In physics, energy is a property of objects which can be transferred to other objects or converted into different forms. The "ability of a system to perform work" is a common description, but it is difficult to give one single comprehensive definition of energy because of its many forms. For instance, in SI units, energy is measured in joules, and one joule is defined "mechanically", being the energy transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. However, there are many other definitions of energy, depending on the context, such as thermal energy, radiant energy, electromagnetic, nuclear, etc., where definitions are derived that are the most convenient.
Common energy forms include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature. All of the many forms of energy are convertible to other kinds of energy. In Newtonian physics, there is a universal law of conservation of energy which says that energy can be neither created nor be destroyed; however, it can change from one form to another.
For "closed systems" with no external source or sink of energy, the first law of thermodynamics states that a system's energy is constant unless energy is transferred in or out by mechanical work or heat, and that no energy is lost in transfer. This means that it is impossible to create or destroy energy. While heat can always be fully converted into work in a reversible isothermal expansion of an ideal gas, for cyclic processes of practical interest in heat engines the second law of thermodynamics states that the system doing work always loses some energy as waste heat. This creates a limit to the amount of heat energy that can do work in a cyclic process, a limit called the available energy. Mechanical and other forms of energy can be transformed in the other direction into thermal energy without such limitations. The total energy of a system can be calculated by adding up all forms of energy in the system.
Examples of energy transformation include generating electric energy from heat energy via a steam turbine, or lifting an object against gravity using electrical energy driving a crane motor. Lifting against gravity performs mechanical work on the object and stores gravitational potential energy in the object. If the object falls to the ground, gravity does mechanical work on the object which transforms the potential energy in the gravitational field to the kinetic energy released as heat on impact with the ground. Our Sun transforms nuclear potential energy to other forms of energy; its total mass does not decrease due to that in itself (since it still contains the same total energy even if in different forms), but its mass does decrease when the energy escapes out to its surroundings, largely as radiant energy.
Mass and energy are closely related. According to the theory of mass–energy equivalence, any object that has mass when stationary in a frame of reference (called rest mass) also has an equivalent amount of energy whose form is called rest energy in that frame, and any additional energy acquired by the object above that rest energy will increase an object's mass. For example, if you had a sensitive enough scale, you could measure an increase in mass after heating an object.
Living organisms require available energy to stay alive, such as the energy humans get from food. Civilisation gets the energy it needs from energy resources such as fossil fuels, nuclear fuel, or renewable energy. The processes of Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun and the geothermal energy contained within the earth.
The universe is made up of matter and energy. Matter — anything that has mass and takes up space — is pretty straightforward and easy to grasp, but energy is a bit more abstract.
In physics, energy is the ability to do work, or the ability to move or elicit change in matter. In effect, the amount of energy something has refers to its capacity to cause things to happen.
Energy has a few important properties. For one, energy is always "conserved" — it cannot be created or destroyed. It can, however, be transferred between objects or systems by the interactions of forces. For example, the energy in vegetables is transferred to the people who digest them.
Another property of energy is that it comes in multiple forms, and can be converted from one form to another. The two most common or basic forms of energy are kinetic energy and potential energy.
Kinetic energy is the energy of motion. A ball has kinetic energy as it flies through the air — it has the ability to do work in that it can act upon other objects with which it collides.
Potential energy is a kind of stored energy that objects have because of their position or configuration. A cup on a table has potential energy; if you knock the cup off the table, gravity will accelerate the cup, and its potential energy will convert to kinetic energy. A stressed bow also has potential energy.
Many other types of energy exist, including electrical, chemical, thermal, electromagnetic and nuclear.
In the early 20th century, scientists theorized that mass and energy are intimately linked. Albert Einstein described this so-called mass-energy equivalence with his famous equation, E = mc2, in which "E" stands for "energy," "m" denotes "mass" and "c" is the speed of light.
Energy is the capacity of a physical system to do work. The common symbol for energy is the uppercase letter E. The standard unit is the joule, symbolized by J. One joule (1 J) is the energy resulting from the equivalent of one newton (1 N) of force acting over one meter (1 m) of displacement. There are two main forms of energy, called potential energy and kinetic energy.
Potential energy, sometimes symbolized U, is energy stored in a system. A stationary object in a gravitational field, or a stationary charged particle in an electric field, has potential energy.
Kinetic energy is observable as motion of an object, particle, or set of particles. Examples include the falling of an object in a gravitational field, the motion of a charged particle in an electric field, and the rapid motion of atoms or molecules when an object is at a temperature above zero Kelvin.
Matter is equivalent to energy in the sense that the two are related by the Einstein equation:
E = mc2
where E is the energy in joules, m is the mass in kilograms, and c is the speed of light, equal to approximately 2.99792 x 108 meters per second.
In electrical circuits, energy is a measure of power expended over time. In this sense, one joule (1 J) is equivalent to one watt (1 W) dissipated or radiated for one second (1 s). A common unit of energy in electric utilities is the kilowatt-hour (kWh), which is the equivalent of one kilowatt (kW) dissipated or expended for one hour (1 h). Because 1 kW = 1000 W and 1 h = 3600 s, 1 kWh = 3.6 x 106 J.
Heat energy is occasionally specified in British thermal units (Btu) by nonscientists, where 1 Btu is approximately equal to 1055 J. The heating or cooling capability of a climate-control system may be quoted in Btu, but this is technically a misuse of the term. In this sense, the system manufacturer or vendor is actually referring to Btu per hour (Btu/h), a measure of heating or cooling power.
What is energy?
Physicists, who are scientists who study force, motion and energy, say that energy is the ability to do work, and work is moving something against a force, like gravity. There are a lot of different kinds of energy in the universe, and that energy can do different things.
Energy can be found in many things, and takes many forms. There is a kind of energy called kinetic energy in objects that are moving. There is something that scientists call potential energy in objects at rest that will make them move if resistance is removed.
The molecules making up all matter contains a huge amount of energy, as Einstein's E = mc^2 pointed out to us. Energy can also travel in the form of electromagnetic waves, such as heat, light, radio, and gamma rays. Your body is using metabolic energy from your last meal as you read this.