Background information for the Solar Energy Lab

• The Difference Between Heat Energy and Temperature
• What happens to the sun's energy when it arrives on Earth?
• Which is easier to heat up: air or water?
• What is the Solar constant and how does it compare for different places on Earth?
• How accurate will our measurement of the Sun's intensity be?
• How does the angle affect the energy received?

• Energy Challenge
• Vocabulary

 The Difference between Heat Energy and Temperature

• What is the difference between temperature and heat energy?

When you touch a hot cup of cocoa, the heat energy of the cup flows into your hand, because the cup has a higher temperature than your hand. You feel the heat, or energy flow from hot to cold. Heat is the thermal energy that is transferred from one body to another and it always flows from a substance at a higher temperature into the substance with a lower temperature. Heat energy is useful for doing many kinds of work. In many situations, part of heat energy is used to do work or it is converted to another form of energy:
• a flame provides energy to break chemical bonds when ice changes to water or water changes to steam
• burning gasoline causes pistons to move in your car that force the car to move,
• burning coal boils water to create steam that will make electrical generators run to light and heat/cool our homes
Adding heat energy to a system DOES NOT mean that its temperature must increase. Often, part of the heat energy is converted to another kind of energy (electrical, wind, water,...) that cannot be measured with a thermometer.

 Scientists have learned there is a limit to how much useful work can be extracted from heat energy and this limit is related to the Second Law of Thermodynamics which basically says that you cannot make a machine that converts 100% of heat energy to useful work.

Temperature, on the other hand, provides a macroscopic measurement of the average random motion of the atoms and molecules of a substance. Temperature is a relative quantity that tells us the direction energy will flow when two objects come into contact. A tea cup of water and a large pot of water can have the same temperature, but the large pot of water will have a lot more heat energy than the tea cup of water. If you put the tea cup and large pot in contact with each other no heat energy will transfer as long as both have the same temperature. Heat energy is measured in calories (lower case 'c'), Calories (used on food labeling) or joules, and temperature is measured in degrees.

1 Joule = 0.2389 calories
1 calorie = 4.186 Joules
1 Calorie = 1000 calories

When you put a pot of water over a flame and watch its temperature rise, the flame gives heat to the pot and the water, and raises their temperature. A bigger pot with more water needs more flame and fuel to get the same increase in temperature in the same time. You could say it needs more heat.

• What happens to the sun's energy when it arrives on Earth?

When you step into the sunshine you notice how warm it feels. Electromagnetic radiation produced on the sun has traveled through the vacuum of space at the speed of light and is absorbed and reflected by your skin. The amount of this radiant energy received by the earth has stayed remarkably constant, even though we experience significant temperature changes during the year. As the seasons change from summer to winter, our location on earth tilts away from the sun's rays so the rays strike at an angle and the density of the energy is less.

The warmth you feel in the sunshine isn't just due to the fact that the sun is hot, it's surface is about 6000 degrees Celsius, which is about the same as the flame of a welding torch. You are warmed because the sun is both hot and very big. It emits an enormous amount of radiant energy which becomes heat energy when absorbed on Earth. Less than one billionth of the Sun's output is intercepted by Earth. The rest travels outward in space to make our sun look like a middle age, average star to the rest of the galaxies in the universe.

When the Sun's radiation does arrive on the Earth, it can be reflected or absorbed and re-emitted. Some of it bounces off back into outer space (reflection) - clouds and snow are good reflecting surfaces. Some of it is absorbed and re-emitted by the rocks, plants, building, lakes, oceans, and even your body lying on the beach. In spite of all this complexity, our planet keeps in an energy balance: all the 'arriving' energy is balanced by 'escaping' energy.
 

• Which is easier to heat up: air or water?

The answer to this question depends on what you mean by "easier to heat up". When water increases in temperature, the volume stays approximately the same. Most liquids expand only slightly when heated. If the water is adequately contained, there will be very little leaking to the surrounding environment. While it is easy to contain, water does require an extraordinary amount of energy to change its temperature: a cubic centimeter of liquid water requires 1 calorie to raise its temperature by 1 degree Celsius. Most other materials require a fraction of a calorie to experience the same 1 degree change.

When air increases in temperature, the air inside a jar will try to expand. This can lead to 2 possible limiting situations:

• If the jar lid is screwed on tightly, then the volume of the air will stay the same and its pressure will increase. In this case, 0.17 calories are needed to increase temperature of 1 gram of air by 1 degree Celsius
• If the jar could expand like a balloon to keep pressure the same , then the gas volume will increase. In this case, 0.24 calories will be needed to increase the temperature of 1 gram of air by 1 degree Celsius.
If we just open the jar, then our air sample becomes part of a open system (the earth's atmosphere). Warm air will rise and cold air will descend, so that mixing occurs. This makes it difficult to estimate actual energy changes. Warm air is associated with lower pressure, since warm air tends to expand and move upward. Cold air is associated with high pressure because it tends to be heavy and move downward due to gravity.

So which is easier to heat up: air or water? For each gram, air provides 4-5 times more degrees Celsius temperature change than water for each calorie invested. The difficulty with air is that it is harder to contain in a jar. As temperature increases, the air will increase pressure and try to expand. If some of the air escapes from the jar, we will get incorrect results. So, in this Energy Lab, we chose to use jar of water to estimate the energy absorbed in a 20 minute time period.
 

• What is the Solar constant and how does it compare for different places on Earth?

The amount of heat energy received each minute over each square centimeter at the top of the earth's atmosphere is called the solar constant. Scientists have measured the solar constant to be about 2.0 calories per minute for each square centimeter. This number should be the same for all parts of the Earth in the sunshine if the energy is measured perpendicular to the sun's rays (background activity #3). However, the amount of energy that actually reaches the ground is affected by the incident angle of the radiation, the thickness of the atmosphere and the presence of clouds. When the sun is low in the sky it's rays have to pass through more of the earth's atmosphere, which means less of the energy actually gets here. When you do the Energy Lab be sure to do your measurement as close as possible to midday (noon). That's when the sun's rays are most perpendicular to the Earth's surface and travel through the least atmosphere. See the diagram which shows that more energy is absorbed by the atmosphere when the sun is low in the sky.
 
• How accurate will our measurement of the Sun's intensity be and why not 100%?

There are several factors that will affect your estimate of the solar intensity:
• ATMOSPHERE THICKNESS: presence of clouds, pollution, aerosols, and dust will cause a smaller temperature change.
• JAR REFLECTION: reflection of light off the curved jar surface will cause a smaller temperature change
• ENERGY LEAKAGE: during the heating period, the jar can lose energy to the surrounding air and this will cause a smaller temperature change
• TILT ANGLE: if the jar is not properly tilted to face the sun, it will collect less energy and this will cause a smaller temperature change.

 
• How does the angle affect the energy received?
In this activity, we want you to set the energy collecting jar so that it directly faces the sun. Why is that important? If the collecting surface does not point directly at the radiation source but is tilted at an angle, less radiation will strike the surface and be collected.

You can experiment with this effect in at least two ways:

• Attach the photocell to your soil moisture resistance meter. Take readings from the meter when the cell is directly pointed towards the sun (or a light) and when it is tilted at an angle.
• Attach a thermometer to a surface that can be adjusted to different tilt angles. See 'set up'. Illuminate the thermometer with a high- Wattage lamp that is kept at a fixed distance from the thermometer and measure the equilibrium temperature for different tilt angles.
Make a graph of your readings (tilt angle vs. photocell readout or equilibrium temperature).

 An Energy Challenge

In your energy experiment, the sun added energy to the water inside the jar. In this activity YOU will add the energy! You will use the increase in temperature to calculate how much energy you've added to a jar of water.
 
Problem: How much energy can you add to a jar of water in 20 minutes by mechanical means only?
Equipment:
• A jar with a tight fitting lid for each team of students.
• 2 cups (300 g) of water for each team
• the shielded alcohol thermometer provided in the GLOBE equipment
• a stopwatch or other timer

 
Procedure:
• Form teams of two or three students.
• Place 2 cups (300 g) of water in each jar.
• Measure the temperature of the water in the jar.
• Tightly close the jar with the lid.
• On the mark, all teams begin to add energy to the jars. If you wish, stagger the starting times by a minute or two to allow time for temperature measurement for each team. The temperature must be increased by mechanical means only. No electrical, solar, flame, or other source of heat can be used. HINT: Teams member can take turns vigorously shaking the jar, rolling it on the floor, jumping up and down with it, etc. etc.
• STOP after 20 minutes. Quickly open the jars and immediately measure the temperature of the water.
• Use the formulas from the Energy Lab to calculate how much heat energy has been added to the water:

Record the temperatures and the energy increase. Send your best results to Mary Ellsworth. We'll see who is the best SOAR-High heat generator!

 Vocabulary

temperature - A measure of the average kinetic energy per molecule in a body. Measured in degrees Celsius or Fahrenheit or Kelvin.
 
 heat - Thermal energy that flows from a body of higher temperature to a body of lower temperature. Measured in calories or Joules.

 calorie - The heat energy required to raise the temperature of one cubic centimeter of water by one degree Celsius.

 The Solar Constant - The amount of heat energy received each minute over each square centimeter at right angles to the sun's rays at the top of the atmosphere.

 absorption - The receiving of radiation by a surface.

 reflection - The return of radiation from a surface at the same angle as the incident radiation.

 closed system - A system which has no significant exchange of energy and matter with the rest of the universe.

 open system - A system that exchanges energy and matter with the rest of the universe.