Lesson C7: Heat Transfer: Radiation

Key Ideas   Unit Checklist

  Video Lesson

Why does the inside of a car become hot on a sunny day? Why do cats love sunbeams? Watch this video to learn more about heat transfer through radiation.

Lesson C7: Heat Transfer: Radiation 

Figure C.2.7.1– Photography with infrared film produces interesting effects.
Figure C.2.7.2– Infrared cameras detect heat escaping from a house.

Infrared Cameras

Most cameras contain film or digital sensors that detect visible light to produce pictures. Some cameras contain film or digital sensors that detect infrared radiation, or heat.

Infrared still photography became popular in the 1930s. Photos taken with infrared sensitive film have interesting artistic effects, such as leafy green trees appearing white.

Today, infrared cameras have many useful purposes. They can detect places on a house where heat is escaping, such as in Figure C.2.7.2. This tells homeowners where to add more insulation or where to seal leaks. Infrared cameras are used by police officers. Police mount infrared cameras to helicopters to detect heat radiating from people, for example. Infrared cameras allow police officers to chase suspects or to see suspicious behaviour, especially at night.
Reading and Materials for This Lesson:

Science in Action 7
Reading: Pages  217–220

Materials:
black plastic garbage bag, white plastic garbage bag, scissors, ruler, 2 clear glasses, water, thermometer, empty cardboard pizza box, clear plastic sheet protectors or clear plastic wrap, black construction paper, tape, wooden skewer, glue, aluminum foil, pen, graham crackers, large marshmallows, chocolate bar
Figure C.2.7.3– Marshmallows roast best when they are not touching the fire directly.
Figure C.2.7.4– Toasters toast bread by means of radiation.

Cooking with Radiation

Some types of food cook best by absorbing heat through radiation. Perhaps you have roasted a marshmallow over a campfire. You might have learned that, if the marshmallow touches the flames directly, it will soon be on fire—and you will have a black crust. Most people prefer to eat soft, golden-brown marshmallows made by roasting the marshmallow slowly by holding the marshmallow close to the red hot embers in the fire. The embers heat the marshmallow by radiation.

Some household appliances heat food by radiation. For example, a toaster has an element that heats up and glows red. Wires surround the bread slot so that the bread does not touch the heating element directly. The toaster element heats the bread by radiation.

  Connections

Connections: Technology
>> Microwave Ovens

Microwave ovens use radiation to heat food very quickly. Microwaves are a type of radiation similar to infrared radiation. A microwave oven generates microwave radiation when it is turned on. Water, fat, and sugar particles inside the food absorb the microwave radiation. The microwave radiation causes the particles in the food to start moving faster, increasing their temperature.

Figure C.2.7.5– Microwave ovens heat food quickly.
Figure C.2.7.6– Microwaves are very easy to use.

 Watch More

Microwave Ovens

This video explains quickly how microwave ovens work.




Watch this video to learn more details about microwave ovens.



Watch this video to learn more about the invention of microwave ovens.


Black Polar Bears

Polar bears appear white, which helps them blend against white snow. However, polar bear fur is not actually white. Polar bear fur is actually transparent and colourless. The fur appears white when it reflects light.

You might be even more surprised to discover that polar bears have black skin! The black colour helps polar bear skin to absorb radiant energy from the Sun. This keeps polar bears warmer in their cold Arctic habitat.
Figure C.2.7.7– Polar bears appear white.

Figure C.2.7.8– When you look closely, you can see that polar bears have black skin.
Figure C.2.7.9– The black skin of polar bears shows on their noses, lips, and paws.

 Watch More

Black Polar Bears

Watch this video to learn more about the appearance of polar bears.

  Try It!


Solar Heating

Try this simple experiment to see how colour affects heating water by radiation.

Materials:

  • black plastic garbage bag
  • white plastic garbage bag
  • scissors
  • ruler
  • 2 clear glasses
  • water
  • thermometer
Take care with scissors; don't cut yourself or anyone else!
Instructions:

    1. Cut a 50-cm by 50-cm square from each garbage bag.

    2. Push each plastic square into a glass so that it acts as a liner.

    3. Fill the 2 glasses with water to the same height.

    4. Fold the plastic square over the top of the water in the glass.

    5. Put both glasses in a sunny window. Leave the glasses for 3 hours.

    6. After 3 hours, measure the temperature of the water in each glass.
The following video is similar to your Try It! Watch to see what happens.
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 glass lined with black plastic has the warmest water. Black-coloured objects absorb lots of radiant energy from the Sun.
The glass lined with white plastic has the coolest water. White-coloured objects reflect lots of radiant energy from the Sun.



Try It!

Solar Oven S’mores

Try this simple experiment to make melted marshmallow s’mores in a homemade solar oven.

Materials:

  • empty cardboard pizza box
  • clear plastic sheet protectors or clear plastic wrap
  • black construction paper
  • tape
  • scissors
  • thermometer (optional)
  • wooden skewer
  • glue
  • aluminum foil
  • pen
  • ruler
  • graham crackers
  • large marshmallows
  • chocolate bar

 Figure C.2.7.9 - The black skin of polar bears shows on their noses, lips, and paws. Photo by Gerry Dincher.
Take care with scissors; don't cut yourself or anyone else!
Instructions:

Watch this video to see how to build your solar oven and make s’mores. You will must place your solar oven outdoors or in a sunny window.

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 black paper absorbed lots of infrared radiation from the Sun. The black paper absorbed heat energy, which it transferred to the surrounding air.
The aluminum foil reflected infrared radiation into the box. The foil directed more radiation toward the black construction paper.
The clear plastic trapped warm air and infrared radiation inside the box. It helped the solar oven to heat faster.



  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 7 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!
|

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.

In the northern hemisphere, the Sun is located to the south. Having several windows on the south side of the house allows lots of sunlight to enter the house. Sunlight contains thermal radiation that enters the house through windows. The thermal radiation is absorbed by objects and air inside the house. This increases the temperature inside the house.
Frozen food coolers are white-coloured because white objects reflect radiant energy. Reflecting radiation reduces warming of the frozen food cooler with the resulting melting of the frozen food.
Concrete is often dark-coloured, especially concrete and paving on roads. Dark-coloured materials absorb well the infrared radiation of the Sun. The heat from concrete can transfer to the surrounding air. Rural areas have fewer dark-coloured materials to absorb radiant energy from the Sun. (Grass is less absorbent of heat than concrete is.)
Lamps in restaurants give off thermal radiation. Thermal radiation from heat lamps is absorbed by the food. This keeps the food warm until servers bring it to the tables.
Chicks must be kept warm because they have not yet grown large insulating feathers. Heat lamps give off thermal radiation. Thermal radiation is absorbed by the chicks, which keeps their bodies warm.