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- Physics LibreTexts - Conservation of Energy (Oct. 17, 2026)
conservation of energy
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What does "conservation of energy" mean in physics?
In physics, the principle of conservation of energy states that within a closed system of interacting bodies or particles, the total energy remains constant. Energy transforms between forms such as kinetic, potential, and thermal energy, without loss or gain.
The concept extends to include various forms like electric current, electromagnetic fields, chemical energy, and mass equivalence as per relativity physics. The first law of thermodynamics expresses this principle, asserting energy is neither created nor destroyed, only converted.
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Why is energy called a conserved quantity?
Energy is called a conserved quantity because, within an isolated system, the total energy remains constant over time. This principle, known as the law of conservation of energy, dictates that energy cannot be created or destroyed but can only be transformed from one form to another. For instance, potential energy can convert to kinetic energy, and vice versa, with the total energy of the isolated system remaining unchanged. This principle extends to include thermal energy, chemical energy, and mass-energy equivalence as described by relativity physics.
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How can energy change from one form to another?
Energy can change from one form to another through various processes. The principle of conservation of energy states that energy is not created or destroyed, but merely changes form.
For example, a car moves when the chemical energy in its gasoline is converted into kinetic energy of motion. When a block slides down a slope, potential energy converts into kinetic energy, and when friction slows it down, kinetic energy converts into thermal energy. Solar radiation can also be converted directly into electric power by solar cells.
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Can energy ever be created or destroyed?
The principle of conservation of energy in physics states that within a closed system, the total amount of energy remains constant, though it may change forms. Energy can be converted from one form to another, such as potential to kinetic or kinetic to thermal energy, but the total energy within the system does not increase or decrease. This principle is also known as the first law of thermodynamics.
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How does the conservation of energy help us understand real-world situations, like a moving car or a falling ball?
The law of conservation of energy states that energy is neither created nor destroyed, but changes from one form to another.
When a car moves, the chemical energy in gasoline converts to the kinetic energy of motion. As a ball falls, its potential energy converts to kinetic energy, increasing its speed. At the top of a pendulum's swing, kinetic energy becomes potential energy, reversing as it swings down.
Accounting for all forms of energy, including kinetic, potential, and thermal energy, shows that the total energy in a closed system remains constant.
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What are some practical examples or experiments that show the conservation of energy in action?
The conservation of energy principle states that in a closed system, the total energy remains constant, though it may change forms.
A simple demonstration is a swinging pendulum. As the pendulum swings upward, its kinetic energy converts to potential energy, and at the top of its swing, the energy is all potential. As it swings down, potential energy converts back to kinetic energy.
Another example is a roller coaster. The initial climb gives the coaster potential energy, which is converted to kinetic energy as it descends, providing the thrilling ride.
When a ball is tossed up into the air, its kinetic energy decreases as it gains gravitational potential energy. As the ball falls back to the ground, its potential energy is converted back into kinetic energy.
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conservation of energy, principle of physics according to which the energy of interacting bodies or particles in a closed system remains constant. The first kind of energy to be recognized was kinetic energy, or energy of motion. In certain particle collisions, called elastic, the sum of the kinetic energy of the particles before collision is equal to the sum of the kinetic energy of the particles after collision. The notion of energy was progressively widened to include other forms. The kinetic energy lost by a body slowing down as it travels upward against the force of gravity was regarded as being converted into potential energy, or stored energy, which in turn is converted back into kinetic energy as the body speeds up during its return to Earth. For example, when a pendulum swings upward, kinetic energy is converted to potential energy. When the pendulum stops briefly at the top of its swing, the kinetic energy is zero, and all the energy of the system is in potential energy. When the pendulum swings back down, the potential energy is converted back into kinetic energy. At all times, the sum of potential and kinetic energy is constant. Friction, however, slows down the most carefully constructed mechanisms, thereby dissipating their energy gradually. During the 1840s it was conclusively shown that the notion of energy could be extended to include the heat that friction generates. The truly conserved quantity is the sum of kinetic, potential, and thermal energy. For example, when a block slides down a slope, potential energy is converted into kinetic energy. When friction slows the block to a stop, the kinetic energy is converted into thermal energy. Energy is not created or destroyed but merely changes forms, going from potential to kinetic to thermal energy. This version of the conservation-of-energy principle, expressed in its most general form, is the first law of thermodynamics. The conception of energy continued to expand to include energy of an electric current, energy stored in an electric or a magnetic field, and energy in fuels and other chemicals. For example, a car moves when the chemical energy in its gasoline is converted into kinetic energy of motion.
With the advent of relativity physics (1905), mass was first recognized as equivalent to energy. The total energy of a system of high-speed particles includes not only their rest mass but also the very significant increase in their mass as a consequence of their high speed. After the discovery of relativity, the energy-conservation principle has alternatively been named the conservation of mass-energy or the conservation of total energy.
When the principle seemed to fail, as it did when applied to the type of radioactivity called beta decay (spontaneous electron ejection from atomic nuclei), physicists accepted the existence of a new subatomic particle, the neutrino, that was supposed to carry off the missing energy rather than reject the conservation principle. Later, the neutrino was experimentally detected.
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Energy conservation, however, is more than a general rule that persists in its validity. It can be shown to follow mathematically from the uniformity of time. If one moment of time were peculiarly different from any other moment, identical physical phenomena occurring at different moments would require different amounts of energy, so that energy would not be conserved.