WHAT IS HEAT?

Consider a mug of steaming tea — it feels warm to my fingers. If I touch it, heat flows from the mug to my hand. That’s the simple story: a hot thing warms up a cooler thing by the transfer of heat or, more strictly speaking, heat energy.

Image courtesy of Wikimedia Commons.

Heat itself isn’t a “thing” but rather a process of energy transfer — a distinction that is difficult for students and generally not fully grasped until middle school or later. People used to think that heat was some sort of weightless thing (called “caloric”) that moved from a hot object (the tea) to the cold object (my hand). But that didn’t explain why, for example, two cold things can heat each other up when you rub them together. We now know that heat is a form of energy, created by the movement of molecules in an object.

So, the most precise way to describe heat flow is this: The mug of tea was hot relative to my hand. When I touched the mug, the molecules in my hand moved faster as heat energy flowed from the mug to my hand. As a result, the mug had slightly less heat energy, and its molecules moved more slowly. The mug becomes “cold” over time. Cold is simply the absence of heat. Cold cannot be transferred. When I pick up an ice cube, my hand feels cold. Which way is heat flowing?

The most important thing for your students to know is that warm things can heat up cooler things. Precise definitions of energy can wait, but students should understand that heat (NOT cold) is transferred, and that heat is not an intrinsic property of an object. They also need to understand that the process of heat transfer continues until everything is the same temperature. If I leave my mug of tea on the table while I check my email, heat will flow from the mug to the room, until the tea is the same temperature as the room (called “equilibrium”). Thank goodness for microwave ovens.

WHAT ABOUT TEMPERATURE?

Temperature is a measure of how much heat energy an object has. When I say that “the mug of tea is hot (relative to my hand),” I’m saying:

1. The mug of tea has a higher temperature than my hand.
2. The molecules in the mug of tea are moving faster than the molecules in my hand.
3. Heat energy will flow in the direction from the tea to my hand (not vice versa).

Those statements all mean the same thing. The distinction between heat and temperature is not important for your elementary students.

Hot coalsWalking on hot coals is a good example of the difference between temperature and heat. Hot coals are at a high temperature, but if you touch them very briefly, only a small amount of heat energy is transferred to your feet. Your feet are not burned (though there’s a bit more to the story than that). The same is true when you sip hot coffee instead of gulping it down. Which action is more likely to burn you?

HEAT TRANSFER: CONDUCTION, CONVECTION, RADIATION

There are three main ways that you can get heat from something else: conduction, convection, and radiation.

Image courtesy of Waifer X via Flickr.

Conduction

Put a metal spoon in a pot on the stove, and the handle gets hot. This is conduction – the handing-off of heat through a material. This happens when the molecules in one part of the substance jiggle faster as they heat up. Then they transfer this heat energy to their neighbors and so on down the line in a sort of “telephone” game. Some materials conduct heat well, like that metal spoon. If you leave a wooden spoon in a pot of soup, the handle doesn’t get hot. Materials that don’t conduct heat well are called insulators. What are some other insulators? Ceramic, wood, plastic foam, wool, and air are all good insulators.

Image courtesy of Wikimedia Commons.

Your students may think that some materials, like blankets or jackets, have the property of “hotness.” However, these things are actually insulators, and thus good at helping to reduce conduction of heat toward or away from an object. Try wrapping one glass of ice cubes in a blanket and leaving another glass of cubes out in the air. Which one melts faster?

Convection

A wood stove warms the air around it, but it doesn’t warm the whole house until you turn on a fan to circulate that warm air. Convection is heat transfer through the movement of liquids or gases. Convection is part of the reason that wind makes you cold – heat flows away from your body more rapidly in a wind. Remember, you don’t “get” cold from the wind, you just lose heat.

Convection currents above a match. Image courtesy of Sebastian Ritter via Wikimedia Commons.

Radiation

Even with no fan, you feel the heat from the wood stove as you bring your hand close. How is that? Invisible electromagnetic waves (called infrared light) and visible light bring energy from the hot stove to your hand. This is called radiation. This is also the reason a car heats up on a sunny day – the energy from the sun is transferred to the car.

Some animals, like snakes, can see in the dark by sensing infrared light. Rescue workers use infrared sensors to find people in emergency situations.

KEEPING WARM

How do we keep warm in an Arctic winter? People are sources of heat – the food we eat helps to fuel our body’s furnace. (People in cold climates eat about twice as many calories as those in warmer climates.) We feel cold when we lose more heat energy than our body can produce. To stay warm, we must reduce the flow of heat away from our body. Blankets, coats, and houses are ways that we reduce heat transfer (from conduction, convection, and radiation) from us to the outdoors. Evaporation is another way that we lose heat.

Hypothermia is what happens when our body temperature drops because the heat flow out from our bodies is more than the body can replace. Hypothermia makes people dizzy and confused, can turn their skin blue, and can be fatal. Frostbite is when some part of the body – like the fingers – freezes, because the body stops circulating blood to that area in order to conserve warmth.

Warm clothing has changed over time. Native peoples wore animal skins, like seal, caribou, polar bear, wolf and fox, to stay warm. Animal skins keep people warm because the fur traps air, and air is a good insulator. A down jacket keeps you very warm because the feathers trap air.

Early explorers to the Arctic used fabrics like wool and cotton, which did not insulate them well enough and were heavy and bulky. Modern outdoor gear is made of light synthetic fabrics worn in layers: long underwear covered with a warm shirt, a jacket, and a windbreaker. Each layer traps air, providing extra insulation. We use boots, socks and mittens to protect the parts of our body that are far from our heart, which provides warmth through blood circulation.

Carl Ben Eielson, US pilot and Arctic explorer, and an Inuit grandmother dress for warmth. Images courtesy of Wikimedia Commons.

Igloos are also used to provide shelter and warmth for travelers and hunters. But how can an igloo keep you warm when it’s made of snow and ice? Snow and ice are actually good insulators. The igloo blocks the wind and, more important, keeps your body heat inside the structure’s air space. The snow on the inside of the igloo typically melts and refreezes, making a protective layer.

Photo courtesy of Wikimedia Commons.

Animals also need to stay warm in polar environments. Marine mammals and penguins have a layer of fat, or blubber, under the skin that provides insulation. The hairs in a polar bear’s fur are actually hollow, providing additional insulation. Other birds, such as snowy owls, have two layers of feathers. These adaptations enable survival in the harsh environments of the Arctic and Antarctic regions.

RESOURCES

AAAS Benchmarks for Science Literacy: Energy Transformations
This section of Benchmarks for Science Literacy details what students at K-2, 3-5, 6-8, and 9-12 should know about energy transformations, including heat.

AAAS Science Literacy Map: Energy Transformations
This portion of the AAAS Science Literacy Energy Transformations map illustrates the concepts that K-5 students should know about the transfer of heat energy.

Temperature and Heat Online Course
An online professional development course on heat and temperature for elementary and middle school teachers.

NATIONAL SCIENCE EDUCATION STANDARDS: SCIENCE CONTENT STANDARDS

The entire National Science Education Standards document can be read online or downloaded for free from the National Academies Press web site. The content standards are found in Chapter 6.

Teaching about heat energy can meet the Physical Science Content Standard for grades K-4 and 5-8:

K-4 Physical Science

• Light, heat, electricity, and magnetism

5-8 Physical Science

• Transfer of energy

Teaching about animals’ adaptations to life in cold environments can meet the Life Science Content Standard for grades K-4 and 5-8:

K-4 Life Science

• Organisms and their Environments

5-8 Life Science

• Regulation and Behavior
• Diversity and Adaptations of Organisms