Convection in Science: Definition, Equation & Examples

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  • 0:01 What Are Convection Currents?
  • 2:19 Equation
  • 3:27 Examples
  • 4:39 Lesson Summary
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Lesson Transcript
David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Expert Contributor
Christianlly Cena

Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

In this lesson you will learn what convection is, go over an equation for convection, and see some examples of convection in everyday life, such as campfires and the Earth's outer core. A short quiz will follow.

What are Convection Currents?

Have you ever held your hand over a pot of boiling water? You probably couldn't keep it there for long. But when you put your hand alongside the same pot, it feels perfectly fine. Why does that happen? Because of convection!

There are three types of heat transfer: conduction, convection and radiation. Convection is a type of heat transfer that can only happen in liquids and gases, because it involves those liquids or gases physically moving.

Convection happens when there is a difference in temperature between two parts of a liquid or gas. The hot part of a fluid rises, and the cooler part sinks. But let's use an example to think about why it happens, lest we assume that the fluid has a mind of its own.

After a day of good, solid learning, it's time for a break. You put the kettle on to make a cup of tea. The kettle heats the water from the bottom, giving the molecules near the bottom more kinetic energy (movement energy). This extra movement allows the molecules to spread apart a little. If they're more spread apart, then that means the water is less dense. Cold water is generally denser than hot water.

Convection currents in boiling water - a saucepan or kettle
Convection in Saucepan

If you put something less dense inside something more dense, what happens? Well, try putting a cork under water. You won't be surprised to see it jump right to the surface. In this same way, the hot water at the bottom of the kettle is less dense than the cold water above it, so it will rise to the surface. Once it gets there, it cools down again because it's further away from the heating element. This causes it to become more dense and sink.

These movements of the water are convection currents, and that's why boiling water moves around so much. The water heats up and becomes less dense, then it rises and cools, becoming more dense again, until it sinks. This process repeats over and over. And it all happens due to a simple temperature difference between the top and bottom of the kettle.

So, just a minute ago, I asked you why it's so hot above the boiling pot of water, when it's perfectly comfortable to put your hand next to it. Convection currents are the reason for this. It's because heat rises. When you put your hand next to the pot, you're receiving energy through other types of heat transfer, like conduction and radiation. But not very much. However, above it you add convection into the mix. The heated air is literally rising up towards you to your hand.


Convection is probably the most complex of the three types of heat transfer, because it involves chaotic fluids. But there is still an equation we can use to represent it. The following equation describes the heat energy transferred to a surface in an area where convection is occurring:

Convection equation
Convection Equation

The coefficient of convection is just a number that represents the properties of the materials involved, and the temperature difference is the difference in temperature between the surface receiving heat energy and the average temperature of the liquid - it is not the same as the difference in temperature between the top and bottom of the liquid.

So for example: Let's say you have a pot of 97 degree Celsius boiling water with a lid on it, and that lid has a surface area of 0.1m and a temperature of 67 degrees Celsius. (This means there is a temperature difference of 97 - 67 = 30 degrees.) The convection coefficient is 20, and you want to know how many Joules of energy is transferred to the lid every second. To calculate this, you would multiply 20 by 0.1 by 30, giving you 60 Joules per second.

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Additional Activities

Convection in Science: Writing Prompts

This activity will help assess your knowledge of the characteristics and examples of convection.


The following statements may or may not be related to convection. In this activity, carefully read each of these statements and answer the questions that follow.

Prompt 1

From seismic reading, it was observed that the Earth's mantle is divided into two regions, namely the lower mantle and the upper mantle. Surprisingly, each of these regions has a distinct structure and composition. In terms of phase, the former is solid since the pressure in this region is far too great for melting to occur. The latter, on the other hand, is a region where rocks behave like a plastic fluid.

  • Which of these two regions will convection take place? Why do you say so?

Prompt 2

In the convection equation, the heat transferred to a system (Q) is practically proportional to the convection coefficient (h), its area (A), and the temperature difference. The change in temperature is obtained from the difference of the surface temperature (Ts) and the ambient air temperature (Ta).

  • From the equation, what would happen to the heat transferred on the material if its area is doubled?
  • Would there be a point where no convection takes place? Explain your answer.

Prompt 3

On a cool night with no wind, people who are facing a campfire would typically feel a breeze on their backs. Using the concept of convection currents, explain how this phenomenon takes place.

Sample Answers

Prompt 1

  • Convection only occurs in fluids, such as liquid and air. Since the lower mantle is solid, convection could not take place in this region. In contrast, convection occurs in the upper mantle due to increased pressure that forces rocks to flow like plastic.

Prompt 2

  • If A is doubled, the value for Q also doubles.
  • The only situation where no heat is transferred via convection is when the temperature difference in the material is equal to zero.

Prompt 3

  • The center of the campfire is hotter compared to the surroundings. With this, the warm air rises to create a change in atmospheric pressure within the environment. At this point, this warm air would spread to a location that is dense to where it cools down. The cooled air pocket would then be drawn back to the campfire and will be felt by those gathered next to it, thereby replacing the warm air that was dissipated awhile ago. This is a clear manifestation of convection currents.

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