# Chapter 5: Energy

## Section 1: The Nature of Energy

• What is energy?

• Every change that occurs—large or small—involves energy.

• Energy: the ability to do work

• Work is done when a force causes something to move.

• Energy also can be defined as the ability to cause change.

• When work is done and a change occurs, energy moves from place to place or changes from one form to another.

• Energy has several different forms—electrical, chemical, radiant, and thermal.

• Energy from the Sun that warms you and energy from the food that you eat are only different forms of the same thing.

• Kinetic Energy: the energy a moving object has because of its motion.

• The kinetic energy of a moving object depends on the object’s mass and its speed.

• Kinetic Energy Equation: kinetic energy (in joules) = 1/2 mass (in kg) X [speed (in m/s)] ^2

• Joule: The SI unit of energy

• Potential Energy: Stored energy due to position

• Energy doesn’t have to involve motion. Even motionless objects can have energy.

• Elastic Potential Energy: energy stored by something that can stretch or compress, such as a rubber band or spring.

• Chemical Potential Energy: Energy stored in chemical bonds

• Gravitational Potential Energy: Energy stored by objects due to their position above Earth’s surface.

• Gravitational Potential Energy Equation: gravitational potential energy (J) = mass (kg) X acceleration due to gravity (m/s^2) X height (m)

## Section 2: Conservation of Energy

• Changing Forms of Energy

• You use many devices every day that convert one form of energy to other forms.

• A light bulb is a device that transforms electrical energy into light energy and thermal energy.

• Fuel stores energy in the form of chemical potential energy.

• When a spark plug fires in an engine, chemical potential energy is converted into thermal energy.

• Some energy transformations are less obvious because they do not result in visible motion, sound, heat, or light.

• Conversions Between Kinetic and Potential Energy

• You have experienced many situations that involve conversions between potential and kinetic energy.

• Mechanical Energy: the total amount of potential and kinetic energy in a system

• mechanical energy = potential energy + kinetic energy

• Energy transformations also occur during projectile motion when an object moves in a curved path.

• Objects that can fall have gravitational potential energy.

• The Law of Conservation of Energy: states that energy cannot be created or destroyed.

• The law of conservation of energy is a universal principle that describes what happens to energy as it is transferred from one object to another or as it is transformed.

• In a swing, mechanical energy is transformed into thermal energy because of
friction and air resistance.

• A special kind of energy conversion—nuclear fusion—takes place in the Sun and other stars. During this process a small amount of mass is transformed into a tremendous amount of energy.

• Another process involving the nuclei of atoms, called nuclear fission, converts a small amount of mass into enormous quantities of energy.

• The process of nuclear fission is used by nuclear power plants to generate electrical energy.

• Mass is converted to energy in the processes of fusion and fission.

• The Human Body - Balancing the Energy Equation

• The complex chemical and physical processes going on in your body also obey the law of conservation of energy.

• Your body stores energy in the form of fat and other chemical compounds. This chemical potential energy is used to fuel the processes that keep you alive, such as making your heart beat and digesting the food you eat.

• Your body has been busy breaking down your breakfast into molecules that can be used as fuel. The chemical potential energy in these molecules supplies the cells in your body with the energy they need to function.

• Your body also can use the chemical potential energy stored in fat for its energy needs.