Can convection occur in both liquids and gases what and the reason?

Answer 1

One Sentence Answer:

Convection can take place in any fluid when the temperature gradient in the fluid is in a different direction than the force of gravity and that includes liquids, gasses and even plasmas.

Conditions for Convection:

Normally when discussing convection the type of convection is assumed to be "natural convection" otherwise known as "thermal convection" caused by heating a fluid. There is also "forced convection" where fluid movement is caused by an artificial mechanism like a fan. Convection ovens, for instance, are actually forced convection where a fan moves heated air. Thus, it is obvious that forced convection can occur in any fluid.

That said, it remains true that natural convection can occur in any fluid.

Natural convection requires three conditions.

1. A Nonuniform temperature in the fluid.

2. A change in density of the fluid with change in temperature.

3. A gravitational field to create buoyancy.

Discussion "natural convection":

We can explain the conditions in order.

0. We could add condition "0" to the list and that is the condition that one has a fluid and not a solid. A solid will maintain its shape under the influence of a force, i.e. a solid will not flow. Convection does not occur in solids. (Some people will call a "glass" a liquid, but that is not appropriate here. The glassy state is still an amorphous solid state. Contrary to some old wives' tales, glass does not flow.)

1. A Nonuniform temperature in the fluid is required because we are discussing thermal convection caused by temperature differences. We could also discuss convection caused by chemical or physical differences in a fluid. For instance, it is well known that if a body of salt water experiences evaporation, the density of the surface regions increases and as a result, the surface water will sink. That is a form of convection that does not require a temperature difference and is not what we call "forced convection." There are other examples where a fluid becomes inhomogenous for reasons other than a temperature gradient, e.g. phase separation, and density changes result in fluid flow.

2. A change in density of the fluid with change in temperature is the driver of convection. If the temperature is uniform and the fluid is therefore uniform in density, there are no buoyant forces and no force to cause motion in the fluid. Of course, one can imagine a fluid which for some reason does not change in density with temperature. Most fluids that decrease in density as temperature increases, gasses for instance. Rarely, a fluid will increase in density as temperature increases, water for instance. (If you increase the temperature of water from 0 C to 4 C, the density increases by 0.013 %.) There is no known material that does not change density with temperature.

3. A gravitational field to create buoyancy is required so that the force pulling on the dense region of the fluid is unbalanced from the force on the less dense regions and the unbalanced force causes motion. This turns out to be an important problem for people living in zero gravity environments. Fluid currents induced by convection are important for heating liquids. In fact, this is also important for combustion. Trying to burn a candle in a zero gravity environment is difficult. (This is not to say that convection due to forces other than gravity is impossible. One can imagine other forces that would penetrate a fluid, e.g. electrical, magnetic, centrifugal. Whatever the force, a kind of convection would occur if there is an imbalance in the force acting on difference regions of the fluid.)

Finally, we should mention plasmas, which exhibit convection the same as any other fluid. The presence of electric and magnetic fields complicate the fluid dynamics, but convection can still take place. The Sun is the obvious example, but only the outer third of the Sun exhibits convection. The dynamics of the Sun's convection zone involves more than temperature, particularly the turbulent outermost region where forces of magnetic fields create dramatic effects. Note that the absence of convection in the deep interior does not violate our basic assertion that convection occurs in all fluids because the gravitational field inside the Sun in very nonuniform, dampening vertical density fluctuations. But, if you want to get picky, then you could claim that the interior of the Sun is one place that fluid convection does not take place!