Why Does Potential Energy Decrease?

Potential energy is the stored energy an object has due to its position or configuration. For example, a ball at the top of a hill has potential energy due to gravity. When the ball rolls down the hill, the potential energy is converted to kinetic energy – energy of motion. The goal of this article is to explain the various forms of potential energy and why they decrease in common situations.

Gravitational Potential Energy

Gravitational potential energy is the energy stored in an object due to its height relative to the ground. An object gains gravitational potential energy when it is lifted upwards against gravity. The higher the object is, the more gravitational potential energy it has.

As an object falls, the gravitational potential energy decreases and is converted into kinetic energy. Kinetic energy is the energy of motion. When the object is at the highest point, it has maximum potential energy and no kinetic energy. As it falls, the potential energy decreases while the kinetic energy increases. By the time the object hits the ground, all the potential energy has been converted to kinetic energy.

The exact amount of gravitational potential energy possessed by an object depends on its mass, its height above the ground, and the strength of gravity. The heavier the object and the higher it is lifted, the more gravitational potential energy it will have. The reason gravitational potential energy decreases as an object falls is because the height above ground is decreasing.

Chemical Potential Energy

Chemical potential energy is the potential energy stored in the bonds between atoms within molecules. It is the energy needed to break bonds and rearrange atoms into different chemical substances. When chemical bonds form between atoms, energy is released as heat. The opposite process, breaking chemical bonds, requires energy input.

During chemical reactions, atoms’ arrangements are reorganized as bonds break and reform. Chemical potential energy decreases as the molecules form more stable, lower-energy bonds. For example, when hydrogen and oxygen gases combine to form water, energy is released because the water molecules have lower potential energy than the separate hydrogen and oxygen atoms. The energy released in the reaction becomes kinetic energy as heat and light. Chemical reactions always proceed in the direction that converts the reactants’ higher chemical potential energy into products with lower chemical potential energy. The decrease in chemical potential energy provides the driving force that enables chemical reactions to occur.

Elastic Potential Energy

Elastic potential energy is the energy stored in objects that can be deformed. This includes springs, rubber bands, stretched strings, and other elastic materials. When these objects are stretched or compressed, the atoms and molecules are forced into positions that increase their potential energy.

For example, when you stretch a spring, you are doing work against the spring’s restorative force. This work gets stored in the spring as potential energy. When you let go, the spring recoils and converts this potential energy into kinetic energy as it returns to its relaxed state. The more a spring is stretched or compressed, the more potential energy it stores.

Elastic potential energy can be calculated using the equation:

E_p = 1⁄2 kx²

Where k is the spring constant, and x is the displacement from equilibrium. This shows that the elastic potential energy increases exponentially with displacement.

When the stretching or compressing force is removed, the objects return to their original shape, converting the potential energy into kinetic energy. This decrease in potential energy is why a compressed spring shoots out, and a stretched rubber band snaps back.

The elastic potential energy decreases because the atoms/molecules are no longer being forced into high energy arrangements. They return to lower energy equilibrium positions when the deforming force is eliminated.

Nuclear Potential Energy

Nuclear potential energy refers to the binding energy that holds the components of an atomic nucleus together. Atoms consist of protons and neutrons clustered together in a tiny nucleus, surrounded by orbiting electrons. The protons and neutrons are bound together by the strong nuclear force, one of the four fundamental forces of nature. This force between the nuclear particles gives rise to nuclear potential energy.

The nuclear binding energy is the amount of energy required to disassemble a nucleus into its individual protons and neutrons. Therefore, the more tightly bound the protons and neutrons are within the nucleus, the greater the nuclear potential energy. When a nuclear reaction occurs and energy is released, such as in nuclear fission or fusion, some of the mass of the nucleus gets converted into energy, resulting in a decrease in nuclear potential energy.

For example, in nuclear fission, a heavy unstable nucleus like uranium-235 can split into two smaller nuclei, releasing neutrons and a large amount of energy. The fission products have less nuclear binding energy than the original uranium nucleus. This mass difference gets converted to energy according to Einstein’s equation E=mc^2, resulting in a decrease in nuclear potential energy.

In nuclear fusion, light nuclei are fused together to form a heavier nucleus, resulting in a decrease in mass and nuclear binding energy, with the lost energy emitted as heat and radiation. For instance, fusing hydrogen nuclei to form helium results in a loss of some mass, leading to a drop in nuclear potential energy, with the released energy heating up the sun and stars.

In summary, nuclear potential energy arises from the binding force within atomic nuclei. Nuclear reactions convert some nuclear mass into energy, decreasing the nuclear potential energy in an energetic process that powers stars and nuclear reactors.

Thermal Potential Energy

Thermal energy, or heat energy, arises from the motions of atoms and molecules in a substance. The faster these particles move, the higher the thermal energy. Thermal potential energy refers to the potential for particles in a high-temperature substance to transfer heat to a lower temperature substance.

When two substances are brought into contact, heat will spontaneously flow from the higher temperature to the lower temperature until they reach thermal equilibrium. This flow results in a decrease in thermal potential energy. The greater the temperature difference between the substances, the more potential energy is available to be transferred.

For example, when a hot cup of coffee comes into contact with the cooler surrounding air, heat flows from the hot coffee to the air, decreasing the coffee’s thermal potential energy. The larger the temperature difference between the coffee and air initially, the more heat can be transferred before the coffee and air reach the same temperature.

Thermal potential energy decreases because temperature differences drive spontaneous heat transfer from hot to cold until equilibrium is reached. The greater the initial temperature difference, the more potential energy is available to be transferred through heat flow.

Electrical Potential Energy

Electric potential energy is the potential energy stored in an electric field. It is the energy that can be transferred into other forms when electric charges are allowed to move. For example, electrons have electric potential energy when they are separated from the positive terminal of a battery. Once the circuit is closed and electrons are allowed to flow, their electric potential energy is converted into other forms like kinetic energy, light, or heat.

When current flows in a circuit, charges like electrons are moving from areas of high potential energy to areas of low potential energy. This flow of charges does work, which means their potential energy is decreasing. For example, electrons moving through a lightbulb use their potential energy to heat the filament and produce light. The current flow depletes the electrons’ electric potential energy.

Electric potential energy can also decrease when charges combine. For instance, when an electron returns to the positive terminal of a battery, its potential energy drops dramatically. The electron is no longer separated from the positive charge, so that stored energy disappears.

In summary, electric current converts potential energy into other forms, while charge recombination eliminates potential energy. Both current flow and charge neutralization lead to decreases in electrical potential energy in a system.

Magnetic Potential Energy

Magnetic potential energy exists when two magnets are positioned in relation to each other in a way that requires work to separate them. The magnetic field exerts an attractive force between the north and south poles of the magnets, pulling them together. As long as the magnets remain separated, they possess potential energy. This potential energy is directly proportional to the strength of the magnetic field and decreases exponentially with increasing distance between the magnets.

When the magnets are allowed to move closer together, their potential energy is converted into kinetic energy as they accelerate towards each other. Once they make contact, the potential energy has been depleted. Separating the magnets again requires work to be done against the magnetic force, which replenishes the potential energy.

The magnetic potential energy can be decreased by changing the orientation or strength of the magnetic field. For example, rotating one magnet so that like poles are aligned will decrease the attractive force between them, reducing the potential energy. Demagnetizing the magnets, such as by heating them above the Curie temperature, eliminates the magnetic field and drops the potential energy to zero. Introducing a non-magnetic material between the magnets can also act as a barrier to decrease the strength of the magnetic interaction. In summary, changing magnetism leads to a direct decrease in magnetic potential energy stored in the system.

Radiant Potential Energy

Radiant potential energy is the potential energy stored in electromagnetic radiation such as light or heat. It refers to the capacity for emitting electromagnetic radiation. This potential energy is decreased when photons (particles of light) are emitted.

Atoms and molecules can absorb energy and become excited. Excited atoms exist at a higher potential energy state. When these excited atoms return to their normal, lower energy state, they release photons. This emission of photons is what decreases the radiant potential energy.

The amount of radiant potential energy that is decreased depends on the wavelength (frequency) of the photons emitted. Higher frequency photons have higher energy, so emitting them leads to a greater decrease in radiant potential energy. As atoms and molecules emit photons, their capacity for emitting further electromagnetic radiation decreases, lowering the overall radiant potential energy in the system.

Conclusion

As we have seen, there are many different types of potential energy, including gravitational, chemical, elastic, nuclear, thermal, electrical, magnetic, and radiant potential energy. While the specifics vary for each type, the overall trend is that potential energy decreases and is converted into other forms of energy.

For example, when an object falls due to gravity, its gravitational potential energy decreases as it accelerates towards Earth and its kinetic energy increases. The potential chemical energy stored in the bonds of molecules and compounds gets released as kinetic energy when those chemical bonds are broken. An extended spring or rubber band has elastic potential energy that is converted into kinetic energy when it contracts. Nuclear potential energy locked inside atomic nuclei gets released as enormous amounts of electromagnetic and kinetic energy in nuclear reactions.

Thermal, electrical, magnetic, and radiant potential energies also tend to decrease as they are converted into other forms like kinetic energy. The key takeaway is that potential energy is converted and used up in various processes, resulting in an overall decrease in potential energy in the system or environment. This trend exemplifies one of the foundational principles of physics – the conservation of energy. The total energy remains constant, while the form that energy takes shifts from potential to kinetic.