The universe is composed of matter in different forms, and all forms of matter contain energy. The different forms of energy on Earth originate from the Sun—the ultimate energy source. For instance, plants capture light energy from the Sun, and through the process of photosynthesis, convert it into chemical energy. This stored energy from plants can be harnessed in many ways. For example, eating plant products as food provides energy for our body to function, and burning wood or coal (fossilized plants) generates heat and electricity. Since all changes of matter involve changes in energy, it is vital to understand how energy flows from one form to another.
Energy is defined as the ability to do work. Work is done when a force applied to an object causes the object to move against an opposing force. For example, work is done when a table is pushed across a room against the resistance from the floor.
Energy can be grouped into two main types: potential energy and kinetic energy. Potential energy is the energy associated with the relative position, composition, or condition of an object. Kinetic energy is the energy associated with the motion of an object. For instance, water held behind a dam possesses potential energy due to its position above the ground. When it flows downward through generators, it gains kinetic energy, which can be used to generate electricity in a hydroelectric power plant.
This text is adapted from Openstax, Chemistry 2e, Section 5.1: Energy Basics.