Physics uses the term energy in the same way that all other branches of science use it. You can best understand energy by its defining properties, especially, how it relates with work:
- Energy is an entity that can be passed from one object to another. Examples: (A) When you eat rice which is rich in carbohydrates (a form of fat, and fat is a chemical energy), energy has transferred from the rice to you. (B) As Sun continues to shine, it passes its radiant energy to the planets in the Solar System including the Earth.
- Energy is never created nor lost – it is only transformed from one type to another. Examples: (A) When you cook using your frying pan, the fuel energy (like LPG) is consumed, not to be lost in space, but only to be transformed into heat energy. (B) When you charge your phone, the electrical energy from your electric company is transformed into chemical energy. When you turn on your phone, the reverse happens – chemical energy is transformed to electric energy.
- Energy can be a result of work. Examples: (A) Do you know how to light a fire by rubbing wood surfaces and using dried leaves? That is possible because the work done to rub the surfaces eventually produce heat energy. (B) By controlling the vibrations in your vocal chords [work is done] you produce sound energy.
- Energy is the capacity to do work. Examples: (A) Without fuel energy, you car will never take you where you want to go. (B) When you kick a soccer ball, you are passing some of your energy to the ball so the ball moves – this depends on how hard you kick (or how much energy you pass to the ball).
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