Is energy always transferred from the source to the object when positive work is done by the source on the object ?
Yes, but be careful. If something else simultaneously does an equal amount of negative work on the object, the net work on the object will be zero.
If so, what energy is transferred from the earth to a freely falling body ?
Gravitational potential energy of the earth/body system is transferred to the freely falling body. Gravity does positive work giving the object kinetic energy per the work energy theorem.
The potential energy is converted to kinetic energy in the process. But I see no transfer of energy between the earth and the body.
Energy transfer to the body comes from the gravitational potential energy of the earth/body system. It does not come from just the earth, but from the earth/body system. Neither the body alone nor the earth alone has gravitational potential energy. It is a property of the earth/body system.
And how does energy transfer takes place in case of negative work ?
In the case of negative work, the force is in the opposite direction as the displacement. The thing doing negative work takes energy away from the thing it does work on, as discussed in the next answer.
When a body is moved across a surface which has friction, the friction does negative work. Does it mean that a sort of energy transfer occurs between the surface (source) and the body (object) ? If so, how ?
Yes. When friction does negative work it takes energy away from the object it does work on. What makes friction interesting, however, is that it involves both energy transfer to the stationary surface upon which the body slides from the object, and energy transfer from the stationary surface to the object.
Consider what happens when you rub your hands together. Take one hand and hold it stationary. Then slide the other hand over the surface of the stationary hand. Both hands feel warm. The temperature, and thus internal microscopic kinetic energy of both hands increases. In the frame of reference of the "moving" hand, the "stationary" hand is moving, and vice-versa. In effect, each does friction work on the other.
If a moving body comes to a stop, part of the lost macroscopic kinetic energy of the body goes into the internal energy of the stationary surface increasing its temperature and thus its internal energy. But part of the lost macroscopic kinetic energy is converted into an increase in the internal energy of the object itself, as reflected by an increase in its temperature. If, after the transfer, the body can be isolated (prevented from transferring heat with its surroundings), that increase in internal energy will be retained.
In my mind at least, friction illustrates that when applying the work energy theorem one must account for changes in both macroscopic and microscopic kinetic energy (internal energy). The overall reduction in kinetic energy of the object is actually the loss of macroscopic kinetic energy minus the gain of microscopic kinetic energy of the object.
Hope this helps.
Work is the transfer of energy that occurs to/from an object when a
force acts on it to cause a displacement.
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.
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?
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.
Best Answer
In thermodynamics, work is the negative of the change in internal energy due to a change in volume, usually holding entropy and particle numbers constant. This takes the form of a force pushing on the walls of the volume, which connects it to our conventional notion of work, $W = F~\Delta x$, as seen for example if we consider a cylinder of cross-section $A$, $$W = F~\Delta x = F~\frac AA~\Delta x = \frac FA~A\Delta x = P ~\Delta V.$$And the change in internal energy is just the negative of the work, $-P~\Delta V,$ due to the law of energy conservation.
In fact we can also define that $W = P~\Delta V$ even when we are not holding entropy and particle numbers constant: but then it is not necessarily the same as the change in internal energy. So for example if you compress an ideal gas it generally heats up; you could still speak of the work as $P~\Delta V$ at constant temperature, but "at constant temperature" means essentially "we squeeze this thing and it wants to become warmer, but we let energy out of the system through the walls until it comes back to the same temperature": there has been a negative work, and perhaps the internal energy has still gone up, but it has not gone up as much as it would have had the walls been thermodynamic insulators. In these cases however we can often define a "free energy" (in this case $E - T S$) which the work is the negative of the change of.