Lesson C6: Heat Transfer: Convection

Key Ideas   Unit Checklist

  Video Lesson

How does air circulate inside a house? Watch this video to learn more about how the process of convection helps move heat inside buildings.

Lesson C6: Heat Transfer: Convection 

Figure C.2.6.1– A convection oven has a fan at the back of the oven to distribute heat.
Figure C.2.6.2– Food bakes more evenly in a convection oven.

Convection Ovens

Convection happens in all types of ovens. Hot air rises from the heating element at the bottom of the oven. The rising hot air forces cooler air at the top of the oven to sink to where it can be warmed, and so the air continues to move.

However, in most ovens, natural convection causes the upper half of the oven to be hotter than the middle of the oven. Food on the top rack of the oven cooks faster than food cooks on the middle rack.

Convection ovens help the process of convection happen faster. A fan at the back of the oven moves air around the oven. Hot air rises faster, and slightly cooler air sinks faster. The fan helps air to move continuously around all the food in the oven so that the food cooks evenly.
Reading and Materials for This Lesson:

Science in Action 7
Reading: Pages 212–216

Materials:
sheet of paper, pencil, scissors, 30-cm piece of string, tape, desk lamp, large clear plastic container, lukewarm tap water, hot tap water, ice cube tray, freezer, blue food colouring, red food colouring, small jar that fits inside the larger container

 Watch More

Convection Ovens

Watch this video to learn more about cooking with a convection oven.

Figure C.2.6.3– A sea breeze forms during the day when warm air rises over the warming land.
Figure C.2.6.4– A land breeze forms every night as warm air rises off of the heated water.

Wind and Convection

Convection currents in the atmosphere cause various types of weather to form. For example, ocean coastlines often are windy. Coastal winds are caused by convection.

On a sunny day, coastal land heats up faster than the ocean water does. Air above the land heats up, rises into the atmosphere, cools over the ocean, and sinks back down, as in Figure C.2.6.3.  This convection current causes wind to blow from the ocean towards the land. This type of wind is called a sea breeze.

At night, coastal land cools faster than the ocean water does. Air above the ocean heats, rises in the atmosphere, cools over the land, and sinks back down, as in Figure Figure C.2.6.4. This convection current causes wind to blow from the land to the ocean. This type of wind is called a land breeze.
Figure C.2.6.5– Food bakes more evenly in a convection oven.

 Watch More

Sea and Land Breezes

If land and sea breezes are difficult to understand, this video breaks it down and explains things step-by-step.

Figure C.2.6.6– Thunderstorms form when hot air rises on hot summer days.
Figure C.2.6.7– Tornadoes sometimes develop from thunderstorms.

Severe Storms

Severe storm such as thunderstorms, tornadoes, and hurricanes are caused by convection. Thunderstorm clouds form when warm moist air near the ground rises into the atmosphere. When the warm air travels high enough, it cools. This causes water drops to condense from the moist air. These water drops form a cloud.

As the convection current of moist air continues, huge thunderhead clouds form. If the convection current starts to move very quickly, the thunderstorm cloud may start swirling into a funnel cloud. When a high-speed funnel cloud touches the ground, it is called a tornado.

Hurricanes, cyclones, and typhoons are different names for the same type of storm. These severe storms start in warm tropical oceans. Warm moist air near the ocean rises into the atmosphere. When the air cools, water drops condense from the moist air into clouds. As the convection current continues, the movement of air eventually forms the swirling clouds of a hurricane.

Figure C.2.6.8– Tornadoes are high-speed swirling winds that cause destruction on land.
Figure C.2.6.9– Hurricanes start forming from rising warm air over oceans.

 Watch More

How Do Tornadoes Form?

Watch these two videos to learn more about tornadoes forming.



Here is some video footage of a real tornado forming.



Hurricanes

This video explains how hurricanes form.

  Try It!

Convection Snake

Try this simple experiment to observe how convection currents form in the air.

Materials:

Take care with scissors; don't cut yourself or anyone else!
Instructions:

    1. Draw a large circle on the sheet of paper, or print out the handout “Convection Snake”.

    2. Cut out the circle or snake. Skip to step 4 if you are using the “Convection Snake” handout.

    3. Inside the circle, draw a spiral. Start from the centre and eventually connect the spiral to the circle’s edge.

    4. Cut out the spiral along the pencil line. For the snake, cut out the white paper all the way to the snake’s tail.

    5. Attach the piece of string to the centre of the spiral, or the snake’s tail.

    6. Lift the string to lift the spiral snake.

    7. Hold the spiral snake above a heat source such as a desk lamp light bulb. The spiral should not touch the light bulb.

    8. Observe the spiral snake.

Questions:

Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.

Air close to the heat source became warm. This hot air rose upwards and travelled through the paper spiral. The hot air eventually cooled and sank back down. The heat source produced a convection current. The convection current of rising and sinking air through the paper spiral caused the spiral to twirl.

  Connections


Connections: Career
>> Meteorologists

Meteorologists are scientists who collect and analyze information about temperature and convection currents in the atmosphere. They use this information to make predictions about the weather. Meteorologists can forecast future weather based on weather patterns that they have seen before.

Meteorology is very important to keep people safe. Meteorologists can inform people in advance when severe weather is approaching. This helps people to prepare for shelter or evacuation.
Figure C.2.6.10– Meteorologists produce maps of hot and cold air to predict the weather.

Figure C.2.6.11– Meteorologists inform people about weather conditions with weather forecasts.
Figure C.2.6.12– These people heard a hurricane weather forecast. They prepare by boarding up their windows.

 Watch More

Meteorologists

Watch this video to hear a meteorologist talk about her job.

Ocean Currents

Huge convection currents form in oceans. This is due to cold, dense water near the North and South poles sinking and warm, less dense water near the tropics rising.

Ocean currents affect climate, especially in places closer to the poles. Ocean water movement can make a climate more mild. The Gulf Stream is an ocean current that flows from the tropics in the Atlantic Ocean towards northern Europe. The Gulf Stream current makes the climate of northern Europe warmer than might be expected.

For example, London, England has the same latitude as Calgary, which is 51° north latitude. However, snow is rare in London because the temperature seldom drops below 0°C. The Gulf Stream makes the weather warmer in London compared to the colder climate of Calgary.


Figure C.2.6.14– Warm water rises to the surface in tropical oceans.
Figure C.2.6.15– Cool water sinks in polar oceans.

  Try It!

Water Currents

Try this simple experiment to observe convection currents in water.

Materials:

  • large clear plastic container
  • lukewarm tap water
  • hot tap water
  • ice cube tray
  • freezer
  • blue food colouring
  • red food colouring
  • small jar that fits inside the larger plastic container
Instructions:

    1. Add 3 drops of blue colouring to one compartment in the ice cube tray.

    2. Fill the ice cube compartment with water and freeze overnight. This will make a blue-coloured ice cube.

    3. Fill the clear plastic container with lukewarm tap water.

    4. Fill the small jar with hot tap water. Add 3 drops of red food colouring.

    5. Place the blue ice cube and the red hot water jar into the large clear plastic container at the same time but on opposite sides of the container.

    6. Observe the motion of the water.

Questions:

Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.

The blue water was cold. The particles in cold water are close together, making the water more dense. The colder, denser blue water sank to the bottom of the container.
The red water was hot. The particles in hot water are further apart, making the water less dense. The hotter, less dense red water rose to the top of the container.



  Make sure you have understood everything in this lesson. Use the Self-Check below, and the Self-Check & Lesson Review Tips to guide your learning.

Unit C Lesson 6 Self-Check

Instructions


Complete the following 6 steps. Don't skip steps – if you do them in order, you will confirm your understanding of this lesson and create a study bank for the future.

  1. DOWNLOAD the self-check quiz by clicking here.

  2. ANSWER all the questions on the downloaded quiz in the spaces provided. Think carefully before typing your answers. Review this lesson if you need to. Save your quiz when you are done.

  3. COMPARE your answers with the suggested "Self-Check Quiz Answers" below. WAIT! You didn't skip step 2, did you? It's very important to carefully write out your own answers before checking the suggested answers.

  4. REVISE your quiz answers if you need to. If you answered all the questions correctly, you can skip this step. Revise means to change, fix, and add extra notes if you need to. This quiz is NOT FOR MARKS, so it is perfectly OK to correct any mistakes you made. This will make your self-check quiz an excellent study tool you can use later.

  5. SAVE your quiz to a folder on your computer, or to your Private Files. That way you will know where it is for later studying.

  6. CHECK with your teacher if you need to. If after completing all these steps you are still not sure about the questions or your answers, you should ask for more feedback from your teacher. To do this, post in the Course Questions Forum, or send your teacher an email. In either case, attach your completed quiz and ask; "Can you look at this quiz and give me some feedback please?" They will be happy to help you!

Self-Check Time!
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Self-Check Quiz Answers

Click each of the suggested answers below, and carefully compare your answers to the suggested answers.

If you have not done the quiz yet – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.

Hot air rises in a house due to convection. On a hot summer day, warm air rises to the top floors of a house while cooler air sinks to lower areas. This is why the basement of a house is cooler than upstairs during the summer.
The candle flame warms the surrounding air. Hot air rises directly above the flame. The hot air rises, cools, and sinks. This produces a convection current with cooler air moving down along the sides of the flame. Thus, your hand feels hotter directly above a candle flame and cooler on the sides of a candle flame.
A house fire produces convection currents. Hot air containing smoke rises in a room. Cooler air without smoke sinks to the bottom of the room. It is safer and easier to breathe the cooler air near the floor.
A room with high ceilings has more distance for hot air to rise. Warm air in the room rises to the ceiling. At the ceiling, the warm air cools and sinks back down, producing a convection current. Hot air rises high into the room, above any people in the room, which is why the room feels cold.
On a hot day, the ground absorbs heat. The hot ground warms air particles close to the ground. The warm air rises, producing convection currents. The air movement from these convection currents near the ground pushes on the airplane. This causes the landing to be bumpy, or turbulent.