Question

Macaroni rising and falling in a pot of boiling water is due to what phenomenon?
(A)conduction
(B)convention
(C)radiation
(D)none of these

Answer 1

Convection is the process of heat transfer by the bulk movement of molecules within fluids such as gases and liquids. The initial heat transfer between the object and the fluid takes place through conduction, but the bulk heat transfer happens due to the motion of the fluid.

Complete answer:
Convection is the bulk movement of the liquid under the driving force of density differences in the liquid. We shall consider the problems raised by convection driven by solutes; heavy solutes cause the liquid to sink, and lighter solutes cause flotation. In this section we shall confine our discussion simply to the effects of temperature: hot liquid will expand, becoming less dense, and will rise; cool liquid will contract, becoming denser, and so will sink.
The existence of convection has been cited as important because it affects the columnar to equiaxed transition There may be some truth in this. However, in most castings, grain structure is of no importance. Hardly any customer specifies the grain structure of their castings, the only critical feature being soundness. Only in very few castings is grain size specified or is in any way noticeably significant in terms of affecting properties. Turbine blades are an exceptional example.
It is of course understood that soundness is of vital importance to nearly every casting, but it is not well known that in certain circumstances convection can give severe soundness problems. This is especially true in counter-gravity systems, and sometimes in investment castings.
Convection is transfer of heat through fluid (liquid or gas) caused by their molecular motion. In convection, heat current flows in a vertically upward direction. Macaroni rising and falling in a pot of boiling water is due to convection. As due to convection, macaroni goes up with heat current and when it reaches the top of the pot it is constrained by the lid and gravitational force takes it down.
The correct answer is “B” .i.e. Convection.

Note:
Convection occurs when heat is transferred through a gas or liquid by the hotter material moving into a cooler area. Discover what convection really is and review several examples of this phenomenon.