Adjusting Your Water Heater to Conserve Energy
Summary
In this lab-based activity, the students will accomplish two sequential goals. The first goal is to understand the law of conservation of energy. This understanding will be reached when the students mix cold and warm water with predetermined volumes and temperatures (thermal energy) only to find out that the thermal energy that was lost by the warm water is equivalent to thermal energy that was gained by the cold water. The second goal is to use their knowledge in part one to determine the temperature of a hot water with a known volume and an unknown temperature that has been mixed with cold water with known temperature and volume.
CollapseLearning Goals
This activity is designed for students to:
- (1) Use mathematics to calculate thermal energy (math skill).
- (2) Use thermometers to measure change of energy when cold and warm water are mixed (data analysis).
- (3) Transfer their knowledge to solve problems involving conservation of energy (critical thinking/synthesis of ideas).
- Concepts Covered:
- (1) The students will understand and calculate kinetic energy.
- (2)The students will learn the difference between temperature (average kinetic energy) and thermal energy (total kinetic energy).
- (3) The students will learn and demonstrate the law of conservation of energy.
- (4) The students will revisit and review physical and chemical changes of matter and decide what type of change is observable in this activity.
- Key Vocabulary Words:
(1) Kinetic energy
(2) Temperature
(3) Thermal energy
(4) Heat capacity
(5) Specific heat
- (1) Use mathematics to calculate thermal energy (math skill).
- (2) Use thermometers to measure change of energy when cold and warm water are mixed (data analysis).
- (3) Transfer their knowledge to solve problems involving conservation of energy (critical thinking/synthesis of ideas).
- Concepts Covered:
- (1) The students will understand and calculate kinetic energy.
- (2)The students will learn the difference between temperature (average kinetic energy) and thermal energy (total kinetic energy).
- (3) The students will learn and demonstrate the law of conservation of energy.
- (4) The students will revisit and review physical and chemical changes of matter and decide what type of change is observable in this activity.
- Key Vocabulary Words:
(1) Kinetic energy
(2) Temperature
(3) Thermal energy
(4) Heat capacity
(5) Specific heat
Context for Use
- This activity is designed for the Physical Science course that is given to 8 or 9 grade students.
- In my classes of 9th graders, this activity is a perfect fit in the part of the curriculum that covers thermal energy, law of conservation of energy, specific heat, and heat capacity.
- The activity is 1 - 2 days lab (about 80 minutes total) given during "Thermal Energy" unit to investigate and demonstrate the law of conservation of energy during the flow of heat from warm to cold water.
- The students will be working in teams of two. Each team needs 2 thermometers that read no higher than 40°C, 3 large beakers, and access to cold and warm water.
- In math skills, the students should be at least at the Pre-Algebra level.
- In my classes of 9th graders, this activity is a perfect fit in the part of the curriculum that covers thermal energy, law of conservation of energy, specific heat, and heat capacity.
- The activity is 1 - 2 days lab (about 80 minutes total) given during "Thermal Energy" unit to investigate and demonstrate the law of conservation of energy during the flow of heat from warm to cold water.
- The students will be working in teams of two. Each team needs 2 thermometers that read no higher than 40°C, 3 large beakers, and access to cold and warm water.
- In math skills, the students should be at least at the Pre-Algebra level.
Description and Teaching Materials
- Introduction: The main concepts that are approached during this activity will be taught before doing the activity. The students will be taught that:
(1) The law of conservation of energy states that energy cannot be created or destroyed, but can be transferred and/or converted from one place/form to another.
(2) The thermal energy change of a substance is related to the mass of the substance, its change in temperature; and its specific heat.
(3) The heat capacity of an object is the capability of that particular object to absorb energy, while its specific heat is the amount of energy required to raise the temperature of 1 g of the substance by 1 °C (or 1 Kelvin), or to raise the temperature of 1 kg of the substance by 1 °C (1 K).
(4) The unit of specific heat is j/(g.°C) or (j/kg.°C).
(5) The specific heat of water is 4180 j/kg.°C when mass is measured in kilograms and it is 4.180 j/g.°C when mass is measured in grams.
(6) The equation that is used to calculate change in energy is
Change in energy (ΔH) = Δt ● m ● Cp
- Materials:
- 2 thermometers that read up to 40°C (or a CBL unit with 2 temperature probes)
- Three beakers labeled A, B, & C (Use Styrofoam cups with covers to minimize heat absorption from or escape to surrounding air)
- Cold water (5-10 °C)
- Warm water (20 - 40°C)
- Hot water (> 60 °C)
- Procedures:
(1) The student will obtain a specific volume from the cold water (100 mL for example) and place in beaker A. They will then obtain warm water with the same volume and place in beaker B and measure the temperature of the 2 samples of water with the 2 thermometers or the temperature probes and record in data table.
(2) Pour cold and warm water samples simultaneously in beaker C and transfer the thermometers (or temperature probes) to beaker C too. Measure the temperature of the mixed water and record in data table.
(3) Use the above equation to calculate the change in energy (ΔH) for cold and warm water.
(4) Repeat the experiment 2 more times using different cold and warm water samples with different temperature values, but the temperature of the warm water should not exceeds 40 °C.
- Conclusions:
(1) What do you notice about the maximum temperature of the cold water in relation to the minimum temperature of the warm water?
(2) Draw (sketch) three different graphs to represent change in temperature of (a) cold water; (b) warm water; and (c) mixed water (combine graph a and b).
(3) Calculate the change in temperature, Δt, for cold and warm water (using the formula: Δt = tmax ― tmin)
(4) Calculate in joules the heat gained by cold water by using the formula:
ΔH = Δt ● m ● Cp
ΔH = Change in thermal energy
Δt = Change in temperature
m = Mass of substance tested (1mL of water = 1 gram of water)
Cp = Specific heat of substance tested (water)
(5) Calculate in joules the heat lost by warm water by using the above formula.
(6) What is the relationship between the energy gained by the cold water and the energy lost by the warm water?
(7) Repeat the experiment 2 more times as was explained in step #4 in the procedure above. Record data.
(8) Do your findings support the law of conservation of energy? Support your answer.
(9) Us,e the knowledge and understanding you have gained from this activity to design and carry out an experiment to solve the following problem (from USAFA Department of Chemistry - Mini-Lab 9):
- "You've just taken a class on energy conservation and you want to do your part. You realize that you can save a lot of energy (and even some money) by turning your water heater down to between 50 and 55 ºC. However, your water heater doesn't have a calibrated control (most don't) and, to make matters worse, your thermometer only reads up to 40 ºC!
- Collect some hot water from the tap (as hot as possible) - it should be well above 40 ºC. Use your knowledge of heat flow to determine the temperature of the hot water.
- What's your major source of error in this experiment?
- Describe at least one way that you could improve the accuracy of your temperature determination.
- Final:
Explain the principles and assumptions you used to determine the temperature of the water." [file 'assignment handout']
(1) The law of conservation of energy states that energy cannot be created or destroyed, but can be transferred and/or converted from one place/form to another.
(2) The thermal energy change of a substance is related to the mass of the substance, its change in temperature; and its specific heat.
(3) The heat capacity of an object is the capability of that particular object to absorb energy, while its specific heat is the amount of energy required to raise the temperature of 1 g of the substance by 1 °C (or 1 Kelvin), or to raise the temperature of 1 kg of the substance by 1 °C (1 K).
(4) The unit of specific heat is j/(g.°C) or (j/kg.°C).
(5) The specific heat of water is 4180 j/kg.°C when mass is measured in kilograms and it is 4.180 j/g.°C when mass is measured in grams.
(6) The equation that is used to calculate change in energy is
Change in energy (ΔH) = Δt ● m ● Cp
- Materials:
- 2 thermometers that read up to 40°C (or a CBL unit with 2 temperature probes)
- Three beakers labeled A, B, & C (Use Styrofoam cups with covers to minimize heat absorption from or escape to surrounding air)
- Cold water (5-10 °C)
- Warm water (20 - 40°C)
- Hot water (> 60 °C)
- Procedures:
(1) The student will obtain a specific volume from the cold water (100 mL for example) and place in beaker A. They will then obtain warm water with the same volume and place in beaker B and measure the temperature of the 2 samples of water with the 2 thermometers or the temperature probes and record in data table.
(2) Pour cold and warm water samples simultaneously in beaker C and transfer the thermometers (or temperature probes) to beaker C too. Measure the temperature of the mixed water and record in data table.
(3) Use the above equation to calculate the change in energy (ΔH) for cold and warm water.
(4) Repeat the experiment 2 more times using different cold and warm water samples with different temperature values, but the temperature of the warm water should not exceeds 40 °C.
- Conclusions:
(1) What do you notice about the maximum temperature of the cold water in relation to the minimum temperature of the warm water?
(2) Draw (sketch) three different graphs to represent change in temperature of (a) cold water; (b) warm water; and (c) mixed water (combine graph a and b).
(3) Calculate the change in temperature, Δt, for cold and warm water (using the formula: Δt = tmax ― tmin)
(4) Calculate in joules the heat gained by cold water by using the formula:
ΔH = Δt ● m ● Cp
ΔH = Change in thermal energy
Δt = Change in temperature
m = Mass of substance tested (1mL of water = 1 gram of water)
Cp = Specific heat of substance tested (water)
(5) Calculate in joules the heat lost by warm water by using the above formula.
(6) What is the relationship between the energy gained by the cold water and the energy lost by the warm water?
(7) Repeat the experiment 2 more times as was explained in step #4 in the procedure above. Record data.
(8) Do your findings support the law of conservation of energy? Support your answer.
(9) Us,e the knowledge and understanding you have gained from this activity to design and carry out an experiment to solve the following problem (from USAFA Department of Chemistry - Mini-Lab 9):
- "You've just taken a class on energy conservation and you want to do your part. You realize that you can save a lot of energy (and even some money) by turning your water heater down to between 50 and 55 ºC. However, your water heater doesn't have a calibrated control (most don't) and, to make matters worse, your thermometer only reads up to 40 ºC!
- Collect some hot water from the tap (as hot as possible) - it should be well above 40 ºC. Use your knowledge of heat flow to determine the temperature of the hot water.
- What's your major source of error in this experiment?
- Describe at least one way that you could improve the accuracy of your temperature determination.
- Final:
Explain the principles and assumptions you used to determine the temperature of the water." [file 'assignment handout']
Teaching Notes and Tips
- Make sure that students know that the amount of heat that is lost by the warm water was absorbed by the cold water.
- In the last three years, I have done the first part of this activity in my physical science classes. However, the final part in #9 in the conclusion is a new piece, suitable for critical thinking, that my students will be doing this year.
- In the last three years, I have done the first part of this activity in my physical science classes. However, the final part in #9 in the conclusion is a new piece, suitable for critical thinking, that my students will be doing this year.
Assessment
- The students will be evaluated on their knowledge and understanding of the concepts by grading their lab reports and making observations during discussions).
Standards
IIC 1. The student will know that potential energy is stored energy and is associated with
gravitational or electrical force, mechanical position or chemical composition.
IIC 5. The student will be able to describe physical and chemical changes in terms of the
law of conservation of energy.
gravitational or electrical force, mechanical position or chemical composition.
IIC 5. The student will be able to describe physical and chemical changes in terms of the
law of conservation of energy.