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(i) Heat
Heat is defined as flow of energy from one point to another.
Thermodynamically heat is defined as flow of energy from one body to another on account of difference in temperature.
♦ If the system absorbs the heat then heat is taken as positive, while of system releases the heat then heat is taken as negative.
♦ Heat is path dependent.
♦ Numerically,

           









(ii) Internal Energy

The system has some amount of energy which is not apparently shown by the system but is capable of doing work. Such type of energy is known as Internal Energy or Intrinsic Energy.
According to Kinetic theory, all the matters are made up of large number of tiny particles, which are in continuous movement inside the body.

The energy due to movement of particles inside the body is called molecular Kinetic Energy, & the energy due to molecular attraction between the particles is called molecular potential energy.
The sum of molecular Kinetic energy & molecular Potential Energy of system is known as Internal Energy of the system.
Here, Internal Energy is denoted by U, molecular Kinetic energy is denoted by Uk  & molecular potential energy is denoted by Up such that :-
    
                      U = Uk  +  Up
♦  Internal Energy is a state function.
♦ Internal Energy of a body is depend on initial & final position & does not depend on path followed.
              
                         U = Uf – Ui
♦ Internal energy depends upon temperature.
♦ In a cyclic process, total internal energy of a complete cycle is zero.
(iii) Work
Work is defined as the way by which system may exchange heat from surrounding.
If force F produces displacement dx, then the work done by the system :-
I hope this tutorial is helpful for you to understand Heat, Internal Energy & Work. Still any question you can ask us in comment section.