Energy on the Move, by Michael Leyden

A scientific inquiry into transferring energy that's good enough to eat.

Popping corn may be a harbinger of good things to come, but it's also a sign that heat energy is being applied to the corn.

This month, we'll use different ways to get the energy into the kernels. One group will pop corn the old-fashioned way, shaking a pot with a lid over a heat source. (Just to be on the safe side, you'd better take over the actual popping yourself.) A second group will use a hot air popper and the third will microwave their batch.

In each case, the kernels absorb energy to make the snack food, but the manner in which the energy is transmitted differs greatly.

Conduction. This is a process where energy is transmitted by vibrating particles. The burner on the hot stove causes particles of metal in the pot to move more rapidly. They collide with the oil and corn particles and cause them to move quickly, too.

Convection. This is the transfer of energy caused by the movement of a gas or a liquid. In the hot air popper, electrical energy is converted to heat, which is transferred to the kernels via circulating air. A hot air popper uses a true convection current because the air's motion is caused by a fan.

In a convection current – e.g., water boiling on a stove – gravitational energy sets the fluid in motion. The hotter particles of water near the bottom of the pot are less dense than the slightly cooler particles at the top. These cooler particles are pulled to the bottom by gravity and force the less dense water upward.

Radiation. This is the transfer of energy via electromagnetic waves. When popcorn is microwaved, radiation is used to get energy inside the kernels.

Conduction and convection occur only when there's a medium for transmission such as solids, liquids and gases. Energy from the sun does not reach earth by either of these two methods simply because space is a vacuum.

Exploding kernels. Getting the energy into the kernels is one thing, but the mechanism responsible for the kernels exploding needs to be discussed. About 13.5% of the kernel's mass is composed of water found in the soft starch inside the hard hull.

When heat is added, the water changes to steam, which takes up more space than an equal mass of cooler water. The steam expands so rapidly that it breaks the hull and the starch inflates with air.

Warning: The released steam is dangerous. When the microwave corn is all popped, use caution to poke a small hole in the bag and insert a thermometer to record the temperature. (Again, this is something that should be done by the teacher.)

Challenges
Math: If 13.5% of the popcorn is moisture, how many grams of water are there in 100 grams of unpopped corn?

Experiment: Wrap some kernels in a paper towel and place the towel on a board. Crack the hulls by hitting the kernels with a hammer. When energy heats the water, the vapor will escape through the cracks; the pressure will not build up and the kernels will not explode. Thus, the yield will be very low (very few will pop).

Multiculturalism: Have students write a narrative on why kernels pop as viewed from a culture that doesn't know about conduction, convection, radiation, expanding water vapor and the modern scientific explanation.

Experiment: In a screw-capped bottle containing about 100 ml of unpopped kernels, add 1 or 2 ml of water. Shake and wait several days. Try to re-pop them.

A simple act of popping corn allows students to see that conduction, convection and radiation are not just abstract ideas, but are tangible concepts used in food preparation.

Michael Leyden is Science Education Consultant, Eastern Illinois University, Charleston, IL.