Energy

Student Expectations

The student is expected to understand energy and its forms, including kinetic, potential, chemical, and thermal energies AND understand the Law of Conservation of Energy and the processes of thermal energy transfer.

Key Concepts

• • Matter may contain one or more types of energy, where energy is defined as the ability to do work. Two of the most basic types of energy include kinetic and potential energy. Kinetic energy, or the energy of motion, is based on the movement of an object or of a substance. Potential energy, known as the energy of position, is either stored energy based on the composition of a substance or the position of an object in space.

  • Chemical energy is a form of potential energy as it is the energy stored in the bonds between the atoms of molecules and ionic crystals. Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction, or the ability of a system (the chemical reaction) to do work during chemical reactions. During chemical reactions, energy is transferred, either producing thermal energy or requiring thermal energy.

  • Thermal energy 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. Temperature is a measure of the average KE within the system where heat is the movement of thermal energy from one substance to another.

  • The law of conservation of energy states that energy can neither be created nor destroyed, but instead is transferred between a system and its surroundings. This energy transfer often occurs in the form of heat flow. Heat, or the movement of thermal energy from a warmer substance to a cooler substance, flows between a system and its surroundings, and can be quantified by measuring the change in temperature of each system.

  • Thermal energy may be transferred in one of three basic ways. Conduction is the transfer of thermal energy by direct surface to surface contact. Convection is the transfer of thermal energy due to the circulation of fluids. Radiation is the transfer of thermal energy due to electromagnetic waves.

ENERGY

Types of Energy

Energy is defined as the ability to do work. Matter may contain one or more types of energy. Two of the most basic types of energy include kinetic and potential energy. Matter may also contain chemical energy (a form of potential energy) or thermal energy (energy that relates to kinetic energy), or a any combination of the above. Within a system, forms of kinetic energy can be transferred to forms of potential energy, and vice versa. The total amount of energy of the system, however, remains the same.

• • Kinetic energy, or the energy of motion, is based on the movement of an object or of a substance. The equation for kinetic energy is as follows:

KE = mv²

where m is mass and v is velocity. In other words, the kinetic energy of any object in motion is directly proportional to the objects mass and velocity. Large objects moving quickly have more kinetic energy than a smaller object moving at the same rate.

• • Potential energy, known as the energy of position, is stored energy based on the composition of a substance or on the position of an object in space. The term potential energy is a very general term, as it exists in all force fields including gravitation fields, electrostatic fields, and magnetic fields. Potential energy actually refers to the energy any object gains due to its position relative to a force field.

  • Chemical energy is a form of potential energy. It is the energy stored in the bonds between the atoms of molecules and ionic crystals. Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction. In other words, it is the ability of a system (the chemical reactants) to do work during chemical reactions.

  • Energy is always transferred during chemical reactions, either producing thermal energy or requiring thermal energy. For instance, the process of photosynthesis is a chemical reaction that occurs in plants. In this chemical reaction, specialized cells called chloroplasts absorb solar energy from the sun. This energy is then used as chemical energy to transform water and carbon dioxide into oxygen gas and sugar.

  • Thermal energy is the total internal energy that a substance possesses as well as the total kinetic energy of the particles in a system. Thermal energy depends on both the temperature of a system. It is very important to note that heat and temperature are not the same thing. Temperature is a measure of the average kinetic energy of the particles within the system, where heat is the movement of thermal energy from one substance to another.

  • For example, if you order a turkey sandwich in a restaurant, it contains a certain amount of thermal energy. If you ask to have your sandwich heated in a microwave oven, the returned sandwich will have a higher temperature than before. The microwave caused the particles within the sandwich to move faster, which increased the average kinetic energy of the particles within the sandwich, which then increased the temperature of the sandwich. As the KE of the particles of the sandwich increased, the thermal energy of the turkey sandwich also increased.

The Law of Conservation of Energy

The law of conservation of energy, also known as the first law of thermodynamics, states that energy can be neither created nor destroyed, but instead is transferred between a system and its surroundings.

This energy transfer often occurs in the form of heat flow. Heat, or the movement of thermal energy from a warmer substance to a cooler substance, flows between a system and its surroundings. Heat flow can be quantified by measuring the change in temperature of each system. As one of the fundamental principles in nature, the law of conservation of energy has many different expressions in different fields. For instance, the Bernoulli equation is the corresponding expression in flowing fluid. Also, Kirchhoff’s voltage law is the statement of the law of conservation of energy in the field of an electrical circuit.

The Transfer of Thermal Energy

The transfer of thermal energy is known as heat. The flow of thermal energy from one object or system to another is known as heat flow. The transfer of thermal energy, or heat flow, may occur in one of three basic ways.

• • Conduction is the transfer of thermal energy by direct surface-to-surface contact. Heat flows 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, thermal energy will be transferred from the metal to your skin.

  • Convection is the transfer of thermal energy due to the circulation of fluids. 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. The circulation creates convection currents. An example of convection is the process that occurs when water boils.

  • 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’s ability to transfer thermal energy in the form of heat to the Earth.