[NOTE: The definition of work that I am using is the following-
Work is the transfer of energy that occurs to/from an object when a force acts on it to cause a displacement.
]
In my physics book it says:
The only type of energy transfer that we have considered is work $W$ done on a system by an external force.
This made me wonder what are the other kinds of energy transfer? While searching about it on the internet I found that $Heat$ transfer is a type of energy transfer which can occur as conduction, convection and radiation. But thinking deeper about these I found that since conduction occurs via interaction caused by the forces that act between different atoms and hence the energy transfer is essentially the work done by the forces. Same goes for convection though here the particles are free to move and the transfer occurs under the influence of gravity. Whereas radiation is transfered via photons which are themselves the force carries of EM force.
So in this way it looks like the transfer of energy requires forces and hence work to be done. So why does the book state so? Am I missing something over here? If whatever I have concluded in the previous paragraph is wrong then what are the different types of energy transfer?
Thanks in advance.
Best Answer
With regard to differentiating energy transfer by heat vs work, the key term in this definition is displacement. In the case of heat transfer, in no case is there a net force causing a net displacement of the atoms and molecules of the substances involved with heat transfer.
Although in the case of heat transfer by convection there is movement the fluid, that movement occurs after heat is transferred by conduction between a surface and thin layer of fluid in contact with that surface. The movement of the fluid away from the thin layer is the result of mechanical work resulting from pressure gradients and gravity.
While both heat and work energy transfers may involve forces, work involves a net force and net displacement of mass. For heat forces may be associated with the collisions of atoms and molecules resulting in transfer of kinetic energy, but there is no net displacement of the center of mass of the collection of those atoms and molecules resulting from those forces.
We are left with two types of energy transfer. Heat and work. There may be various forms and mechanisms associated with them, they are nonetheless different.
Hope this helps.