Learning Lesson: Melts in you bag, not in your hands


OBJECTIVE Show the transfer of heat through radiation.
OVERVIEW The students will learn how the sun transfers heat to the earth through radiation.
TOTAL TIME 20 minutes
SUPPLIES Two (2) small pieces of chocolate
Two (2) small resealable snack bags
PRINTED/AV MATERIAL None
TEACHER PREPARATION This works best on a hot sunny day. You can also complete this demonstration indoors on a sunny winter day.
SAFETY FOCUS Summer weather Safety


Background

The earth receives its heat from the sun in the form of radiation. Many in the animal kingdom lay out in the sun to absorb this form of energy to warm their bodies. This form of energy is vital to life on this planet.

Procedure
  1. Place one piece of chocolate in a bag, seal it and label it with an 'A'.

  2. Do the same with the second piece of chocolate but label that bag with a 'B'.

  3. Take both bags outside and place bag 'A' in the sun and bag 'B' in the shade. Suspend bag 'A' in such a way to ensure is not touching the ground or located near a wall to limit any transfer of heat by convection or conduction. (If this experiment is done indoors, place bag 'A' in the window, exposed to the sun and keep bag 'B' in the shade.)

  4. 20 minutes later, inspect the chocolate in both bags.

  5. Ask the students to explain any change in consistency of the chocolate.
Discussion

Depending on how hot it is, the chocolate in the shade may also be softened or even partially melted. However, the chocolate in bag 'A' will be more melted. The bulk of the heating that takes place in bag 'A' is from direct solar radiation. This radiation is what causes objects, such as the metal on automobiles, to become hot. Radiation also causes sunburns.

Fast Facts
The initial melting temperature of chocolate is approximately 122°F (50°C). Americans eat an average of 11 lbs. (5 kg) of chocolate per person per year. The Swiss eat an average of 26 lbs. (12 kg) per person per year.


Live Weatherwise

Summer Weather Safety
Melanoma is the most serious type of skin cancer, and accounts for more than 75% of the deaths due to skin cancer. In addition to skin cancer, sun exposure can cause premature aging of the skin, wrinkles, cataracts, and other eye problems. When you play outdoors, there are five important steps you can take to protect against UV radiation and skin cancer:

  • Cover up. Wear clothing to protect as much of your skin as possible. Wear clothing that does not transmit visible light. To determine if clothing will protect you, try this test: Place your hand between the fabric and a light source. If you can see your hand through the fabric, the garment offers little protection against sun exposure.

  • Use a sunscreen. Experts recommend products with a Sun Protection Factor, or SPF, of at least 15. The SPF number represents the level of sunburn protection provided by the sunscreen. Products labeled "broad spectrum" block both UVB and UVA radiation. Both UVA and UVB contribute to skin cancer.

  • Wear a hat. A wide-brimmed hat is ideal because it protects the neck, ears, forehead, nose and scalp. A baseball cap provides some protection for the front and top of the head, but not for the back of the neck or the ears where skin cancers commonly develop.

  • Wear sunglasses. UV-absorbent sunglasses can help protect your eyes from sun damage. Ideal sunglasses do not have to be expensive, but they should block 99 to 100 percent of UVA and UVB radiation. Check the label to make sure they do. Darker glasses are not necessarily the best. UV protection comes from an invisible chemical applied to the lenses, not from the color or darkness of the lenses.

  • Limit direct sun exposure. UV rays are most intense when the sun is high in the sky, between 10 a.m. and 4 p.m. If you are unsure about the sun's intensity, take the shadow test: If your shadow is shorter than you, the sun's rays are the strongest. Seek shade whenever possible.

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