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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. Plants capture light energy from the Sun, and, via 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. Therefore, since all changes of matter involve changes in energy, it is vital to understand how energy flows from one form to the other. 

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 the 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 put to work to generate electricity in a hydroelectric power plant.

Potential Energy

Potential energy is also known as energy at rest or stored energy. Common types of potential energy include the gravitational potential energy stored in an apple hanging from a tree,  the electrical potential energy stored in an object due to the attraction or repulsion of electric charges, or the chemical potential energy stored in the bonds between atoms and molecules. Additionally, the nuclear energy stored in an atomic nucleus and the elastic energy stored in a stretched spring due to its configuration are types of potential energy.

Usually, objects or systems with high potential energy tend to be less stable, and thus move towards lower energy levels to attain stability. For instance, the radioactive element Uranium-235 (U²³⁵) has an unstable nucleus. To gain stability, it splits into smaller but stable elements and releases the stored nuclear energy. This released energy can then be used to generate electricity in nuclear power plants. 

Kinetic Energy

The amount of kinetic energy of an object depends on its mass and speed. Consider two balls of different masses rolling down an inclined plane at the same speed. The heavier ball will have more kinetic energy. Similarly, when two balls of the same mass roll down an inclined plane at different speeds, the ball that moves faster has more kinetic energy. 

Different forms of kinetic energy also exist, including mechanical, electrical, radiant, sound, and thermal energy. Mechanical energy is associated with the motion of an object. The faster an object moves, the more mechanical energy it has.  For example, a bullet fired from a gun or water flowing down a dam are examples of mechanical energy. Electrical energy is attributed to the flow of electric charges, as observed in the case of lightning strikes during thunderstorms or the everyday electrical circuits and devices. Radiant energy is the form of kinetic energy that travels as electromagnetic waves and can be experienced in the form of light and heat. Sunlight is an example of radiant energy.

Thermal energy is associated with the random motion of atoms and molecules. When the atoms and molecules in an object move or vibrate quickly, they have a higher average kinetic energy (KE), and the object is said to be “hot.” When the atoms and molecules are moving slowly, they have lower average KE, and the object is designated as “cold.” Thus, thermal energy can be observed through the temperature changes of an object. Assuming that no chemical reaction or phase change (such as melting or vaporizing) occurs, increasing the amount of thermal energy in a sample of matter will cause its temperature to increase. Similarly, assuming that no chemical reaction or phase change (such as condensation or freezing) occurs, decreasing the amount of thermal energy in a sample of matter will cause its temperature to decrease.

The Law of Conservation of Energy

Energy can be converted from one form to another, but the total energy present before a change always exists in some form even after a change. This observation is expressed as the Law of Conservation of Energy. The Law of Conservation of Energy states that energy is neither created nor destroyed, although it can be changed in form. Thus, the total energy of a system remains constant. For example, chemical energy (a type of potential energy) is stored in the molecules that compose gasoline. When gasoline is combusted within the cylinders of a car’s engine, the rapidly expanding gaseous products of this chemical reaction generate mechanical energy (a type of kinetic energy) when they move the cylinder’s pistons.

This text is adapted from Openstax, Chemistry 2e, Section 5.1: Energy Basics.