Astronomy Answers
AstronomyAnswerBook: Energy

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This page answers questions about energy. The questions are:

• [553] Where does a permanent magnet get the energy to maintain its magnetic field?
• [503] Why does the Moon slowly move away from the Earth, even though that takes energy?
• [328] According to Einstein, mass is equivalent to energy. Can we use mass a a source of energy?
• [293] Where does the energy of a star such as the Sun come from?
• [288] How fast does energy travel through space?
• [173] How is energy transported out of the Sun?

1. Energy

Energy is the ability to do things. The more energy something has, the further it can go or the faster it can go or the greater the influence is that it can have. There are many different kinds of energy. A few of them are listed below.

• kinetic energy or speed energy is energy due to the speed of the object. Kinetic energy is proportional to the square of the speed (except for speeds near the speed of light).
• potential energy is energy due to place or situation. The more you go agains a conservative force (for example, the force of gravity), the more potential energy you get.
• thermal energy is energy in the form of heat. Thermal energy is always coupled to matter, and is equal to the kinetic energy of the particles of matter.
• chemical energy is potential energy contained within the structure of atoms, ions, and molecules. Chemical energy is released or locked up in chemical reactions.
• nuclear energy is potential energy contained within the structure of the nucleus of atoms. Nuclear energy is released or locked up in nuclear reactions.
• radiation energy is energy in massless elementary particles such as photons (in electromagnetic radiation) and neutrinos.
• mass-energy is energy that is associated with the rest mass of matter. Einstein discovered that mass and energy are equivalent, according to his famous formula \(E = mc^2\).

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2. Energy Transport

There are three ways in which you can transport energy:

1. by convection, in which temperature differences make hotter material move to colder areas and colder material to hotter areas, which means that there is net heat (and so energy) transport from hot to cold areas. The energy moves at the speed of the material.
2. by conduction, in which heat travels through a material without making the material itself move. The energy moves at a speed that depends on the characteristics of the material.
3. by radiation, in which energy travels through empty space (for example, as electromagnetic radiation such as light). The energy moves at the speed of light.

All three kinds of energy transport happen when you heat a pan of soup on a fire. The hot gases from the fire go up because of convection. The fire produces heat radiation, which you can block with your hand. The outside bottom of the pan is heated and conduction makes the heat reach the inside bottom of the pan, where it heats the soup. At first, the heat travels up the soup only by conduction, but later (when the soup is boiling) also by conduction.

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Last updated: 2021-07-19