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Convection

Convection is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Convection rather than just read about it. In short: Convection is the transfer of heat through the physical movement of fluids (liquids or gases), where warmer, less-dense material rises and cooler, denser material sinks. It is a single-phase or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid.

Convection — main illustration
Convection — illustration

Key takeaways

  • Convection belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Convection to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Convection from memory before moving on to harder problems.

Reference excerpt

Convection is the transfer of heat through the physical movement of fluids (liquids or gases), where warmer, less-dense material rises and cooler, denser material sinks. It is a single-phase or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid. When the cause of the convection is unspecified, convection due to the effects of thermal expansion (change in density) and gravity/buoyancy can be assumed (see convection in heat transfer).

Convective flow may be transient (such as when a multiphase mixture of oil and water separates) or steady state (see convection cell). The convection may be due to gravitational, electromagnetic or fictitious body forces. Heat transfer by natural convection plays a role in the structure of Earth's atmosphere, its oceans, and its mantle. Discrete convective cells in the atmosphere can be identified by clouds, with stronger convection resulting in thunderstorms. Natural convection also plays a role in stellar physics. Convection is often categorised or described by the main effect causing the convective flow; for example, thermal convection. Convection may also take place in soft solids or mixtures where particles can flow. Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Granular convection is a similar phenomenon in granular material instead of fluids. Advection is the transport of any substance or quantity (such as heat) through fluid motion. Convection is a process involving bulk movement of a fluid that usually leads to a net transfer of heat through advection. Convective heat transfer is the intentional use of convection as a method for heat transfer.

History

In the 1830s, in The Bridgewater Treatises, the term convection is attested in a scientific sense. In treatise VIII by William Prout, in the book on chemistry, it says:

[...] This motion of heat takes place in three ways, which a common fire-place very well illustrates. If, for instance, we place a thermometer directly before a fire, it soon begins to rise, indicating an increase of temperature. In this case the heat has made its way through the space between the fire and the thermometer, by the process termed radiation. If we place a second thermometer in contact with any part of the grate, and away from the direct influence of the fire, we shall find that this thermometer also denotes an increase of temperature; but here the heat must have travelled through the metal of the grate, by what is termed conduction. Lastly, a third thermometer placed in the chimney, away from the direct influence of the fire, will also indicate a considerable increase of temperature; in this case a portion of the air, passing through and near the fire, has become heated, and has carried up the chimney the temperature acquired from the fire. There is at present no single term in our language employed to denote this third mode of the propagation of heat; but we venture to propose for that purpose, the term convection, [in footnote: [Latin] Convectio, a carrying or conveying] which not only expresses the leading fact, but also accords very well with the two other terms.

Later, in the same treatise VIII, in the book on meteorology, the concept of convection is also applied to "the process by which heat is communicated through water".

Terminology Today, the word convection has different but related usages in different scientific or engineering contexts or applications. In fluid mechanics, convection has a broader sense: It refers to the motion of fluid driven by density (or other property) difference. In thermodynamics, convection often refers to heat transfer by convection, where the qualified natural convection is used to distinguish the fluid mechanics concept of convection (covered in this article) from convective heat transfer. Some phenomena which result in an effect superficially similar to that of a convective cell may also be (inaccurately) referred to as a form of convection; for example, thermo-capillary convection and granular convection.

Mechanisms Convection may happen in fluids at all scales larger than a few atoms. There are a variety of circumstances in which the forces required for convection arise, leading to different types of convection, described below. In broad terms, convection arises because of body forces acting within the fluid, such as gravity.

Natural convection

… excerpt ends here. Continue reading the full article.

Illustrations

Convection: Simulation of thermal convection in the Earth's mantle. Hot areas are shown in red, cold areas are shown in blue. A hot, less-dense material at the bottom moves upwards, and likewise, cold material from the top moves downwards.
Simulation of thermal convection in the Earth's mantle. Hot areas are shown in red, cold areas are shown in blue. A hot, less-dense material at the bottom moves upwards, and likewise, cold material from the top moves downwards.
Convection: Thermal image of a newly lit Ghillie kettle. The plume of hot air resulting from the convection current is visible.
Thermal image of a newly lit Ghillie kettle. The plume of hot air resulting from the convection current is visible.
Convection illustration
Convection illustration
Convection: This color schlieren image reveals thermal convection originating from heat conduction from a human hand (in silhouette) to the surrounding still atmosphere, initially by diffusion from the hand to the surrounding air, and subsequently also as advection as the heat causes the air to start to move upwards.
This color schlieren image reveals thermal convection originating from heat conduction from a human hand (in silhouette) to the surrounding still atmosphere, initially by diffusion from the hand to the surrounding air, and subsequently also as advection as the heat causes the air to start to move upwards.

Worked examples

Example 1 — a first encounter with Convection

Start with the simplest possible case. Write down what Convection claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Convection before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Convection ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Convection

In research
Convection appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Convection in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Convection is common in secondary-school and first-year university syllabi. It links to neighbouring topics Convection, Fluid mechanics, Physical phenomena, so understanding it makes those chapters shorter.
In everyday life
Look for Convection outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Convection in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Convection means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Convection out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Convection in simple terms?

Convection is the transfer of heat through the physical movement of fluids (liquids or gases), where warmer, less-dense material rises and cooler, denser material sinks. It is a single-phase or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterog…

Why does Convection matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Convection?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Convection.

Tags

  • Convection
  • Fluid mechanics
  • Physical phenomena

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