What exactly is energy in physics?

Energy is a measure of material movement and transformation, referred to as "energy". Everything in the world is in constant motion. Of all the properties of matter, motion is the most basic property, and other properties are the concrete manifestations of motion. Energy is a measure of the ability of a physical system to do work.

Corresponding to the various forms of motion of matter, there are various forms of energy, which can be transformed into each other in a certain way.

In mechanical motion, it is manifested as the mechanical energy of an object or system as a whole, such as kinetic energy, potential energy and acoustic energy. In thermal phenomena, it is the internal energy of the system, the sum of the kinetic energy of irregular movement of each molecule in the system, the potential energy of intermolecular interaction and the energy in atoms and nuclei, but it does not include the mechanical energy of the whole system. For thermal motion energy (heat energy), people know it through the mutual conversion between it and mechanical energy (see the first law of thermodynamics).

Spatial attribute is a broad manifestation of material movement; Time attribute is the continuous embodiment of material movement; Gravitational property is the embodiment of the interaction caused by uneven mass distribution in the process of motion; Electromagnetic properties are the external manifestations of charged particles in the process of motion change, and so on. There are various forms of motion of matter, and each specific form of motion of matter has a corresponding form of energy.

The energy form corresponding to the mechanical motion of macroscopic objects is kinetic energy; The energy form corresponding to molecular motion is heat energy; The energy form corresponding to atomic motion is chemical energy; The energy form corresponding to the directional motion of charged particles is electric energy; The energy form corresponding to photon motion is light energy, and so on. Besides these, there are wind energy, tidal energy and so on. When the motion forms are the same, the motion characteristics of objects can be described by some physical or chemical quantities. The mechanical motion of an object can be described by physical quantities such as speed, acceleration and momentum. Current can be described by physical quantities such as current intensity, voltage and power. But if the motion forms are different, the only physical quantity that can describe and compare the motion characteristics of substances is energy, which is the common feature of all moving substances.

Therefore, energy can be defined as:

In ancient Greek, energy means "activity and operation", which is an indirectly observed physical quantity and is regarded as the ability of a physical system to do work on other physical systems. Work is defined as the spatial cumulative effect of force displacement in the direction of force, which is equal to the product of force and displacement transmitted in the direction of force.

The total energy contained in an object is based on its total mass. Like mass, energy will neither be produced nor destroyed out of thin air. Like mass, energy is a scalar. In the international system of units, the unit of energy is joule, but sometimes other units such as kilowatt-hours and kilocalories are used, which are also units of work. Energy is a unified measure of the motion scale of all substances.

System A can transfer energy to system B through simple matter transfer (because the mass of matter is equal to energy). If energy is not transferred by matter, but by other means, it will change system B, because system A does do work on system B, and the effect of doing work is like a force acting on an energy-receiving system at a certain distance. For example, system A can radiate electromagnetic radiation to system B, so that the particles in system B that absorb radiant energy generate thermal motion. A system can also transfer energy through collision. In this case, the collided object will be stressed within a certain distance and get the energy of motion, which is called kinetic energy. The transfer of heat energy can be generated in the above two ways: heat energy can be transferred by radiant energy, or it can be directly converted into kinetic energy by the collision of particles between systems.

Energy can be stored in a system without being expressed as matter, kinetic energy or electromagnetic energy. When a particle moves a distance in the interacting field (it needs external force to move), the energy required for the particle to move to a new position in this field is stored. Of course, the particle must be kept in its new position by external force, otherwise the field it is in will push or pull the particle back to its original state. This kind of energy stored by changing the position of particles in the force field is called potential energy. For a simple example, the work required to lift an object to a certain height in a gravitational field is potential energy.

Any form of energy can be converted into another form. For example, when an object moves freely to different positions in a force field, potential energy can be converted into kinetic energy. When energy exists in the form of non-thermal energy, it can be efficiently or even perfectly converted into other kinds of energy, including the generation of electricity or new material particles. But if it is thermal energy, when it is converted into another form, as described by the second law of thermodynamics, there will always be a limit to the conversion efficiency.

In all energy conversion processes, the total energy remains the same, because the energy of the total system is to transfer energy between systems. When one system loses energy, another system will definitely gain the lost energy, resulting in a balance between gain and loss, so the total energy will not change. This law of conservation of energy was put forward at the beginning of19th century and is applicable to any isolated system. According to Nott's theorem, the conservation of energy is a natural result, because the laws of physics will not change with time.

Although the total energy of a system will not change with time, the value of its energy may vary with different reference frames. For example, a passenger sitting on an airplane has zero kinetic energy relative to the airplane; But relative to the earth, the kinetic energy is not zero and cannot be compared with the earth with single momentum.

Energy is one of the basic concepts in physics. From classical mechanics to relativity, quantum mechanics and cosmology, energy has always been a core concept.

In popular words or popular science books, energy refers to a system that can be released or obtained from it, which can be equivalent to doing certain work. For example, 1 kg gasoline contains 12 kWh of energy, which means that if all the chemical energy in 1 kg gasoline is released, 12KWh of work can be done.

Energy is a physical quantity that describes a system or process in physics. The energy of a system can be defined as the sum of the work from zero energy state to the current state of the system. How much energy a system has is not a definite value in physics, it changes with the description of the system. In the process of human life activities, all life activities need energy, such as the synthetic reaction of material metabolism, muscle contraction, gland secretion and so on. And these energies mainly come from food. Nutrients contained in animal and plant foods can be divided into five categories: carbohydrates, lipids, protein, minerals and vitamins, and water is six categories. Among them, carbohydrates, fats and protein can release energy through in vivo oxidation. These three are collectively referred to as "productivity nutrients" or "heat source substances".

The law of conservation of energy shows that energy will not be produced or disappeared out of thin air, but can only be transformed from one form to another, and the total amount of energy remains unchanged. Energy is a scalar, not a vector, and has no direction. As for positive matter and antimatter, it is not the positive or negative mass, but the opposite electrical properties of the nucleus, and the masses are converted into energy when they meet. Any exercise needs energy. There are many forms of energy, such as light energy, sound energy, heat energy, electric energy, mechanical energy, chemical energy and nuclear energy. For example, observe the energy of a solid with a mass of 1Kg:

In classical mechanics, its energy is the sum of the work done by accelerating from rest to the existing speed.

In classical thermodynamics, its energy is the sum of the work done by heating the existing temperature from absolute zero.

In physical chemistry, its energy is the sum of the work added to the raw materials when synthesizing this solid.

In atomic physics, its energy is the sum of the work done by atomic energy from zero state to the present state.

The energy contained in this solid can also be defined in the opposite way. Give two examples:

The internal energy of a solid is the sum of the work released by cooling it to absolute zero.

The atomic energy of a solid is the kinetic energy of the combined energy released in nuclear fission or fusion reaction and converted into reaction products.

Although energy is a common and basic physical concept, it is also an abstract physical concept. In fact, physicists didn't really understand the concept of energy until the middle of19th century, and before that, they were often confused with the concepts of force and momentum.

There is a lot of energy, such as:

chemical energy

Energy released or absorbed when a substance undergoes a chemical change (chemical reaction). For example, the discharge of dry batteries and storage batteries is the conversion of chemical energy into electrical energy; Charging a battery is to convert electrical energy into chemical energy. Its essence is the energy released by the change of electrons in the outer layer of atoms, which leads to the change of electron binding energy. The annihilation of positive and negative electron pairs into photons means that the static energy of electrons is converted into the energy of photons.

heat energy

The kinetic energy of thermal motion of atoms and molecules in matter, the higher the temperature, the greater the heat energy contained in matter. A heat engine is an expanding steam, which changes thermal energy into kinetic energy.

electric energy

Due to the action of electricity, the potential energy between positive and negative charges can be expressed by the strength of electric field. The electric energy density (electric energy per unit volume) in vacuum is electric field energy density w = E2/2; The electric energy density in the medium w = e d/2, where d is the electric displacement vector and e is the electric field intensity. The extraction of electric energy is to convert electric potential energy into kinetic energy of charged particles, such as current in a conductor or charged particle beam in an accelerator. Magnetic energy refers to the magnetic field energy, and the magnetic energy density w = h b/2, where H is the magnetic field strength and B is the magnetic induction strength. The sum of electric energy density and magnetic energy density is electromagnetic energy density (electromagnetic field energy density) w = (e d+h b)/2.

radiation energy

Refers to the energy of light and electromagnetic waves (photon energy).

nuclear power

The binding energy of nucleons in the nucleus can be released into kinetic energy of reaction products in the fission or fusion reaction of atoms. According to the special theory of relativity, there is a mass-energy relationship between the mass m and the energy e of an object, E=mс2(с is the speed of light in vacuum). Therefore, an object also has energy at rest. The energy and quality of matter are closely related. The mass of a nucleus is less than the total mass of its constituent nuclei, that is, when a free nucleus combines into a nucleus, it releases energy, which is called the binding energy of the nucleus. Heavy nuclear fission with lower binding energy (the average binding energy of each nucleus in a nucleus) forms lighter nuclei with higher binding energy, or several light nuclei with lower binding energy are polymerized into heavier nuclei with higher binding energy, and the released energy is atomic energy.