Labs from YSI 95:
How Much Energy Does the Earth Receive from the Sun?

Todd Duncan
CARA Yerkes Summer Institute, August 1995

This is the staff copy of the lab and other brainstorming/reflections.

Teacher's Guide:

(Note that this lab is to be used in conjunction with the "Photometry" lab, also from the 1995 YSI.)

Materials:

Outline of Class:

(This activity was designed for a 2 hour class session. Notes here are rough suggestions of what to say to the class.)

Before class: prepare a bucket of water with the dye, cover with foil, leave outside in the shade to equilibrate to ambient air temperature. (This will save about 30 minutes of class time).

Introductory discussion: We're experimenting with sunlight, but this activity is really all about understanding energy.(After discussing their answers to questions): Energy is really a very elusive concept. A standard definition is something like "Energy is the ability to do work." But this definition doesn't tell us very much at all: What is work, exactly? How do I know if I have enough energy available, to do the work I want to do?
I'm not going to give you a precise, general definition for energy. Instead we're just going to work with some examples of it, and of how you can put the concept to practical use. Once you've worked with it awhile, you can come up with your own definition, that makes sense to you. Hopefully, you'll also see how powerful and useful the concept of energy is. But first a few key points about energy:

  1. Every activity that you do in the course of a day requires a certain amount of energy to make it happen. Energy is needed to run the radio that wakes you up in the morning, to lift yourself out of bed (sometimes this seems to take a lot of energy!), to run the heater to give you hot water for a shower, to transport you to school, etc. If the activity requires more energy than you have available, it won't happen. (example of air-conditioners during heat wave, leading to Commonwealth Edison asking people to turn off lights and computers)
  2. Energy can exist in many different forms. (ask them to list different kinds of energy they know about). (things they might think of are kinetic, potential, chemical, electrical, heat,....encourage as many ideas as possible, even if some are really duplicates of the same thing). Show them how to calculate the amount of energy for a few cases, maybe kinetic and gravitational potential.
  3. Energy is conserved. You can take energy from one place and move it somewhere else, or change it from one form into another (e.g. from energy in sunlight to energy in the form of hot water). But if you add everything up, making sure nothing slipped away unnoticed, the total amount of energy stays the same. However, some forms of energy are much more useful than others for the things you want to do. Gasoline stores energy in the form of chemical energy that can be burned in your car to provide the energy to make it move. When you brake, that energy is converted to heat, which is basically useless for making your car go.
Now here's the really amazing and useful thing about energy: Remember a few minutes ago we talked about all the different forms of energy? For each of those cases, the setup is totally different. In one case we're talking about a cup of hot water, in another we might be talking about a tank of gasoline, in another a book about to fall from the edge of a table, in another a baseball flying through the air. And for each of these cases, the way we use the properties of the system to calculate how much energy it has is totally different (refer to examples above: one cases uses height and mass, another speed and mass, etc.). But once we've used these formulas to calculate this mysterious quantity we call energy for one setup, then we can forget about the specific details of that setup. This single quantity, the energy, is all we need to know to determine it could make our car run, heat the water to a certain temperature, etc.

This is a difficult concept, so a few examples are needed to show how the concept of energy gives you a common ground for talking about seemingly unrelated things: temperature and velocity, for example, are both involved with energy, though they seem very different things.

Discuss standard for measuring amounts of energy: temperature change produced in a certain volume of water. Define 1 calorie = amount of energy required to raise 1 gram of water by 1 degree C. Emphasize again the connection between different forms of energy. Knowing how many calories you have eaten (say 2000 kilocalories) tells how much water could be heated by the energy stored in that food. Or how much water could you heat to 100 C with the energy used to launch a rocket out of earth's orbit.

Return focus to energy from the sun: what are two things that would be different about earth if we didn't have the sun around? (no light, no heat)...forms of energy.

Energy from sun has to travel through empty space (a lot of space: 90 million miles!!) to get to us. It does this in the form of light (which is another name for radiation). We said before that one way we can tell there is energy flowing is that it will heat something up. So lets go outside and see if it feels like there is heat coming from sunlight...(establish their qualitative understanding of the effect we're going to measure quantitatively: remind them that you can really feel the heat transmitted by sunlight.

Next we need some good way to measure more accurately just how much energy there is: emphasize that number only matters as a standard, so that people uniformly can use that number to figure out what they can do with that energy (boil water, heat their house, power a car, run a computer, etc. ...remember Apollo 13, big problem was to be able to run the equipment they needed with the small amount of energy they had available. Try to lead them into suggesting how to do the measurements, then show them materials for building what THEY have suggested.

This afternoon we're going to build a calorimeter to convert the light from the sun into heat. We'll measure how much the sunlight heats some water, and use that information to figure out how many calories or joules are in the sunlight.

See procedure on student activity sheet for how to build the calorimeter, and carry out the rest of the experiment and data analysis.

Mention that same kind of calorimeter is used in chemistry labs

Student Worksheet

Introduction:

The aim of this lab is to help you understand more about energy: What is it? Why do we need it? Where can we find it? How do we measure how much of it we have? We'll be experimenting with the energy that reaches us in the form of sunlight. To get started, let's see what we already know about energy:

1)  What do you think "energy" is?





2)  List some things that require energy to make them work (tools, appliances, games, etc.; 
any kind of activity that you think requires energy): 







3)  For each of the things you listed above, try to identify the source of the energy it uses 
(e.g. a battery,...): 







4)  List anything you can think of that does NOT require energy to make it work: 






A Few Facts About Energy:

A standard definition of energy is something like "Energy is the ability to do work." That's a good start, but this definition doesn't really tell us very much: What is work, exactly? How do I know if I have enough energy available, to do the work I want to do?

I'm not going to give you a precise, general definition for energy. Instead we're just going to work with some examples of it, and of how you can put the concept to practical use. Once you've worked with it awhile, you can come up with your own definition, that makes sense to you. Hopefully, you'll also see how powerful and useful the concept of energy is. But first a few key points about energy:

  1. Every activity that you do in the course of a day requires a certain amount of energy to make it happen. Energy is needed to run the radio that wakes you up in the morning, to lift yourself out of bed (sometimes this seems to take a lot of energy!), to run the heater to give you hot water for a shower, to transport you to school, etc. If the activity requires more energy than you have available, it won't happen.
  2. Energy can exist in many different forms, and can be converted from one form to another. (We'll come up with some examples of different forms in class...you might want to list a few of them here):

  3. Energy is conserved. You can take energy from one place and move it somewhere else, or change it from one form into another (e.g. from energy in sunlight to energy in the form of hot water). But if you add everything up, making sure nothing slipped away unnoticed, the total amount of energy stays the same. However, some forms of energy are much more useful than others for the things you want to do. Gasoline stores energy in the form of chemical energy that can be burned in your car to provide the energy to make it move. When you brake, that energy is converted to heat, which is basically useless for making your car go.

    In order to compare amounts of energy which appear in different forms, we need to set up some standard way of measuring the amount of energy. One convenient way to do this is to use energy in the form of heat as a standard. A common unit of energy is the calorie, which is defined as the amount of energy needed to raise one gram of water by one degree Celsius. Another common unit of energy is the Joule, which is 4.186 calories.

Procedure:

We'll split up into 4 groups. All groups will do the same experiment, except for one thing: Two groups will cover their styrofoam cups with aluminum foil, while the other two will cover theirs with plastic wrap.

Record the surface area of the top surface of the water in the styrofoam cup: _______

____________________________________________________________________________


Measure out about 200 ml of dyed water into a styrofoam cup.  Be sure to note 
ACCURATELY how much water you used, and record the volume of water here:

	volume of water used: ______________________ milliliters

	
Attach the probe of the thermometer so that the tip is about in the middle of the cup, 
away from the sides or bottom, but well submerged in the water.  Secure the probe by 
taping it to the lip of the cup.

Cover the mouth of the cup with plastic wrap (aluminum foil), and secure the cover 
with tape or a rubber band.

Get another styrofoam cup, and cut off the top part of the cup (about 1/2 inch wide).  
Tape this piece to the top of your original cup, and then attach another layer of plastic 
wrap (aluminum foil) to the top of this.  

You should now have a cup full of black water, with a thermometer probe in the water.  
The opening of the cup should be covered by 2 layers of plastic wrap (aluminum foil), 
one layer separated from the other by about half an inch.  This is your calorimeter, 
which you will use to record how much energy we are receiving from the sun.

Move the calorimeter into the shade outside, and monitor the temperature until it 
stabilizes, and you observe no temperature change for several minutes.  

Now go out in the sun and prepare a stand for the calorimeter.  Use the cardboard to 
make a ramp on which the calorimeter will be placed.  Push a straight pin into the top 
surface of the ramp, making sure it is perpendicular to the surface.  Adjust the angle of 
the ramp until the shadow of the pin disappears.  This indicates that the sun is shining 
directly onto the surface of the ramp.   

You are now ready to begin the real experiment.  Attach your calorimeter to the ramp 
using a bit of the putty.  Record the starting time, and the temperature reading on your 
digital thermometer:

	starting time: _____________			starting temperature: ___________C


Keep monitoring the experiment until the temperature has increased by about 3 C or 
more.  (or until the sun is covered by a cloud!).  Record the ending time and 
temperature.

	ending time: _____________			ending temperature: ___________C

Now we have all the data we need, to figure out how much energy your cup of water 
absorbed from the sun:
	
First subract the start time from the ending time, and convert to seconds, to get the 
elapsed time (ET):
	
	ET = time(end) - time(start) = ____________ seconds

Then subtract the starting temperature from the final temperature, to get the temperature 
increase (TI) in your water:

	TI = T(end) - T(start)= ___________ C

We learned earlier that for each milliliter of water and for each degree C that bit of water 
has its temperature raised, 4.186 Joules of energy are needed.  So multiply TI by the 
volume of water you used, and then multiply by 4.186, to find out how many Joules of 
energy were transferred from the sun, into your water:
	
	Total energy transferred = (TI) x (4.186) x (volume of water in ml) = 

_____________Joules

This tells you the total amount of energy that was absorbed over the whole area of the 
surface of the water, in the roughly 20 minute time period of the experiment.  To get the 
amount that was transferred in each second, over each square cm of surface, you need 
to do some division:

	Energy flux =  (total energy transferred)/ (surface area) / (elapsed time)
	
	= ______________________ Joules/cm2/sec

Record here the energy flux values for each of the 4 groups, indicating whether plastic 
wrap or aluminum foil was used in each case:








Find the average of the 2 values obtained from the plastic wrap experiments.  This is 
your final result for the energy flux from sun:





Questions about the experiment and your results:

1)  The standard value for the "average" amount of energy from the sun which reaches 
the top of the earth's atmosphere, per second, and per square centimeter, is 0.14 
Joules/sec/cm2 .  If our value came out higher or lower than this standard number, 
explain why you think this happened. 





	
2)  Why did we make the water black, rather than, say, white?  [Hints:  1) Think about 
what happened in the experiments where the cup was covered with aluminum foil, vs. 
plastic wrap.  2) What happens to the light that hits a black object, to make it look 
black?  What happens to the light hitting a white object, so that it looks white?]






3)  Look at the number you recorded for the TOTAL amount of energy you collected 
from the sun.  (If you did the aluminum foil experiment, use the results from someone 
who did the plastic wrap experiment).  How long could you keep a 60 Watt lightbulb lit 
with the energy you collected?  (A Watt is one Joule per second):


What does this tell you about the difficulties in using solar energy as a major power 
supply?




Important Disclaimers and Caveats