Heat Transfer Convection - Definition, Examples, Types, FAQs

Imagine if you were about to pour water into a glass and then it kept behaving as if it had always been and remained a rock, or vice versa—that picking up a rock means finding it flowing through your fingers as if it was water. Such simple scenarios just remark on the amazing contrast between solids and liquids, two states of matter in our everyday lives. In this chapter, we shall look at properties that make solids rigid and liquids fluid, by examining molecular structures and forces which produce them. Knowing these properties will allow us to understand everything from why the ice cubes form in our freezer to how hydraulics work in heavy machinery.

This Story also Contains

1. What is Convection?
2. Heat Transfer By Convection
3. Solved Examples Based On Heat Transfer By Convection
4. Summary

In this article, we will cover the concept of Heat Transfer By Convection. This concept falls under the broader category of Properties of Solids and Liquids which is a crucial chapter in Class 11 physics. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE and more.

What is Convection?

Convection is the process of heat transfer through a fluid (liquid or gas) caused by the movement of the fluid itself. It occurs when warmer, less dense portions of the fluid rise, and cooler, denser portions sink, creating a circulation pattern. This transfer of heat can happen naturally due to temperature differences (natural convection) or be forced by external means, such as a fan or pump (forced convection).

Examples of Convection

1. A radiator is a device that emits warm air from the top and draws cooler air in from the bottom. A steaming cup of hot tea - When you drink a cup of hot tea, the steam you observe signals that heat is being transported into the air. Ice melts because heat from the air is transferred to the ice.
2. When water boils, the molecules that are denser sink to the bottom, and the molecules that are less dense rise, resulting in a circular motion of the molecules, which heats the water.
3. Warm water goes towards the poles as it approaches the equator, while cooler water moves towards the equator.
4. Warm-blooded animals use convection to circulate their blood, which helps to regulate their body temperature.

Heat Transfer By Convection

Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids. Convection is of two types -

1. Natural Convection

The main cause of this is the difference of densities at two places and is a consequence of gravity because on account of gravity, the hot light particles rise up (density becomes less) and cold heavy particles (density becomes high) try setting down. It mostly occurs on heating a liquid/fluid.

2. Forced Convection

If a fluid is forced(by means of a fan or draught or any external means) to move to take up heat from a hot body then the convection process is called forced convection. In this case, Newton's law of cooling holds good. According to which rate of loss of heat from a hot body due to moving fluid is directly proportional to the surface area of body and excess temperature of body over its surroundings.

Application of Convection

1. Convection currents in the water pipes cool car engines. Water is an excellent conductor of heat away from the engine and into the radiator.
2. Convection currents are what cause land and sea winds.
3. Convection currents are rising air over land that glider pilots employ to maintain their gliders in the air.
4. Air conditioners are placed towards the room's ceiling to allow for the formation of convection currents. Cool, dry air is blown into the room by the air conditioner. Cool air sinks because it is denser. Warm air rises because of is less dense. The air circulated, and the temperature of the air fell to the ideal level over time.

What Does the Term "Thermal Convection" Mean?

Heat convection is the physical movement of a fluid (liquid, gas, or plasma) from one area to another to transfer thermal energy. In liquids and gases, heat convection is frequently the primary mechanism of energy transmission. Convection is one of three primary ways of heat transport, along with conduction and radiation.

Recommended Topic Video

Solved Examples Based On Heat Transfer By Convection

Example 1: It is hotter for the same distance over the top of fire than it is on the side of it, mainly because

1) Air conducts heat upwards

2) Heat is radiated upwards

3) Conversion takes more heat upward.

4) Conversion, conduction and radiation all contribute significantly to transferring heat upwards.

Solution:

Convection: Mode of transfer of heat through migration of material particles of the medium.

Convection significantly transfers heat upwards. The top of a fire is hotter than its sides because hot air rises, creating a column of intense heat directly above the flames, while the heat is more dispersed on the sides.

Hence, the answer is the option (2).

Example 2: The layer of the atmosphere is heated through

1) Convection

2) Conduction

3) Radiation

4) 2 & 3 both

Solution:

Natural Convection

This arises due to the difference in densities at the two places.

wherein

The heating of the atmosphere is mainly because of natural convection.

Hence, the answer is the option (1).

Example 3: Heating of a room using a blower is an example of

1) Conduction

2) Natural convection

3) Forced convection

4) Radiation

Solution:

Forced Convection

If a fluid is forced to move to take up heat from a hot body.

wherein

So, heat transfer with the help of a blower is an example of heat transfer through forced convection.

Hence, the answer is the option (3).

Example 4: The rate of heat transfer in forced convection is

1) proportional to $\left(\theta-\theta_o\right)$
2) proportional to $\left(\theta-\theta_o\right)^2$
3) proportional to $\left(\theta-\theta_o\right)^3$
4) proportional to $\left(\theta^4-\theta_o^4\right)$

Solution:

Forced Convection
$
\begin{aligned}
& \frac{Q}{t} \propto\left(\theta-\theta_0\right) \\
& \Rightarrow \frac{Q}{t}=h A\left(\theta-\theta_0\right)
\end{aligned}
$
Where,
$\mathrm{h}=$ Constant of proportional
$\theta=$ Temperature of body
$\theta_0=$ Temperature of surrounding

Hence, the answer is the option (1).

Summary

Convection is a form of heat transfer through the movement of fluids, either naturally due to differences in density or by external forces like fans (forced convection). It plays a crucial role in everyday phenomena like boiling water, weather patterns, and regulating body temperature in animals. Convection currents are key in applications such as cooling car engines, air conditioning, and creating winds. Solved examples demonstrate how convection significantly transfers heat, both naturally and by forced mechanisms, with practical implications in various scenarios.