What are Convection Currents? - Definition & Examples

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  • 0:00 What Are Convection Currents?
  • 0:52 Example: Soup for Lunch
  • 2:10 Example: Campfires
  • 2:31 Driving the Weather
  • 3:15 Magma and Magnetism
  • 3:57 Lesson Summary
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Lesson Transcript
Instructor: 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.

This lesson will define convection, explain how it works, and describe how convection creates currents. We will also go through some examples of convection in the world around us: campfires, weather systems, and convection in the earth's mantle.

What Are Convection Currents?

Have you ever wondered why it's so much hotter above a campfire than next to it? Or, why when you boil a pot of water, the liquid moves around so rapidly? Both of these things are due to convection currents.

Convection is one of the three main types of heat transfer, the other two being conduction and radiation. Unlike the other two, convection can only happen in fluids. This includes liquids and gases and is because the molecules have to be free to move.

Heat energy can transfer by convection when there is a significant difference in temperature between two parts of a fluid. When this temperature difference exists, hot fluids rise and cold fluids sink, and then currents, or movements, are created in the fluid. But why does this happen? Let's look at an example of convection in action to get a better understanding.

Example: Soup for Lunch

Imagine it's lunchtime, and you're hungry. You decide to heat up some soup on a stovetop. As the soup increases in temperature, you are giving the soup's molecules more movement, or kinetic, energy. This extra movement energy will cause the molecules to spread further apart. Because of this, hotter fluids are less dense than colder fluids. The soup near the bottom of the saucepan is hottest, because it is closer to the burner. This means that the soup is less dense at the bottom.

Now, let's think about what happens when you put a cork into some water. The cork is less dense than the water, so it pops to the surface. The same thing happens with the soup. The hotter, less dense soup at the bottom rises above the colder, denser soup. As the soup rises and gets further away from the burner, it starts to cool and become denser. Before long, the soup is dense enough that it sinks again--passing the rising hot soup on its way--and the whole process repeats.

These movements in the soup are called convection currents, and they are the reason that a pot of boiling water moves around so rapidly. The water is heated, becomes less dense and rises, cools, becomes denser, and sinks, over and over again. All because the burner creates a temperature difference between the top and bottom of the pot.

Example: Campfires

Convection currents are also the reason that it is hotter above a campfire than next to it--heat rises! If you put your hands in front of a campfire, there are fewer convection currents heating your hand. A lot of the heat comes from a different type of heat transfer--radiation. But when you put your hands over the top of a campfire, there are lots of convection currents rising up towards you.

Driving the Weather

If you've ever sat on a hot, sandy beach and suddenly felt a lovely, cooling breeze, you have convection currents to thank!

Convection is a big part of how the earth's weather systems work. Warm fronts, being less dense, will meet colder fronts and rise above them. If those warm fronts contain enough water vapor, the water will cool, condense into a liquid, and form rain.

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