Conservation of EnergyStudent Guide

All matter possesses energy, and there are various types of energy that matter may possess. This energy can be transferred from one form of matter to another, or from a system to its surroundings, and vice versa. The Law of Conservation of Energy states that when energy is transferred, it is neither created nor destroyed in an ordinary chemical or physical process. During the energy transfer, the starting energy must always equal the ending energy.

So what is energy? Simply put, it is the ability to do work. Matter may contain one or more types of energy. It may contain kinetic energy (KE), or the energy of motion. It may also contain potential energy (PE), or the energy of position.

Think of a child swinging on a swing. When the child is at the top of their swing (in either direction), there is a moment when motion stops in the upward direction, and the child and swing are not moving at all. At this peak of the swing, the child has no kinetic energy, but a maximum of potential energy. As the child begins to swing in the opposite direction, the kinetic energy of the child increases and the potential energy decreases. When the child reaches the bottom of the swing, the child has maximum kinetic energy and no potential energy. As the child begins to move upward, the kinetic energy decreases as the potential energy increases until the child reaches the top of the swing in the other direction.

Both potential (PE) and kinetic energy (KE) are involved in other types of energy transfers. Chemical energy is a form of PE, in that chemical energy is the energy stored in molecules and ionic crystals, and it is the potential of a chemical substance to undergo a transformation through a chemical reaction. In other words, chemical energy is the ability of a system (the chemical reactants) to do work during chemical reactions. During chemical reactions, energy is transferred, either producing thermal energy or requiring thermal energy. In other words, chemical reactions will either absorb or release energy, usually in the form of heat and/or light. Thermal energy is another form of energy that matter may have. It is the total internal energy that a substance possesses as well as the total KE of the particles in a system. It depends on the temperature of that system and relates to both heat and temperature; though it is critical to note that heat and temperature are not the same thing!

Temperature is the "hotness" or "coldness" of matter. It is not actually energy, but a measure of the average KE within the system. The faster the molecules move, then the "hotter" the substance will be. In other words, the energy (heat) added to the substance will increase the kinetic energy of the molecules of the substance (temperature).

Heat, on the other hand, is actually energy and is the movement of thermal energy from one substance to another. It flows from a system to its surroundings and can be measured by the change in temperature of each system. Heat always flows from warmer temperatures to colder temperatures.

Thermal energy may be transferred through three different methods. Conduction is the transfer of heat through direct contact. Thermal energy is transferred from a warmer object to a cooler object until equilibrium is reached. For example, metal is a very good conductor of heat. If you touch a piece of hot metal, heat will be transferred from the metal to your skin.

Convection is a method of heat transfer that occurs in liquids and gases. The warmer liquid or gas will rise to take the place of the cooler liquid or gas. The cooler substance then moves to the bottom and the cycle is repeated continuously. An example of convection is the process of boiling water. Radiation is the transfer of thermal energy by electromagnetic waves through empty space. When the wave comes in contact with a substance, it transfers thermal energy to the substance. An example of radiation is the Sun transferring thermal energy to Earth.

The change in KE of the particles can lead to a change of state of a substance as the molecular movement either increases or decreases. When thermal energy is added to a substance, the added energy will increase the KE of the individual particles.

For example, when water changes from a solid state (ice) to a liquid state (water), heat (or the transfer of thermal energy) moves from the surroundings to the ice, increasing the KE of the molecules. The temperature of the ice will then increase until a phase change, where the ice changes from a solid to a liquid. During the change of state process, where the water goes from the solid form to the liquid form, the temperature of the substance does not increase. Why not?

When thermal energy is added to the ice, the KE of the molecules will increase, increasing the temperature of the ice. At the state change, however, the average KE of the ice is the same as the average KE of the water, and they are therefore at the same temperature. The added energy in the form of heat is no longer increasing the KE of the molecules, but is instead used to break the bonds between the molecules. As the ice melts, energy is being absorbed into the system. If the water were solidifying (turning into ice), then energy would be released out of the system. A temperature-energy graph highlights the heat flow of a pure substance as it changes states.

Answer these questions:

1. Define kinetic energy (KE) and potential energy (PE).________________________________________________________________________________________________________________________________________________________

2. Briefly describe how the KE and PE change as a child swings on a swing.________________________________________________________________________________________________________________________________________________________

3. What is chemical energy, and where is it stored?________________________________________________________________________________________________________________________________________________________

4. What is thermal energy, and what does it depend on? Be specific.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

5. What is the difference between heat and temperature?________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

6. Briefly describe what occurs as a substance changes states from a solid to a liquid.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________

7. Give an example of each type of heat transfer: conduction, convection, and radiation.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________