In simple terms energy is defined as the ability to do work. Energy is the extensive property of the system or substance as it depends on the quantity. Energy is commonly denoted by symbol E and its unit of measurement is J or Joule.
In nature energy can be found in various forms like thermal (heat), mechanical, electrical, chemical (in fuels), light, nuclear etc. As per the first law of thermodynamics energy can neither be created nor it can be destroyed and the total energy of the universe remains constant. However, the energy can be converted from one form to the other form. Hence chemical energy can be converted into thermal energy, which can be converted into mechanical energy. Similarly nuclear and hydraulic energy can be converted into electrical energy which can alter be converted to light and mechanical energy.
Energy is stored inside the substance or the system. The Three forms of energy which are stored inside the system are:
1) Kinetic Energy
2) Potential Energy
3) Molecular internal energy
The energy of the system due to its motion is called as kinetic energy. For instance the vehicle moving at the fast speed possesses kinetic energy. The potential energy is usually due to the height or position of the system. For instance water falling from height in the dams possesses potential energy. It falls on the fans of the turbine causing their motion, thus converting potential energy into kinetic energy, which is then converted into electric energy.
The energy of the substance or the system stored inside the molecular and atomic structure is called as internal energy of the system. The atoms and molecules inside the system are in continuous state of motion and there are also various forces between them. The individual atoms and molecules have posses various energies like kinetic, vibrational, electronic, nuclear, binding etc. The total amount of energy possessed by the atoms and molecules is called as internal energy of the substance or system.
The total internal energy of the system, denoted by U is depends on the temperature of the system, its mass and its state. As the temperature of the system increases the internal movements of the molecules also increases so internal energy of the system also increases. One cannot measure the internal energy of the system directly, and it is measured as the change in internal energy.
>>>Also Read:
Units of Measurement of Energy
First Law of Thermodynamics
Book: Engineering Thermodynamics by P K Nag