Spice of Life Class 6 – Spring 2012
Chocolate, Energy Transfer, and The Speed of Light.
Overview
Microwave ovens have made cooking many types of food fast and easy. Even better –
they provide the opportunity to learn physics concepts, ranging from kinetic energy and
energy transfer to wave structure. In this activity, you will learn about wave structure,
and how microwaves heat food via kinetic energy changes and conduction. Based on
your knowledge of wave structure, you will then design and carry out experiments that
will allow you to determine the length of a microwave and to calculate the speed of light.
Scientific Background
Introduction
“In recipes that involve cooked food, ingredients are combined, heated, and transformed
into a totally new food product. Life is good.”
Now let's translate that into science talk:
Energy Sources. As we learned during the last class period, most of the energy for
cooking is supplied by electromagnetic radiation, which is a form of energy that exhibits
wave-like behavior (see next section) as it travels through space. Electromagnetic
radiation can be classified according to its wavelength (below).
What are Waves? A wave is an oscillation that travels through space and time,
accompanied by the transfer of energy. As you can see in the animation below, while
waves transfer energy from one location to another, they do not generally permanently
displace the particles of the medium through which they travel. In the animation the blue
dot moves up and down, but does not move to the right as the wave energy passes in that
direction.
The structure of a typical electromagnetic wave is shown below. The distance between
two adjacent crests (or any other identical parts of a wave) is called its wavelength.
Waves also have a frequency, which is the number of oscillations that pass a fixed point
in one second. The frequency of the wave is often determined by the wave source. For
example, most commercial microwaves have a frequency of 2450 Megahertz (MHz).
Finally, the power of a wave is related to its amplitude. Waves with higher amplitudes
have greater power. If you have ever played in ocean waves, this should make sense!
Figure from http://www.answers.com/topic/wavelength-3
How Are Waves Related to Cooking? Chemical reactions lie at the heart of many
kitchen recipes – particularly those that involve cooking. To start a typical chemical
reaction in the kitchen, energy (called activation energy) must be applied to the
ingredients. As energy is transferred into food molecules, their chemical potential energy
(a form of potential energy related to the structural arrangement of atoms or molecules,
including chemical bonds) is converted into kinetic energy (energy of motion). As more
energy is applied, the food molecules begin to vibrate…moving faster and faster.
Eventually this kinetic energy is sufficient to break apart old chemical interactions and to
allow new chemical reactions to occur. New food products result. The energy profile of
a typical chemical reaction is depicted in the figure below.
One common form of activation energy used in recipes is thermal energy. When waves
of electromagnetic radiation (such as infrared or microwave radiation) strike molecules in
your food, they cause the food molecules to oscillate and gain thermal energy, increasing
in temperature. In cooking, the two most important forms of electromagnetic radiation
that generate thermal energy are infrared radiation and microwave radiation. Another
common form of energy in cooking is mechanical energy (such as when you whip egg
whites to make lemon meringue pie topping). We’ll play with mechanical energy later
this semester.
So, to sum it all up:
In recipes that involve cooked food, ingredients (reactants) are combined, and an
energy source provides electromagnetic radiation. This radiation transfers its
energy directly into the food (or into a fluid or solid surrounding the food) in the
form of thermal energy. Ultimately, the thermal energy is transferred through the
food via conduction, and the kinetic energy and temperature of the food increases.
The kinetic energy provides the activation energy for dozens or more chemical
reactions. New, piping hot food products are produced. Life is still good.
Microwave ovens
Microwave radiation differs from infrared radiation in that it does not heat the
surrounding air, so cooking with microwaves does not involve convection. Instead,
microwaves act on the water molecules in food (and to a lesser extent some fats). As we
already discussed in class, water molecules are polar, that is, they have a positive charge
at one end of the molecule and a negative charge at the other end. As the microwaves
move through the oven cavity, they generate rapidly changing magnetic fields. The water
molecules spin over rapidly from end to end, trying to stay in line with the magnetic
fields. This spinning generates thermal energy, which heats the outer layers of your
food. Conduction then heats the inside…so microwaves do NOT cook from the inside
out! Substances like plastic do not vibrate at all, so microwaves will not cause them to
heat up – but conduction from your hot food sure will! The structure of metal is such that
microwaves simply bounce off it, which is why you have a metal grate in the see-through
door of your microwave.
A brief divergence into the pleasure of chocolate
Through the ages, chocolate has been an important part of human culture. It has played
an important role in religious ceremonies in South America, where it also served as a
form of currency, and even today is an important part of traditions in the U.S., ranging
from Valentine’s Day to birthdays. Why do we love chocolate soooo much? Here are a
few possibilities:
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Chocolate contains cannabinoid chemicals (active ingredient similar to that in
marijuana). However, these chemicals are present in trace amounts.
Chocolate contains phenylethylamine (a chemical that has amphetiamine-like
effects). However, so do other fermented foods like sausages.
Chocolate contains two stimulants: caffeine and theobromine.
Chocolate contains a bunch of sugar and fat. Unfortunately, this is the most likely
reason for our chocolate cravings!
The Activity
On the table in front of you, you will see:
1. several chocolate bars
2. microwave oven
3. metric ruler
Instructions:
1. Microwave radiation travels in waves through a microwave oven, as shown in the
figure below.
2. A formula for the speed of light is λν = c where λ is the wavelength of light, ν is
the frequency of light (in Hz, expressed in s-1) and c = the speed of light
(expressed in m s-1). The frequency of this microwave oven is 2450 MHz. Using
only the materials on the table in front of you, calculate the speed of light.
Team Concept Application Questions and Report
Names:
1. Analyze and summarize the results of your experiments below. Don’t forget to
show your calculations. What is the speed of light? What did you calculate? Did
you come close? If not, why not?
2. If you were to use a form of radiation with a shorter wavelength for this
experiment, how would you expect the frequency of that wave to change?
3. The final step on all microwave food package instructions is to let the food sit for
1-2 minutes before eating. Explain the purpose of this step, correctly
incorporating the terms kinetic energy, heat, temperature, radiation and
conduction.
4. Whole eggs in their shells normally explode in microwave ovens. A new kitchen
gadget has been invented that prevents this phenomenon. It has an outer plastic
shell and an inner metal chamber that holds the egg. The space between the inner
and outer layers is filled with water, as depicted below:
Plastic coating
Water
Egg
Metal lining
Again, explain how this gadget safely cooks an egg, correctly incorporating the
terms kinetic energy, heat, temperature, radiation, reflection, convections and
conduction.