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Energy is the capacity to do work. We all "know" what work is, because we expend considerable energy trying to avoid doing it!
Energy is manifest in the world in many different ways: a car battery, radiant light from the Sun, the explosive potential of gasoline. But ultimately, all manifestations of energy can be categorized as one of three types: potential energy, kinetic energy, and radiative energy.
Potential energy is "stored energy by virtue of position". Do some work by lifting your textbook off the surface of a table: the work you did against the force of gravity is now "in" the book, stored as gravitational potential energy.
Kinetic energy is energy of motion. If some object is moving with respect to you, it has kinetic energy with respect to you. For example, a moving bus has kinetic energy: don't step in front of it!
Radiative energy is energy delivered by particles of light. The light particles can be visible or not. The amount of energy in a light particle is proportional to its frequency. For example, blue light has more energy per particle than red light, because blue light has a higher frequency than red light.
From a classical perspective, that is, from the perspective of a physicist prior to the twentieth century, energy (alone) is said to be conserved. You don't really create it or destroy it, you just move it around. It may stay in the object you manipulated (like your textbook mentioned above), or it may dissipate into the environment, as heat, sound, or something else. Conservation of energy can be very valuable, especially when trying to understand motion problems.
For example, suppose you wanted to know how fast you'd have to go, in order to escape Earth, and enter outer space. The minimum speed you'd need is called the escape velocity of Earth, and it can be calculated using conservation of energy. The escape velocity of Earth is 11 kilometers per second, which is over 24,000 miles per hour! This escape velocity is the same for all objects, regardless of their masses (and weights). Because it is difficult to achieve the escape velocity of Earth, it is common for space probes to first orbit the Earth, and then use the gravitational forces of the Moon and Sun to "slingshot" itself out of the Earth-Moon system, and head towards its final destination.From a modern perspective, that is, from the perspective of a physicist trained in the twentieth century and beyond, energy and mass together are conserved. Mass is how much stuff an object is made of. It is not the same thing as weight.
1) an atomic bomb goes off, such as what happened in 1945 at Hiroshima and Nagasaki, Japan, or,
2) to a lesser extent, when a nuclear power plant accident happens, such as what happened in Chernobyl, Ukraine, in 1986.
Fusion is also a process wherein potential energy stored in bonds between atomic particles is released, but with fusion, clusters of atomic particles are not split in two, but instead pairs of them are smashed together. Vast amounts of energy are released in the fusion process. The fusion bomb is known as the hydrogen bomb (because of the fuel involved), as well as the thermonuclear bomb. No hydrogen bombs have been used in warfare, and no nuclear power plants as yet use the fusion process to generate electricity.
Einstein showed (in 1905) that mass could be converted into energy, and vice-versa. It is this concept that explains how the Sun works, how atomic energy works, and how the fission and fusion bombs work.
Fission is a process wherein potential energy stored in bonds between atomic particles is released. Fission is the splitting of heavy atoms into lighter ones. Fission occurs naturally, but can be amplified in a controlled fashion to produce heat energy that can in turn be converted to electrical energy. This is the process that occurs at a nuclear power plant.
If fission is allowed to happen in an amplified but uncontrolled fashion, a nuclear explosion occurs. This is what happens when:
On a more mundane level, internal energy (sometimes referred to as heat) in an object is just the total mechanical energy (kinetic and potential) of the atoms or molecules in that object. When we measure an object's temperature, we are indirectly measuring the average kinetic energy per molecule of that object.
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