Module 1

PlanningGuide

Lab 1.2

Lesson 1.1
Lesson 1.2
Lesson 1.3
Lesson 1.4
Lab 1.1
Lab 1.2
Lab 1.3
Lab 1.4
Project 1


Physics Lab 1.1B Solar Energy

Experiment 1.1.3 Solar Energy

Use a thermometer, a glass jar and a light bulb to estimate the amount of solar radiation reaching the earth’s surface at a particular time of day.

Background
Solar energy is radiant energy. It spreads out as it travels away from its source. The amount of radiant energy that lands on an object depends on the area of the object that faces the source and also its distance from the source. The amount of radiant energy that "lands" on an object decreases with the square of the distance from the source.

Light and other forms of radiant energy are also emitted from a light bulb. The amount of energy from a light bulb that "lands" on an object will depend on its area and the distance from the bulb.

If we place a flat object that has an area of 50 square centimeters facing a 100-Watt light bulb at a distance of 30 centimeters from the center of the bulb, we can estimate the amount of energy intercepted by the flat surface as follows:

Assuming that we could place a ball with a radius of 30cm around the light bulb – with the bulb at the center of the ball;

All of the energy from the bulb (100 watts) would "land" on the inside of the ball.

The surface area of the ball is 4pi.r2 = 4.pi.(30cm)2

(pi = 3.414)

The amount of energy intercepted by the 50cm2 object facing the bulb at a distance of 30 cm would then be a fraction of the energy intercepted by the ball.

This would be: 50 cm2 / [4.pi.(30cm)2] x 100 watts = 0.442 watts.

All objects absorb radiant energy.

If we place a piece of aluminum in the sun, its temperature will increase because it absorbs radiant energy from the sun. If we paint it black and place it inside a glass jar, its temperature will increase further because it will absorb and retain energy more efficiently.

We can estimate the amount of energy absorbed from the sun by measuring the temperature of the object and then placing it in the path of radiant energy from a light bulb. We can estimate the amount of energy absorbed from the bulb from the power of the bulb and the distance between the bulb and the object. If we adjust the distance between the object and the bulb until it reaches the same temperature as it did when exposed to the sun, we can assume that the energy absorbed from the bulb is about the same as the amount of energy absorbed from the sun.

Materials & Equipment

Thermometer

Glass jar

Black paint

Aluminum sheet (or empty aluminum can)

Scissors

Tape measure / ruler

Light bulb and holder

Safety Instructions Be very careful when using the aluminum foil – especially if you choose to use a soda can. The thin metal is extremely sharp and can cut very easily.

Procedure

  1. Make a simple radiometer by cutting a piece of aluminum sheet (painted mat-black) to a suitable shape, attaching it to a thermometer as shown in the diagram and placing it in a sealed glass jar.
  2. Measure the area of the aluminum that will be exposed to the radiation source.
  3. Place the radiometer in the sun in such a way that it intercepts the maximum amount of solar energy - i.e. the absorbing surface directly faces the sun.
  4. Measure the maximum temperature on the thermometer.
  5. Measure the temperature of the surrounding air.
  6. Place a 100-watt light bulb in a suitable holder on a flat surface in such a way that the radiometer can be placed directly in the path of the radiation from the bulb’s filament.
  7. Ensure that the air temperature is similar to the previous air temperature. [in 5) above]
  8. Adjust the distance between the bulb and the radiometer until the maximum temperature reading is the same as it was when placed in the sun.
  9. Calculate the amount of energy absorbed from the bulb at this distance.
  10. Assume that this is the same amount of energy that was absorbed from the sun.

Results and Conclusion

Use the following to record your results

RADIOMETER PLACED IN SUNLIGHT:

  1. Time …………………….
  2. Area of aluminum facing the sun ……………… square meters
  3. Air temperature: ..……………..ºC
  4. Maximum Temperature – Radiometer: ……………….ºC

RADIOMETER PLACED IN PATH OF 100-WATT BULB

4) Air Temperature ……………..ºC

5)

Distance From Bulb (cm) Maximum Temperature (ºC)
   
   
   
  1. Distance that provide same temperature as in 3) above
  2. Energy intercepted by aluminum surface (watts per square meter) = …………………

Conclusion
The amount of energy that can be absorbed from the sun is limited. Very roughly 1000 watts per square meter reaches the earth’s surface at noon near the equator. The position of the sun in the sky varies continually and the further we are away from the equator, the lower the sun’s average position in the sky. If we intend to use solar energy to distill water, heat water or create electricity, we need to mount our collecting device in such a way that the maximum amount of energy is absorbed during the course of the day. This means that solar collecting devices usually "point" towards the equator and are mounted at an angle that depends on the latitude. The further the collected is away from the equator, the larger the angle between the horizontal and the collector’s surface.

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