Learning Lesson: Canned Heat


OBJECTIVE Show light and dark colored objects absorb the sun's radiation at different rates.
OVERVIEW The demonstration will show that water in a dark colored can will have a higher temperature after exposure to the sun than water in a shiny can.
TOTAL TIME 2 hours
SUPPLIES Two empty coffee cans, (3 lbs size)
Thermometer
"Flat Black" spray paint
PRINTED/AV MATERIAL None
TEACHER PREPARATION One coffee can needs to have a shiny metallic surface inside and out. The other can should be painted flat black inside and out.
SAFETY FOCUS Summer safety rules


Background

Different colored objects absorb energy at different rates. That is partially due to albedo. Albedo is the amount of reflection from a surface.

Procedure
  1. Fill the two cans with about two inches of water.

  2. Measure the temperature of the water in each can. (The readings should be the same.)

  3. Remove the thermometer and place the cans in a sunny location where they will not be disturbed and receive two hours of sunlight.

  4. After two hours, measure the temperature of the water in each can.
Discussion

The darker and duller the object, the more energy that object absorbs. The lighter colored or the shiner the object is, the less energy that its absorbs. Since more energy was reflected from the shiny can, less energy is absorbed than in the black painted can. But how does the water become warmer?

The water is heated by the energy emitted by the cans. Some objects, like glass, seem to absorb light hardly at all - the light goes through. For a shiny metallic surface, the light isn't absorbed either, it gets reflected. For a black material, light and heat are almost completely absorbed.

Heated bodies radiate energy, operating in reverse. The more an object absorbs, the more it radiates. The water in the shiny can was not as warm as in the black can because it absorbed less energy and therefore has less energy to radiate into the water. This is called "black body radiation".

Live Weatherwise

Summer Weather Safety
Avoid the Heat. Stay out of the heat and indoors as much as possible. Spend time in an air conditioned space. Only two hours a day in an air-conditioned space can significantly reduce the risk of heat-related illness. Shopping Malls offer relief if your home is not air-conditioned. If air conditioning is not available, stay on the lowest floor out of the sunshine. Remember, electric fans do not cool, they just blow hot air around.

Dress for the heat. Wear loose-fitting clothes that cover as much skin as possible. Lightweight, light-colored clothing reflects heat and sunlight and helps maintain normal body temperature. Protect face and head by wearing a wide-brimmed hat. Avoid too much sunshine. Sunburn slows the skin's ability to cool itself. Use a sun screen lotion with a high SPF (sun protection factor) rating.

Drink FOR the Heat. Drink plenty of water and natural juices, even if you don't feel thirsty. Even under moderately strenuous outdoor activity, the rate your body can absorb fluids is less than the rate it loses water due to perspiration. However, if you have epilepsy or heart, kidney, or liver disease; are on fluid-restrictive diets; or have a problem with fluid retention, you should consult a doctor before increasing liquid intake.

Do not drink IN the Heat. Avoid alcoholic beverages and beverages with caffeine, such as coffee, tea, and cola. Alcohol and caffeine constrict blood vessels near the skin reducing the amount of heat the body can release. Although beer and alcohol beverages appear to satisfy thirst, they actually cause further body dehydration.

Eat for the Heat. Eat small meals more often. Avoid foods that are high in protein because they increase metabolic heat. Avoid using salt tablets, unless directed to do so by a physician.

Living in the Heat. Slow down. Reduce, eliminate, or reschedule strenuous activities such as running, biking and lawn care work when it heats up. The best times for such activities are during early morning and late evening hours. Take cool baths or showers and use cool, wet towels.

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