Chapter 12 Review Notes

13-1 Atomic Theory of Matter

On a microscopic scale, the arrangements of molecules in solids (a), liquids (b), and gases (c) are quite different.

13-2 Temperature and Thermometers

Temperature is a measure of how hot or cold something is.

Most materials expand when heated.

13-2 Temperature and ThermometersThermometers are instruments designed to measure temperature. In order to do this, they take advantage of some property of matter that changes with temperature.

Early thermometers:

13-2 Temperature and Thermometers

Common thermometers used today include the liquid-in-glass type and the bimetallic strip.

13-2 Temperature and Thermometers

Temperature is generally measured using either the Fahrenheit or the Celsius scale.

The freezing point of water is 0°C, or 32°F; the boiling point of water is 100°C, or 212°F.

13-3 Thermal Equilibrium and the Zeroth Law of Thermodynamics

Two objects placed in thermal contact will eventually come to the same temperature. When they do, we say they are in thermal equilibrium.

13-6 The Gas Laws and Absolute Temperature

The volume is linearly proportional to the temperature, as long as the temperature is somewhat above the condensation point and the pressure is constant:

Extrapolating, the volume becomes zero at −273.15°C; this temperature is called absolute zero.

13-6 The Gas Laws and Absolute Temperature

The concept of absolute zero allows us to define a third temperature scale – the absolute, or Kelvin, scale.

This scale starts with 0 K at absolute zero, but otherwise is the same as the Celsius scale.

Therefore, the freezing point of water is 273.15 K, and the boiling point is 373.15 K.

14-1 Heat As Energy Transfer

We often speak of heat as though it were a material that flows from one object to another; it is not. Rather, it is a flow of energy.

Unit of heat: calorie (cal)

1 cal is the amount of heat necessary to raise the temperature of 1 g of water by 1 Celsius degree.

Don’t be fooled – the calories on our food labels are really kilocalories (kcal or Calories), the heat necessary to raise 1 kg of water by 1 Celsius degree.

14-1 Heat As Energy Transfer

If heat is a way to transfer energy, it ought to be possible to equate it to other forms. The experiment below found the mechanical equivalent of heat by using the falling weight to heat the water:

14-1 Heat As Energy Transfer

Definition of heat:

Heat is energy transfer from one object to another because of a difference in temperature.

• Remember that the temperature of a gas is a measure of the average kinetic energy of its molecules.

14-2 Internal Energy

The sum total of all the energy of all the molecules in a substance is its thermal (or internal) energy.

Temperature: measures molecules’ average kinetic energy

Thermal (internal) energy: total energy of all molecules

Heat: transfer of energy due to difference in temperature

13-11 Distribution of Molecular Speeds

These two graphs show the distribution of speeds of molecules in a gas, as derived by Maxwell. The most probable speed, vP, is not quite the same as the rms speed.

As expected, the curves shift to the right with temperature.

14-3 Specific Heat

The amount of heat required to change the temperature of a material is proportional to the mass and to the temperature change:

(14-2)

The specific heat, c, is characteristic of the material. Some values are listed at left.

14-4 Calorimetry – Solving Problems

Closed system: no mass enters or leaves, but energy may be exchanged

Open system: mass or energy may transfer

Isolated system: closed system where no energy in any form is transferred

For an isolated system,

Energy out of one part = energy into another part

Or: heat lost = heat gained

14-4 Calorimetry – Solving Problems

The instrument to the left is a calorimeter, which makes quantitative measurements of heat exchange. A sample is heated to a well-measured high temperature, plunged into the water, and the equilibrium temperature measured. This gives the specific heat of the sample.

14-5 Latent Heat

Energy is required for a material to change phase, even though its temperature is not changing.

14-5 Latent Heat

Heat of fusion, HF: heat required to change 1.0 kg of material from solid to liquid

Heat of vaporization, HV: heat required to change 1.0 kg of material from liquid to vapor

14-5 Latent Heat

Heat of Fusion: Q = ± mHF

Heat of Vaporization: Q = ± mHv

Pay attention the sign. If you are melting something, Q is positive. If you are freezing something Q is negative.

The latent heat of vaporization is relevant for evaporation as well as boiling.

On a molecular level, the heat added during a change of state does not go to increasing the kinetic energy of individual molecules, but rather to break the close bonds between them so the next phase can occur.

14-6 Heat Transfer: Conduction

Heat conduction can be visualized as occurring through molecular collisions.

14-6 Heat Transfer: Conduction

The constant k is called the thermal conductivity.

Materials with large k are called conductors; those with small k are called insulators.

14-7 Heat Transfer: ConvectionConvection occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced; both these examples are natural convection.

14-7 Heat Transfer: Convection

Many home heating systems are forced hot-air systems; these have a fan that blows the air out of registers, rather than relying completely on natural convection.

Our body temperature is regulated by the blood; it runs close to the surface of the skin and transfers heat. Once it reaches the surface of the skin, the heat is released through convection, evaporation, and radiation.

14-8 Heat Transfer: Radiation

The most familiar example of radiation is our own Sun, which radiates at a temperature of almost 6000 K.

14-8 Heat Transfer: Radiation

If you are sitting in a place that is too cold, your body radiates more heat than it can produce. You will start shivering and your metabolic rate will increase unless you put on warmer clothing.

15-1 The First Law of Thermodynamics

The change in thermal (internal) energy of a closed system will be equal to the energy added to the system minus the work done by the system on its surroundings.

This is the law of conservation of energy, written in a form useful to systems involving heat transfer.

15-3 Human Metabolism and the First Law

If we apply the first law of thermodynamics to the human body:

we know that the body can do work. If the internal energy is not to drop, there must be energy coming in. It isn’t in the form of heat; the body loses heat rather than absorbing it. Rather, it is the chemical potential energy stored in foods.

15-3 Human Metabolism and the First Law

The metabolic rate is the rate at which internal energy is transformed in the body.

15-4 The Second Law of Thermodynamics – Introduction

The absence of the process illustrated above indicates that conservation of energy is not the whole story. If it were, movies run backwards would look perfectly normal to us!

15-4 The Second Law of Thermodynamics – Introduction

The second law of thermodynamics is a statement about which processes occur and which do not. There are many ways to state the second law; here is one:

Heat can flow spontaneously from a hot object to a cold object; it will not flow spontaneously

from a cold object to a hot object.

15-5 Heat Engines

It is easy to produce thermal energy using work, but how does one produce work using thermal energy?

This is a heat engine; mechanical energy can be obtained from thermal energy only when heat can flow from a higher temperature to a lower temperature.

15-5 Heat Engines

We will discuss only engines that run in a repeating cycle; the change in internal energy over a cycle is zero, as the system returns to its initial state.

The high temperature reservoir transfers an amount of heat QH to the engine, where part of it is transformed into work W and the rest, QL, is exhausted to the lower temperature reservoir. Note that all three of these quantities are positive.

15-5 Heat Engines A steam engine is one type of heat engine.

15-5 Heat Engines

The internal combustion engine is a type of heat engine as well.

15-5 Heat Engines

The efficiency of the heat engine is the ratio of the work done to the heat input:

Using conservation of energy to eliminate W, we find:

(15-4a)

(15-4b)

15-5 Heat Engines

For an ideal reversible engine, the efficiency can be written in terms of the temperature:

(15-5)

From this we see that 100% efficiency can be achieved only if the cold reservoir is at absolute zero, which is impossible.

Real engines have some frictional losses; the best achieve 60-80% of the Carnot value of efficiency.

15-6 Refrigerators, Air Conditioners, and Heat Pumps

These appliances can be thought of as heat engines operating in reverse.

By doing work, heat is extracted from the cold reservoir and exhausted to the hot reservoir.

15-6 Refrigerators, Air Conditioners, and Heat Pumps

15-8 Order to Disorder

Entropy is a measure of the disorder of a system. This gives us another statement of the second law of thermodynamics:

Natural processes tend to move toward a state of greater disorder.

Example: If you put milk and sugar in your coffee and stir it, you wind up with coffee that is uniformly milky and sweet. No amount of stirring will get the milk and sugar to come back out of solution.

15-8 Order to Disorder

Another example: when a tornado hits a building, there is major damage. You never see a tornado approach a pile of rubble and leave a building behind when it passes.

Thermal equilibrium is a similar process – the uniform final state has more disorder than the separate temperatures in the initial state.

15-7 Entropy and the Second Law of Thermodynamics

Definition of the change in entropy S when an amount of heat Q is added:

(15-8)

Another statement of the second law of thermodynamics:

The total entropy of an isolated system never decreases.

15-9 Unavailability of Energy; Heat Death

Another consequence of the second law:

In any natural process, some energy becomes unavailable to do useful work.

If we look at the universe as a whole, it seems inevitable that, as more and more energy is converted to unavailable forms, the ability to do work anywhere will gradually vanish. This is called the heat death of the universe.