Name: ______Hour: ______
ENERGY LOSS THROUGH HEAT:
How energy-efficient are you?
Introduction: There’s no such thing as a free lunch; every organism on the planet must get its energy from something else. Autotrophs (such as plants, algae and cyanobacteria) get their energy from the sun. Herbivores, such as cows, get their energy from eating autotrophs, carnivores get their energy from eating herbivores (or other carnivores) and saprovores get their energy from anything that dies. Ultimately, however, all of an organism’s energy comes from the sun (either directly or indirectly).
As energy is passed up a food chain, much of it is lost. Just as an engine loses some of its energy as heat, living organisms also radiate much of their consumed energy in the form of heat. Thus, not all of the energy that is eaten by an individual may be passed onto the thing that eats it.
Purpose: To calculate the amount of energy put into our bodies (food Calories), and to estimate the amount of that energy that is lost through heat.
Hypothesis: Predict how much of your food’s energy will be lost as heat:
Instructions:
Day One:
1. Keep a log of all of the foods (and amounts) that you eat between now and tomorrow. Record Calories if amount is known. We will use this data to calculate the total number of nutritional Calories (energy) that you consume in a 24-hour period.
Day Two:
Step one: Calculating total energy consumed through food
1. Using the computers in the classroom, go to https://www.supertracker.usda.gov/foodtracker.aspx
2. Using your log, enter in the items that you have eaten over the past 24 hours. Be sure to enter everything and approximate quantities accurately. For instance, if you had cereal, be sure to include the milk as well.
3. For each food that you’ve entered, add it to your running total.
4. Once you have entered everything from your log, look on the right side of the screen to find the total calories eaten. This is the total amount of energy that you have consumed in the past 24 hours. Enter this amount below:
Total energy consumed = ______Calories (kilocalories)
Note: Food Calories (kilocalories) are actually 1000 calories, so we need to multiply this amount by 1000 to convert to calories (for later calculations).
Total energy consumed = ______calories
Step two: Measuring heat loss from your hand
1. Obtain a test tube and carefully add 15 mL of room temperature water. Set in rack.
2. Insert the Vernier temperature probe into the test tube.
3. Record your initial water temperature in the data table below.
4. Hold the test tube in your hand. Wait 10 minutes. Then record your final water temperature in the data table below.
Initial water temp (ºC) / Final water temp (ºC)Step three: Estimating total heat loss (calculators needed)
1. Calculate the total change in water temperature (DT) by subtracting the initial temperature from the final temperature:
______- ______= ______
final temp initial temp DT (ºC)
2. A calorie is a unit of energy equal to the amount needed to increase 1 mL of water by 1º Celsius. Since we used 15 mL of water in this lab, we need to calculate the total amount of energy lost by your hand in 10 minutes:
______x ____15__mL____ x 1 calorie/(mL* ºC) = ______calories DT(ºC) volume of water conversion energy lost by hand
3. The palm surface of your hand is approximately 1% of the total surface area of your body[1]. Thus, to estimate your total surface area, you would multiply the surface area of your palm by 100. In order to estimate the total amount of heat you lost in 10 minutes, we will assume that you can multiply the energy lost through your hand by 100 (you will discuss problems with this assumption and others later).
______x 100 = ______calories
energy lost by hand total body heat loss (10 min)
4. Since we only measured heat loss over a period of 10 minutes, we need to estimate the total heat lost by your body over the course of an entire day (24 hours). 10 minutes is 1/6 of an hour, so we will multiply total body heat loss by 6 to estimate heat lost in an hour and then multiply by 24 to estimate heat lost in a day.
______x 6 x 24 = ______calories
total body heat loss (10 min) total body heat loss (24 hours)
5. Finally, we need to calculate the percentage of energy consumed (through food) that was lost by heat. To do this, divide the amount of energy lost through heat (in 24 hours) by the amount of energy consumed as food (calories in 24 hours; not kilocalories) and multiply by 100 (to convert to a %).
______/ ______x 100 = ______%
total body heat loss (24 hours) total energy consumed % energy lost to heat
Answer the following questions in complete sentences:
1. How does your estimated % energy lost to heat compare to your predicted value?
2. What made the temperature of the water increase?
3. When you bundle up in a sweatshirt, coat or wool pants, you feel warmer. Where does this warmth (energy) come from: the clothes, the environment or your body?
4. Where does all of your energy come from?
5. More accurate measurements in nature have revealed that most organisms (plants, animals, humans…) lose about 90% of their consumed energy.
- Explain some possible reasons why your estimate might be different.
- Pick two assumptions being made in these calculations and discuss why they may not be appropriate to make.
- How could this experiment be improved to give more accurate results?
Show your work for questions 6 – 8!
6. Assuming that you eat 2000 Calories worth of Oreos™ (mmmmm) and then lose 90% of that energy, how many of those Oreos™ Calories would a tiger get if he gobbled you up?
7. If you eat 5 oz of cooked fish, worth 160 Calories, how many food Calories did the fish have to eat to make that 5 oz chunk of meat? Remember, an organism loses about 90% of its consumed energy (mostly through heat).
8. You enjoy a 1-pound steak one evening, worth 2000 Calories. The cow that “donated” this steak was corn-fed.
- How many Calories did that cow have to eat (from corn) in its lifetime to create that 1-pound steak?
- How many Calories of sunlight were needed to create the corn that fed the cow that fed you? Remember, this is just the corn whose energy was used to create your steak, not the corn that was used to build the remaining 1999 pounds of cow.
- What happened to the sun’s energy as it was transferred from corn to cow to you?
[1] Sheridan, et al., The Journal of burn care & rehabilitation, 1995,vol.16,no 6,pp.605-606.