CON EDISON
WEB-BASED MIDDLE SCHOOL ACTIVITY
Shaking Water
What Happens When You Add Energy?
Overview
This activity demonstrates several fundamental science and energy concepts. Students
vigorously shake a small container of water and measure the change in temperature. In so
doing, they discover that they are converting potential energy from their muscles into
kinetic energy, which transfers thermal energy into the water as a result of the friction of
the molecules bumping against each other and the sides of the container. The heat energy
in the water is not “created”; rather, it is transferred from their muscles, which have stored
energy from food, which ultimately received energy from the sun. The inefficiency of the
energy conversion (from muscle power to heated water) becomes evident through the generation of “waste” heat: the shakers get warm, and they may even start to perspire.
While students are vigorously shaking the water, you have ample time to discuss energy
conversions and the relationship between temperature and molecular motion. When
they stop shaking, they compare the water temperature to the starting temperature. (If
they shake it vigorously for 15 - 30 minutes, the temperature may go up by as much as
5˚ or 10˚C). They will consider where the heat energy that went into the water came
from, and they will relate the temperature change to the weight of the volume to determine the amount of heat energy that was transferred to the water.
The activity works best as a class demonstration.
Objectives
By doing this activity students will understand that
• Energy is not created or destroyed.
• Energy exists in many forms, but ultimately all energy derives from the sun.
• Energy can be transferred from one form to another.
• Heat is one form of energy, and it is a function of the motion of molecules.
• The conversion of energy involves some inefficiency or loss.
Time Requirements
One class period
Materials
• Small container (such as a 2- or 3-ounce food storage container) with a
tight fitting, leak-proof lid
• Water at room temperature
• Thermometer (Celsius)
• Graduated cylinder (1 cc = 1 gram)
• Student worksheet (provided)
1.
Procedure
1. Fill the container about one-third full with room temperature water.
2. Measure the volume of water by pouring it into a graduated cylinder. Since one cubic
centimeter of water weighs one gram, you know exactly how much your water
weighs.
3. Measure the temperature of the water and then put the lid on tightly.
4. Shake the bottle vigorously for a minute or two, then pass the container around the
room, so each student has a chance to shake. (In that way, the shaking will remain
vigorous and everyone will remain involved.) the longer and more vigorously your
class shakes the container, the more dramatic the temperature rise will be!
NOTE: If students grip the container tightly in the palm of their hand,
there will be a significant transfer of body heat. Have them hold the
container with just their fingertips to minimize this conduction.
5. As students are shaking and passing the container, ask them to predict what will
happen to the temperature of the water? Will it go up, down, or stay the same? If
they predict a temperature change, ask them by how much, record their estimates on
the board, and ask them to explain the logic they used in making these estimates.
(The guesses may vary widely.)
You may also begin to explain (or review) that temperature is actually a
measure of molecular motion. The crashing of the water against the sides
of the container creates friction and increases the molecular motion, thus
raising the temperature. (In addition, a small amount of energy is transferred
by conduction from skin to the water via the walls of the container.)
6. Once everyone in the class has shaken the container, measure and record the water
temperature again.
7. Ask students if they noticed other changes as a result of shaking the water, such as
changes in body temperature or perspiration? What do these changes indicate about
the efficiency of the transfer of energy from their muscles to the water molecule?
These changes show the inefficiency of energy transfers. Your students’
muscles generated body heat, which is “wasted” energy if their goal was to
heat the water.
8. Relate the rise in temperature to the amount of energy that entered the water. A calorie is defined as the amount of energy needed to raise the temperature of one gram of
water by one degree Celsius.
Degrees of temperature rise (C) X volume of the water (g) = calories
2.
Food Calories Versus Energy Calories and
Inefficiencies of Energy Transfers
A food calorie (which is sometimes spelled with a capital “C”) is 1,000 times larger
than an energy calorie (which is always spelled with a lower case “c”). If 300 calories
of energy went into the water, that is the equivalent of .3 food Calories. The ratio of
energy used over energy expended represents the efficiency of the operation, so if your
body actually used 3 food Calories to add 300 calories to the water, the operation would
be 10% efficient. (3 X 1,000) ÷ 300 = 10%)
A great deal of wasted energy went into the students’ bodies (which they felt as a
warming up in their arms and muscles), and other heat energy was released to the room
from the students’ bodies as their heated muscles were cooled by the air.
Extenders
1. Have students vary the types and sizes of the containers, as well as the fullness of the
container. Find an optimum level and container type, then discuss possible reasons
for the variation. (You will find that smaller volumes of water heat up better, so the
container should not be too full, and harder containers yield better results than softer
containers.)
Background Science
Students may have heard of the “Law of conservation of energy,” which states that
“energy is not created or destroyed. Energy may be transformed from one kind into
another, but its total magnitude remains the same.”
Even when energy seems to explode out of nowhere, as in lightning or fireworks, the
light and heat energy is really being transferred from other forms of stored energy. The
energy that went into the lightning was stored as electrical charges in the atmosphere.
The energy that went into the fireworks display was stored in the chemicals in the firecracker. When lit, the chemicals released that energy in the form of light and sound.
Energy comes in many different forms. In addition to heat (thermal energy) and light
(radiant energy), there is also electrical energy, chemical energy, and physical (kinetic)
energy. All energy derives ultimately from the sun, and it is transferred through many
processes. The sun, for example, heats up the atmosphere to create wind and drive the
water cycle. Plants convert energy from the sun and store it chemically as starch.
Animals consume the starch and transform that stored energy into heat energy, kinetic
energy, electrical energy (in our neurons), and even chemical energy (from our excrement). In this activity, students transfer some of their potential chemical energy into
kinetic energy, where it quickly becomes thermal energy.
3.
Review
1. What kind of energy did you use to shake the water?
Muscle power: kinetic energy.
2. Where did that energy come from?
From the chemical energy stored in the food we ate.
3. What happened to that energy as you shook the water?
It increased the motion of the water molecules, which made the water heat up.
4. What does temperature measure?
Heat energy.
5. What is heat?
Molecules in motion.
6. Explain how this activity demonstrated the concept of energy conversion
and transformation.
Answers should explain how the food/chemical energy stored in our bodies was
converted to kinetic energy in our muscles, which transferred energy to water,
where it was converted into heat energy.
7. Discuss whether this was an efficient method of energy transformation.
Answers should discuss the waste energy that did not get transferred to the
water, such as warmed muscles or sweating palms.
4.
Worksheet
Name: _______________________
Starting Conditions
Initial water temperature: _____˚C
Volume of water (in cc’s): _____cc’s
Weight of water: ______ grams
Prediction
The temperature of the water will:
___ go up ___ go down ___stay the same after shaking.
The temperature will change by ____˚C.
This temperature change will occur because:
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
Observations and Explanation
Temperature of water after shaking: ____˚C
The temperature in the container changed by: ______˚C.
This change occurred because
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
How many calories were transferred directly into the water? ______ calories
(Temperature change in ˚C X volume of water in cc’s)
What other changes did you notice as a result of shaking the water, such as
changes in body temperature or perspiration?
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
How can you explain these other changes?
_______________________________________________________________
_______________________________________________________________
5.