# How is energy of an isolated system defined ?

1. Dec 24, 2012

### apratim.ankur

(energy is the capacity of a physical system to perform work and,
an isolated system is a physical system without any external exchange – neither matter nor energy can enter or exit.)
so,
how is the energy of an isolated system defined?
can it even be defined in principle??

2. Dec 24, 2012

### Studiot

Yes energy is stated to be the capacity to do work in elementary science.

However not all forms of energy are fully available to do work. So you can never convert other forms of energy totally to work.

The energy of an isolated system is defined as the sum total of all forms of energy present or contained in the system.

This is, of course, the same as the definition of the energy for any system.

However for an isolated system no energy may be added or withdrawn, so the total energy is constant.