Chapters 9 and 10 will develop the basic energy model, a powerful set of ideas for using energy. A key distinction is between the system, which has energy, and the environment. Energy can be transferred between the system and the environment or transformed within the system.

《 LOOKING BACK Section 7.1 Interacting objects

Three important forms of energy:

• Potential energy is energy associated with an object’s position.

• Kinetic energy is energy associated with an object’s motion.

• Thermal energy is the energy of the random motion of atoms within an object.

Energy is measured in joules.

A process that changes the energy of a system by mechanical means—pushing or pulling on it—is called work.

Work $W$ is done when a force pushes or pulls a particle through a displacement, thus changing the particle’s kinetic energy.

Working with energy is very much like accounting: A system’s energy $E$ changes by the amount of work done on the system. The mathematical statement of this idea is called the energy principle:

Power is the rate at which energy is transferred or transformed. For machines, power is the rate at which they do work. For electricity, power is the rate at which electric energy is transformed into heat, sound, or light. Power is measured in watts, where 1 watt is a rate of 1 joule per second.

Energy is one of the most important concepts in science, engineering, and society. Some would say it is the most important. All life depends on energy, transformed from solar energy to chemical energy to us. Society depends on energy, from industry and transportation to heating and cooling our buildings. Using energy wisely and efficiently is a key concern of the 21st century.