Energy is a concept thrown around so much that we all have an intuitive idea of what it is about, but we can’t exactly define it. There are so many types of energy and they are closely related, yet different from each other. e.g. the Sun has energy because of the nuclear reaction going on in it. It radiates that energy towards us, in the form of heat and light. Plants utilise that light to synthesize food and we, humans get energy from eating those plants (or animals that eat plants). In the whole process, energy gets converted as Nuclear->Thermal & Radiation->Chemical. We, after eating food, are able to do work.
In fact, when you say that “I can’t do work, I feel tired and low on energy”, you are very close to the way it is defined in Physics. In mechanics, energy is defined as the ability to do work. This is the most agreeable definition we could come up with, at this level (In special relativity, Energy and matter are seen as equivalent).
As mentioned before, energy comes in many forms. It can transform again and again into different forms, either naturally or by an effort made by us. When you use electricity to power a light bulb, the energy in electricity gets converted to light(and heat). Some different forms of energy include:
- Thermal energy
- Nuclear energy
- Heat (Energy in transfer by virtue of a temperature difference)
- Chemical energy
- Potential energy
- Kinetic energy
- Gravitational energy
- Elastic energy
- Radiant energy
- Binding energy
- Rest energy etc.
Note that these terms are only defined in a particular context, and are related. e.g. Mechanical Energy is usually the sum of kinetic and potential energies.
Energy can also be understood abstractly as some quantity that stays constant in all the processes and reactions(except nuclear reactions, where the total of mass and energy stays constant). In other words,
Energy can neither be created nor be destroyed.
We may never know what Energy is, but we do understand how to utilise it to get our work done. If we also develop ways to optimize our energy usage, we can progress tremendously.