Chapter 12: Thermal Energy

Secton 1: Temperature and Thermal Energy

Thermal Energy

Thermal Energy and States of matter

What makes a Hot Body Hot?
Although the effects of fire have been known since ancient times, only in the eighteenth century did scientists begiin to understand how a hot body differs from a cold body. They proposed that when a body was heated, an invinsible fluid called " Caloric" was added to the body. Hot bodies contained more caloric than cold bodies. The Caloric Theory could explain observatons such as the expansion of obejects when heated,but it could not explain why hands get warm when they are rubbed together.

A new theory was developed by scientists, this theory is based on the assumption that matter is made up of many tiny particles that are always in motion. In a hot body, the particles move faste, and thus have a higher energy than particlesin a cooler body. The theory is called the Kinetic-molecular theory.

The sum of the kinetic and potential energies of the internal motion of particles that make up an object is called the internal energy or thermal energy of that object.

Thermal Energy Transfer:

Thermal energy is transferred in three ways
  • Conduction= most common in solids, involves transfer of kinetic energy when the particles of an object collide.

  • Convection= The movement of fluids caused by their diffrent densities at different temperatures transfers heat.

  • Radiation= Theraml energy can be transferred through space in the form of electromagnetic waves.

  • Thermal Energy and Temperature:

    Hotness, measured on some definite scale, is a property of an object called it's temperature.

    The thermal energy in an object is proportional to the number of particles in it, but it's temperature is not.

    Equilibrium and Thermometry:

    The thermometer is made of a glass tube. When the cold glass touches your hotter body, the particles in your body hit the particles in the glass. These collisions transfer energy to the glass particles by conduction. The thermal energy of the particles that make up the thermometer increases. As the particles in the glass become more energetic, they begin to transfer enrgy back tot he particles in your body. At some point, the rate of the transfer of energy back and forth between the glass and your body is equal. Your body and the thermometer are in thermal equilibrium.

    A thermometer is a device to measure temperature. It is placed contact with an object and allowed to come to thermal equilibrium with the object.

    Temperature Scales;Celcius and Kelvin:

    Temperature scales were developed by scientists to allow them to compare their temperature measurements with those of other scientists. A scale based on the properties of water was devised in 1741 by the Swedish astronomer and physicist Anders Celsius (1704-1744).

    There can be no lower temperature than -273.15 degrees Celcius. This is called absolute zero.

    The Kelvin temperature scale is based on absolute Zero. Absolute zero is the zero point of the kelvin scale.

    Kelvin is equal to one celcius degree.


    C + 273.15 = K

    Heat and Thermal Energy:

    Heat is thermal energy transferred because of a diffrence in temperature. Heat flows spontaneously from a warmer to a cooler body.

    Heat is the energy that flows as a result of a difference in temperature.

    Q is the symbol for heat.

    Heat, like any other form of energy, is measured in joules.

    The amount of increase depends on the size of the size of the object. It also depends on the material from which the object id made. The specific heat of a material is the amount of energy that must be added to raise the temperature of a unit mass one temperature unit.


    Q = mC(delta)T
    delta is in the symbol of a little triangle
    Q is the heat gained or lost, m is the mass of the object, C is the specific heat of the substance, and delta T is the change in temperature.

    Calorimetry: Measuring Specific Heat:

    A carolimeter is a device used to measure changes in thermal energy. A measured mass of a substance is heated to a known temperature and placed in the calorimeter. The calorimeter contains aknown mass of cold water at temperature. for the resulting increse in water temperature, the change in thermal energy of the substance is calculated.

    The calorimeter depends on the conservation of energy is isolated, closed systems.

    The total energy of the system is constant;

    Ea + Eb = constant

    The change in thermal energy is equal to the heat transferred:


    Section 2: Change of States and Laws of Thermodynamics

    The branch of physics is called thermodynamics, it explores the properties of thermal energy.

    Change of State:

    The three most common states of matter are solids, liquids, and gases

  • Melting Point> When a substance is melting, added thermal energy increases the potential energy of particlers, breaking the bonds holding them together. The added thermal energy does not increase the temperature.

  • While a substance is melting, added energy does not increase the temperature.

  • Heat Fusion> The amount of energy needed to melt one kilogram of a substance is called heat fusion of that substance.

  • Boiling Point> At a specific temperature, known as the higher boiling point, further addition of energy causes another change of state. It does not raise the temperature; it converts particles in the liquid acquire enough energy to break free from other particles in the vapor or gaseous state.

  • Heat of Vaporization> is the energy needed to vaporize one kilogram of a substance.

  • Q which equals the heat, equals the mass (m) times the heat of fusion.


    Q = mHf

    Q which is the heat equals the mass (m) times the heat of fusion.


    Q = mHv
    Q which is equal to heat, equals the mass (m) times the heat of vaporization.

    The First Law Of Thermodynamics:
    Thermal energy can be incresed either by adding heat or by doing work on a system. Thus, the total increase in the thermal energy of a system is the sum of the work done on it and the heat added to it. This is the study of the changes of the thermal properties of matter. The first law is merely a restaement of the law of conservation of energy.

    The conversion of mechanical energy to thermal energy, as when you rub your hands together, is easy. The reverse process, conversion of thermal to mechanical energy continuously is called a heat engine.

    A heat engine accepts heat from a high temperature source, performs work, and transfers heat out at a low temperature.

    The Second Law of Thermodynamics:

    In the 19th century, the french engineer Sadi Carnot studied the ability of engines to convert heat to mechanical energy. He developed a logical proof that even an ideal engine would generate even more waste heat. Carnot's result is best described in terms of a quantity called entropy.

    Entropy, like thermal energy, is contained in an object. If heat is added to a body, entropy is increased. If heat is removed from a body, entropy is decreased.

    The second law of thermodynamics states that natural processes go in a direction that increases the total entropy of the universe. Entropy and the second law can be thought of as statements of the probability of events happening.

    The second law predicts that heat flows spontaneously only from a hot body to a cold body.

    The second law and entropy also give new meaning to what is commonly called the "energy crisis".

    Chapter 13: States of Matter

    Section 1: The Fluid States

    A fluid is any material that flows and offers little resistance to a change in its shape when under pressure. Both liquids and gases are fluids.<.p>


    The Kinetic-molecular theory is based on three simplified assumptions.

    Gases are made up of large number of very small particles. The particles are in constant, random motion. They are widely seperated and make only elastic collisions with one another. The particles make perfectly elastic collisions with the walls of the container that holds them.

    The kinetic theory is a microscopic theory.

    Pressure is the force on each unit area of a surface.

    In the SI system, the unit of pressure is the pascal, Pa, one newton per square.

    Fluids at Resty -Hydrostatics:

    Blaise Pascal, a french physician, noted that the shape of a container had no affect on the pressure at any given depth. He was the first to discover that any change in pressure applied to a confined fluid at any point is transmitted undiminished throughout the fluid. This discovery became known as Pascal's principle.


    F1= F2
    A1 A2

    The buoyant force is equal to the weight of the fluid displaced.

    Archimedes' Principle states that an object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. It is important to note that the buoyant force does not depend on the weight on the submerged object, only the weight of the displaced fluid.


    F(buoyant) = P(water)Vg
    Wapparent = mg - F(buoyant)

    Fluids in Motion-Hydrodynamics:

    The relationship between the velocity and pressure Bernoulli's Principle which states as the velocity of a fluid increases, the pressure exerted by that fluid decreases.

    Liquid vs Gases:

    Although liquids and gases are grouped together as fluids, liquids are different from gases in several ways.

    A liquid has a definite value;a gas takes the volume of any container that holds it.

    A liquid is practically incompressible; a gas is easily compressed.

    The particles of a liquid are very close together-the volume of the particles makes up almost all the volume of a liquid; the particles of a gas takes up takes up relatively liitle space-a container of gas is mostly empty space.

    Although the particles of an ideal liquid are totally free to slide over and around one another, in real liquids the particles do exert electromagnetic forces of attraction on eachother.These forces are called cohesive forces, and they directly affect the behavior of a liquid.

    Surface Tension:

    Surface tension is a result of the cohesive forces among the particles of a liquid to contract to the smallest area.

    A force similar to cohesion is adhesion. Adhesion is the attractive force that acts between particles of different substances. Like cohesive forces, adhesive forces are electromagnetic in nature.

    Evaporation and Condensation:

    Evaporation cools the remaining liquid, and is also the escape of particles.

    A liquid that evaporates quickly is called a volatile liquid.

    If a molecule looses enough energy, the cohesive force will be strong enough to prevent the particle's escape. This process is called condensation.


    The gaslike state of negatively-charged electrons and positively-charged ions is called a plasma. Plasma is another fluid state of matter.

    Section 2: The Solid State

    Solid Bodies:

    The particles in a true solid are fixed in a crystal lattice.

    A substance that has no regular crystal structure but have definite volume and shape are called amorphous solids.

    Elasticity of Solids:


    is the ability of an object to return to its original form after external forces are removed.

    If too much deformation occurs, the object will not return to its original shape-its elastic limit has been exceeded.

    Elasticity depends on the electromagnetic forces that hold the particles of a substance together.

    Thermal Expansion of Matter:

    Most materials expand when heated and contract when cooled. This property is known as thermal expansion.

    The length, L, of a solid as temperature T is given by;



    The dimensions of a are

    length unit
    (length unit/temp. unit)

    References and Links

    Merrill,Physics Principles and Problems;Textbook published by Paul W. Zitewitz, Mark Davis, and Robert F. Neff

    Renewable Energy;Edited by Godfrey Boyle

    Physics Principles and Problems by Zitewtz, Elliot Haase, Harper, Herzrog, Nelson, Schuler, Zorn

    Physics 5th Revised Edition; by Douglas C. Giancoli Stages of matter Chapter 10. Page 275

    This book provides a comprehensive overview of the principal types of renewable energy-including solar, thermal photovoltaics, bioenergy, hydro, tidal, wind, wave, and geothermal. In addition, the text explains the underlying physical and technological principles of renewable energy and examines the environmental impact and future prospects of different energy sources.

    Articles of Thermal Energy

    PF online feature Article-Thermal Energy Method

    Thermal Energy-Wikipedia Encyclopedia

    Equiparition of Energy