Photovoltaic cells, also called solar cells, convert the energy in light into an electric current. They power communications satellites, deep space probes, calculators and road signs. By connecting a solar cell to a voltmeter, you can measure light's intensity, test the cell's sensitivity to colour and find the relationship between the cell's output and its orientation to the sun.
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White light consists of all visible colours added together. Each colour has a wavelength and an associated energy. The silicon in a solar cell is sensitive to different colours, depending on the silicon and the transparent cover protecting it. Obtain a bright light bulb and several different-coloured light filters. Connect the solar cell to a voltmeter, shine light on the cell, starting with white and then trying different filters. Observe the voltage the solar cell produces for each colour and write down the results. One or two of your colours may produce a higher voltage than others.
The more light you shine on a solar cell, the more electricity it produces, but only up to a point. The silicon in the cell has a limited capacity to move electric charges; this limit is called the saturation point. You can use bright sunlight and an iris diaphragm or movable shutter to vary the brightness of the light falling on the solar cell. Connect the cell to a 100-ohm resistor and connect a voltmeter in parallel with the resistor. Starting with total darkness, vary the amount of light falling on the cell and measure the voltage on the meter. At some point, the cell will produce a maximum voltage; more light will not increase it.
A solar cell produces the most electricity from sunlight if it faces the sun directly. As the angle between the sun and the solar cell increases, the cell's output decreases. You can measure this by connecting the cell in parallel with a 100-ohm resistor and voltmeter. In bright sunlight, start with the solar cell at 90 degrees to the sun, so it faces away from the sun's direct rays. Read the voltmeter and turn the solar cell a few degrees at a time until you reach a maximum voltage. Write down the voltages at each angle. Graph the voltage versus the angle on a computer and see what shape the curve makes.
As you move away from a light bulb, the light's intensity decreases rapidly according to a relationship called the Inverse Square Law. The light you receive on a fixed area 2 feet away from the light is one-fourth as intense as it is 1 foot away. You can see this for yourself by setting up a small light in a darkened room. Do not use a flashlight, because its reflector alters the light. Place a couple of yardsticks end to end or a measuring tape leading up to the light. Connect the solar cell to a 100-ohm resistor and voltmeter in parallel. Set the solar cell a few inches from the light; read the voltage on the meter; and write the distance and voltage down. Move the cell away a few inches at a time, writing down information at each point. Stop when you've reached 4 or 5 feet, and plot a graph of voltage versus distance. You should see a curve that drops sharply with increasing distance.
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