any combination of the two is possible. You could save it all, spend it all, or save some and spend some.

This would be a good time to point out that a formula for work that will come in handy when working with gases.

Work = pressure × change in volume

Example 1

The initial internal energy of a system is 45 J. 32 J of energy are added to the system as heat, as the system does 21 J of work on its surroundings. What is the final internal energy of the system?

Given: Ui = 45 J      Q = 32 J      W = 21 J

Find: Uf


Starting with our original equation: Q = ΔU + W

We isolate the change in internal energy: ΔU = Q – W

Because ΔU = Uf – Ui, we get: Uf = Ui + (Q – W)

Solution: Uf = Ui + (Q – W) = 45 J + (32 J – 21 J) 56 J

Use the money analogy to check our answer. Suppose you had $45 in the bank (45 J) and you earned an extra $32 (Q = 32 J). You used $21 to buy something (W = 21 J). How much money would you have left? $45 + $32 – $21 = $56.

The Second Law of Thermodynamics

The second law of thermodynamics has several statements associated with it. The ones that are likely to be stressed in your physics class will be shown here.

1. Heat will not spontaneously flow from a colder body to a warmer body. If you go swimming in cold water, heat is transferred from your body to the water, not the other way around.

2. The entropy (disorder) of the universe is always increasing. If your mother asks you why you can’t keep your room clean you can reply, “Mom, even I can’t violate the second law of thermodynamics!” However, if she knows her physics, she may point out that your room isn’t a closed system.

3. No cyclic process that converts heat entirely into work is possible. Some energy is always “lost” due to heat. No heat engines are 100-percent efficient. It is impossible to construct a perpetual motion machine.

The Third Law of Thermodynamics

Absolute zero is the theoretical temperature at which the kinetic energy of the particles of a substance will reach zero. The third law of thermodynamics states that the entropy (disorder) of a substance approaches zero as its temperature approaches absolute zero. This temperature has never been reached experimentally, but scientists have cooled things to within a fraction of 1 K.

Thermodynamic Processes

There are several specific thermodynamic processes associated with the ideal gases that you should become familiar with.

 Isothermal process: A process in which the temperature of the gas remains the same.

 Isobaric process: A process in which the pressure of the gas remains the same.

 Isometric process: A process in which the volume of the gas remains the same.

 Adiabatic process: A process in which no heat is gained or lost by the system.

Lesson 9–5 Review

1. The pressure of an ideal gas is maintained at 2.45 × 105 Pa while its volume increases by 0.0345 m3. How much work does the gas do on its surroundings?

2. A system that absorbs 340 J of heat from its surroundings does 80 J of work as it expands. Find the change in its internal energy.

3. In a(n) ___________________ process, the pressure of the gas in question remains the same.