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.