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Activity

Dynamic Balance in a Bottle

Background:
Many people are concerned that humans are harming the natural balance of life on Earth. What does the word balance mean when we use it to describe our planet? Most of us think of balance in terms of standing straight and not falling down. Clearly we are not worried that the Earth will fall down.

Another common way that we use the word balance is to talk about the amount of money in a bank checking account. Say you have $400 in the bank. That is called your balance. You write some checks to pay bills and now you only have $50. Perhaps you earn money or sell something or get a gift and you put that money in the bank. Now your balance is $700. Then you buy some stuff, pay some more bills and it goes back down to $400.

There are a lot of things about our planet that are like that. There are influences that cause increases and there are influences that cause decreases, but the over-all result is that the total amount of the "planet thing" tends to remain the same.

Pages 114-119 of Dr. Art's Guide to Science provides an example of dynamic balance. The amount of water in each of the reservoirs of the water cycle stays fairly constant even though water is constantly flowing into and out of that reservoir.

We call this kind of situation dynamic balance. Dynamic means things are happening. Balance means there is no total change. Water in the ocean is dynamic - it keeps leaving through evaporation and returning through precipitation and runoff. Yet, the total amount of water in the ocean is not changing, so we say it is in balance.

Purpose:
Many different Earth systems demonstrate stability even though matter and energy are constantly flowing through them. These systems are examples of dynamic balance. The "Dynamic Balance in a Bottle" experiment illustrates the phenomenon of dynamic balance. It provides direct experience of how changes to a system can alter the existing dynamic balance. This experiment can help explain a wide variety of phenomena including each of the cycles of matter, Earth's energy budget, and the issues of ozone depletion and global climate change.

Equipment and Supplies:
For each person or group:
1 empty 2-liter plastic bottle with paper label removed
1 measuring cup (2 cup size is better than 1 cup size) or 500 ml graduated cylinder
Tub or sink with faucet (must be deep enough for 2-liter bottle)
1 Thumb tack
1 Funnel
Duct tape or other good tape
Stopwatch or clock
Ruler

Safety Issues:
Be careful with the thumb tack!!

Duration:
50 minutes

Procedures:
PART A

  1. Carefully using the thumb tack, poke 20 evenly-spaced holes in the bottle along the outside about 1/2 inch up from the bottom.
  2. Adjust the flow of water from the faucet so it is coming out at a rate of 400 ml per minute or about 1.5 cups per minute.
  3. Put the bottle (and funnel if it helps) under the tap so all the water flows into it.
  4. Carefully watch what happens with the water in the bottle. Write down your observations from the beginning until there is no longer any measurable change. If you do not get a steady level, adjust the flow rate so you do.
PART B
  1. Experiment with increasing the rate at which water enters the bottle. Determine a rate that increases the level without overflowing the bottle. Measure that rate and record it. Record any observations that may help explain your result.
  2. Reduce the flow rate back to the rate used in step 2. Using the tape, experiment with covering some of the holes in the bottle. Determine a number of holes covered that increases the level without overflowing the bottle. Record your procedure and results.
  3. Remove the tape. Poke more holes along the side near the bottom of the bottle. Record your procedure and results.

Analysis

  1. Discuss whether Part A of this experiment models dynamic balance. Justify your answer.
  2. Describe how the exit flow rate from the holes changes as the level of water in the bottle increases. Discuss how this change in rate affects the establishing of a stable level.
  3. Compare the effects of increasing the flow rate, covering the holes, and increasing the number of holes.

Conclusions and Discussion

  1. Which of the experiments provides a model of how the amount of water in the atmosphere remains the same even though water is constantly entering and leaving the atmosphere? (ANSWER)
  2. One example of something that humans are doing to change an existing balance on planet Earth is that we are burning oil, coal and gas. This results in extra carbon dioxide going into the air. Before humans started burning large amounts of these fossil fuels, the amount of carbon dioxide in the atmosphere had been fairly constant for many thousands of years. Which experiment models a change in dynamic balance due to increasing the rate of in-flow? (ANSWER)
  3. These experiments can also be used to model Earth's energy budget. In that case, what does the rate of water flowing into the bottle represent? What does the rate of water leaving the bottle represent? What does the level of the water in the bottle represent? (HINT: see pages 141-142 of Dr. Art's Guide to Science) (ANSWER)
  4. Which experiment models how increasing the greenhouse effect may affect the Earth system? Based on that experiment, what would you predict will be the result of increasing the greenhouse effect? (HINT: see pages 232-233 of Dr. Art's Guide to Science) (ANSWER)
  5. Which experiment models the effect of CFCs on the amount of ozone in the upper atmosphere? (HINT: see pages 227-229 of Dr. Art's Guide to Science) (ANSWER)

Appendix
DON'T WASTE THE WATER: Many of us are concerned about wasting water. If you do these experiments and it does not change the way you think about our planet and what you do in your daily life, then maybe you shouldn't bother doing the experiments. Don't waste the water. The idea is that doing these experiments helps all of us understand that there are natural balances in the kinds and amounts of things and living organisms on our planet. The amount of fresh, usable water is limited. It is the result of a dynamic balance among processes that increase that amount and processes that decrease it. Now that you understand dynamic balance, hopefully you will do things that save lots more water than the amount you used in these experiments.

 



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