What Changes The Energy Of A System Physics?

Energy is the ability to do work. A system is any collection of matter and energy. The law of conservation of energy states that energy can neither be created nor destroyed within a system, it can only be transferred or changed from one form to another. The goal of this article is to understand the different ways that energy can be transferred or transformed within a system, which changes the total energy of that system.

Kinetic Energy

Kinetic energy is the energy of motion. An object that has motion – whether it is vertical or horizontal motion – has kinetic energy. The amount of kinetic energy depends on the mass and velocity of the object. The greater the mass and velocity, the more kinetic energy the object will have.

Some examples of kinetic energy in everyday life include:

  • A car driving down the road has kinetic energy due to its motion.
  • A baseball being thrown or hit has kinetic energy.
  • Wind has kinetic energy due to the motion of air molecules.
  • Flowing water in a river has kinetic energy.

The key point is that kinetic energy changes with velocity. An object at rest has no kinetic energy. But as the object starts moving, it gains kinetic energy proportional to its mass and the square of its velocity. This is described by the equation:

Kinetic Energy = 1/2 x mass x velocity2

So the faster something moves, the more kinetic energy it has. This is why it’s important to consider the velocity of an object when looking at its kinetic energy.

Potential Energy

potential energy depends on an object's position or arrangement.

Potential energy is the energy an object has due to its position or arrangement. There are several types of potential energy:

  • Gravitational potential energy – This is the energy an object has due to its height above the ground. For example, a book sitting on a shelf has gravitational potential energy. As the book falls, this potential energy gets converted into kinetic energy.
  • Elastic potential energy – This is the energy stored in elastic materials that are stretched or compressed. For example, a stretched rubber band has elastic potential energy. When released, this energy is converted into kinetic energy as the band snaps back.
  • Chemical potential energy – This is the energy stored in the chemical bonds of substances. This energy can be released through chemical reactions like burning. The stored chemical energy in gasoline is converted to kinetic energy to power a car engine.

In all cases of potential energy, the energy is stored and ready to be released and converted into other forms like kinetic energy. Understanding potential energy is key to explaining how energy transfers and transforms in physical systems.


Heat is a transfer of thermal energy between objects or systems due to a temperature difference. When two objects or systems with different temperatures come into contact, thermal energy will spontaneously transfer from the hotter system to the colder system. This occurs because temperature is a measure of the average kinetic energy or molecular motion of the atoms or molecules in a system. Systems with higher temperatures have atoms/molecules that move faster on average.

When a hot system comes into contact with a colder system, the faster moving atoms/molecules in the hot system collide with the slower ones in the cold system. These collisions transfer kinetic energy from the hot system to the cold system, increasing the speed and kinetic energy of the atoms/molecules in the cold system. This increase in molecular motion corresponds to an increase in thermal energy and temperature of the cold system.

Over time, thermal energy will transfer until the two systems reach thermal equilibrium, meaning they are at the same temperature and there is no net heat transfer. Heat flows spontaneously from hot to cold, raising the thermal energy and temperature of colder objects while cooling hotter objects, until equilibrium is reached.


Work is the transfer of energy that occurs when an external force moves an object. The amount of work done is equal to the magnitude of the force times the distance moved in the direction of the force. For example, when you lift a book from the floor up onto a table, you are doing work to raise the book against the force of gravity. The work you do equals the book’s weight times the height you raise it. In physics, work is defined as force multiplied by distance:

Work = Force x Distance

So if you apply a force of 5 Newtons to move an object 3 meters, you’ve done 5 x 3 = 15 Joules of work. The units of work are Joules (J) in the metric system and Foot-pounds in the English system. Work involves transferring energy from one place to another or one form to another. When you lift the book, chemical energy from your body is transferred into gravitational potential energy of the book-Earth system.

In order for work to be done, the force must cause movement or displacement in the direction of the force. If you push on a wall but it doesn’t move, no work is done. The force and distance must be in the same direction for their vectors to multiply. Work can transfer energy into or out of a system. When net work is done on a system, its kinetic or potential energy changes. This work-energy theorem is important because it lets you calculate changes in a system’s energy by calculating just the work. The work-energy theorem shows that the net work on a system equals its change in kinetic energy.

Chemical Reactions

Chemical reactions involve the breaking and formation of chemical bonds between atoms and molecules. These chemical bonds store potential energy. When bonds break, energy is absorbed. When new bonds form, energy is released.

The amount of energy absorbed or released in a chemical reaction depends on the strength of the bonds broken and formed. Strong chemical bonds like those in hydrocarbon fuels or explosives release large amounts of energy when broken. Weak bonds require less energy to break and release less when formed.

Exothermic reactions involve net energy release as the energy from new bond formation exceeds the energy needed to break existing bonds. This excess energy is given off as heat, light, or motion. Examples include combustion, metabolism, and acidic reactions with metals.

Endothermic reactions absorb net energy as more energy is required to break existing bonds than is released when new ones form. These reactions feel cold as they draw in thermal energy from the surroundings. Examples include photosynthesis, dissolving ammonium nitrate in water, and thermal decomposition reactions.

The energy changes in chemical reactions follow the law of conservation of energy. The total energy in the system remains constant. Chemical reactions just change the form of energy from stored chemical potential energy to heat, light, motion, or new chemical bonds.

Nuclear Reactions

Nuclear reactions such as fission and fusion involve changes in the nuclei of atoms. In these reactions, a small amount of mass is converted into a very large amount of energy according to Einstein’s famous equation E=mc2.

In nuclear fission, a heavy unstable nucleus like uranium-235 breaks apart into two smaller nuclei, releasing neutrons, gamma rays, and energy. Nuclear power plants use the heat released from fission reactions to produce electricity. The fission of 1 gram of uranium-235 releases about 22.5 million kilojoules of energy, which is equal to the kinetic energy of a baseball pitched at 100 km/h!

Nuclear fusion occurs when two light atomic nuclei fuse together to form a heavier nucleus, converting some of the mass into energy. Fusion powers the sun and other stars as hydrogen nuclei fuse into helium. Scientists are researching fusion as a future energy source on Earth, but a sustained fusion reaction requires immense pressure and heat, making it very difficult to achieve. Just a few kilograms of fusion fuel can provide as much energy as 10,000 tons of fossil fuels.

In both fission and fusion reactions, mass is converted directly into nuclear potential energy according to E=mc2. Even tiny amounts of mass loss, when converted by this equation, release tremendous amounts of useful energy. Nuclear processes are the most concentrated forms of energy available.


Friction is a force that occurs when two surfaces rub against each other. The friction between two surfaces converts the kinetic energy of motion into thermal energy, or heat. Here’s how it works:

When two surfaces are in contact and moving relative to one another, the irregularities on their surfaces “grip” each other through adhesive and cohesive forces at the microscopic level. This grip provides resistance to the surfaces sliding, which we experience as friction.

To keep the surfaces in motion, an external force must be applied to overcome this frictional force. The work done by the external force is converted into heat. The faster the surfaces move, the stronger the frictional force becomes. This causes more kinetic energy to be converted into thermal energy.

The heating caused by friction results in an increase in the temperature and internal energy of both surfaces. The thermal energy produced can be observed in examples like rubbing hands together to warm them up. This heating effect is why friction is useful in applications like brakes and matches.

In summary, friction is a mechanism that transforms the kinetic energy of motion into thermal energy whenever two surfaces rub against each other. This energy transformation plays an important role in mechanical systems by producing heat and slowing motion.


Sound is a form of energy that is transmitted through vibrations of matter. When an object vibrates, it causes the particles or molecules around it to vibrate as well. These vibrations create waves of energy that travel through a medium like air, water, or solid objects.

As an object vibrates, it alternately compresses and rarefies the medium near it. Compressing the medium pushes the particles closer together while rarefying creates a region of lower pressure by spreading the particles apart. The regions of compression and rarefaction travel outward from the vibrating object in a wave-like pattern. The frequency of the vibrations determines the frequency of the sound wave and corresponds to the pitch we perceive.

The propagating sound wave carries energy through the medium. As the particles vibrate back and forth, they transfer some of their kinetic energy to neighboring particles. The alternation between compressed and rarefied regions also represents potential energy being stored and released. The energy is transmitted forward with limited actual motion of the particles themselves. This allows sound to travel distances without mass being displaced.

The properties of the medium affect how sound travels through it. Denser mediums enable sound to propagate faster and farther. The elasticity, which affects how well particles transmit vibrations, also impacts the speed of sound. When a sound wave encounters a new medium, part of it can be reflected at the boundary while the rest is transmitted through to the new material. Sound can only travel through matter in a medium, it cannot propagate through a vacuum.


Light can transfer energy in two main ways. First, light carries radiant energy that can be absorbed by matter. When light is absorbed, its energy is transformed into other forms like thermal energy and chemical energy. For example, sunlight provides the energy needed for photosynthesis in plants and solar heating systems can capture sunlight to warm buildings.

Second, light exerts radiation pressure which can do work. Light particles called photons carry momentum. When photons strike an object, they transfer some of their momentum to that object. This creates a small force that pushes the object. While the force of radiation pressure from a single photon is tiny, the cumulative effect from trillions of photons can enable light to do work like propelling objects in space.

Some examples of light doing work through radiation pressure include:

  • Comet tails point away from the sun because sunlight exerts pressure that pushes dust away
  • Solar sails could use sunlight for propulsion of spacecraft
  • Radiation pressure from lasers can be used to manipulate small objects

So in summary, light is a form of energy that can transfer into other types through absorption and can exert forces to do work through radiation pressure.

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