Name______________________________
Experiments with Solar Cells
Introduction:
What happens when light shines on a solar cell? The light knocks electrons loose in the p-layer and
sends them into the n-layer . The barrier, which acts like a one-way door, lets the electrons cross into
the n-layer, but stops them from going back to the p-layer. This gives the n-layer a negative charge and
the p-layer a positive charge. It is as if light were turning the solar cell into a kind of battery.
When the solar cell is connected in a circuit with, for example, a small light bulb, the electrons flow as
electric current from the n-layer, through the wire and the bulb, to the p-layer. As electrons arrive at
the p-layer of the cell, light sends them back into the n-layer, where they again flow through the circuit.
Electric current will continue to flow as long as light is shining on the cell. The current can be used to
supply electricity.
Today we are going to see what happens when we change the distance between the light source and
the solar panel. We want to see what kind of mathematical relationship there is between the distance
and either current or voltage.
Newton’s Inverse square law:
According to Newton’s inverse square law, the power of the light will be inversely proportional to the
square of the distance. So if we take a distance of 2 and square it, we get 4, the inverse of which would
be
1
or rather, a quarter of the original power.
4
This idea is used with the current of a solar panel as well. It will follow the equation:
current 
k
dis tan ce 2 .
Materials Needed:
Small solar photovoltaic cells with at least a 0.4 volt output
DC ammeter with a range of approximately 0 to 10 amps
DC voltmeter with a low rating (1 to 5 volts DC minimum rating is fine)
Multimeter (can be used for ammeter and voltmeter)
Graph paper
A bright, directional light source, such as a shaded desk lamp or a clip-on reading lamp, with a 100 watt
bulb
meter stick
Connecting the Solar Cell to an Ammeter and a Voltmeter
In this experiment you will be measuring the current and voltage produced by the solar cell. The cell’s
current is measured using an ammeter, while the voltage is measured using a voltmeter. The device we
will use will measure both.
1. Connect the 2 “alligator” clips to the bottom of your solar panel.
2. Plug the black cord into the middle (COM) of the multimeter.
3. The red cord will be plugged into the left (10 ADC) when you are measuring amps, and
plugged into the right for volts. PLEASE MAKE SURE THE SWITCH AT THE TOP IS OFF WHEN
YOU ARE CHANGING BETWEEN AMPS AND VOLTS
4. Connect the multimeter to the solar cell via the alligator clips. When you are measuring
amps, set the ammeter on 10 amps (move the switch to the bottom right) and the voltmeter
on 20 volts (move the switch to the top left.)
5. Now you are ready to gather data!
Experiment: Solar Cells and Light Intensity
Gathering the Data:
Teacher Notes:
Make sure students do not hold the solar panel under the light too long… it gets hot!
If students get negative readings… switch the cords, or just make the data positive
This experiment investigates how changes in light intensity, (distance from the light source) affects the
amount of current and voltage a solar cell can produce.
Procedure:
1. Connect the multimeter to the solar cell. Set the ammeter on 10 amps and the voltmeter on 20
volts. You can use the same multimeter for both, make sure you change the connectors on the
bottom to represent either the voltmeter or the ammeter for the 10 amps.
2.
Place the solar cells 1 inch from a bright, directional light source other than the sun. The solar
cell should directly face the light source.
3. Measure the current and the voltage of the cell at a distance of 1 inch. Record results on the
light intensity table in the appropriate column.
4. Repeat steps 2 and 3 for distances of 3 inches, 6 inches, 9 inches, 12 inches, and 15 inches.
Light Intensity Table
Distance
from
Light Source
Current (I)
(amps)
Voltage (V)
(volts)
Power
P = VI
1 inch
3 inches
6 inches
9 inches
12 inches
15 inches
.65
.41
.23
.12
.07
.04
4.38
4.04
3.82
3.74
3.6
3.37
2.84
1.66
.88
.45
.25
.13
Analyzing the Data - Current
1. Using the Light Intensity Table and graph, state the relationship between the current produced
by the solar cell and the cell’s distance from the light source.
As distance increases, current decreases, therefore it is an inverse relationship.
2. Sketch a graph that shows the relationships between current and the solar cell’s distance from
the light source. Label the horizontal axis of the graph “distance from light source” and mark it
with distances shown in the Light Intensity Table. Label the vertical axis “current” and mark it
based on the readings from the Light Intensity Table. Plot the values of current corresponding
to each distance on the graph. (You may also use your graphing calculator for this. Enter the
distance in L1 and current in L2. Use ZoomStat to find a good window that clearly shows all data
points)
3. Predict what type of graph this would be.
Many students will say this is a parabola (quadratic) or an exponential decay graph. These are
great ideas, and creates the opportunity to have a discussion about analyzing data. This is not
quadratic, it will not go back up and create a parabola. It is not exponential decay, there is no
initial value, there is actually an asymptote at x = 0. It is a rational function with vertical and
horizontal asymptotes, and as stated in the introduction, it follows Newton’s Inverse Square
Law.
4. a. Using the formula given in the introduction, calculate three different k values using the
distance and current.
Using distances 9, 12, and 15, k = 9.72, 10.08, and 9
b. What is your average k value? K= 9.6
c. Use this k value to write an equation for your data. Current = 9.6/distance2
d. Put your equation into your calculator, was it a good fit?
e. Sometimes this will not be an exact fit. Explain why the results are not always going to give
an exact fit to your graph. In an experiment, there are many outside variables and human
error that could affect the data.
f.
What are the asymptotes for your graph? Why? X = 0, and y = 0
Questions:
1. Based on your results, would the solar cells produce more electricity on a sunny day or a
cloudy day? Why? Sunny… light is more intense
2. Predict the time of day when the solar cells will produce the most electricity. Explain your
reasoning. Noon – light is more intense
3. Complete the Light Intensity Table above for Power. (Power = volts * amps)
Multiply the current by the voltage for each given distance. (If you are using your graphing
calculator, you can do this in L4. Arrow up so L4 is highlighted, then type L2*L3… and the
data table will be filled in!)
4. What happens to the power as you increase your distance away from the light source?
Explain your reasoning. Power also decreases, light is less intense and hot, therefore there
is less power.
5. Use your equations from the” Analyzing the Data” portions above to predict what the
current will be if you are 8 inches away. Test to see how accurate you are. Explain your
results.
Extension:
Analyzing the Data – Volts
Complete steps 1-4 above to analyze the relationship between distance and volts.
(If you are using a graphing calculator… put the volt data in L3, and graph the data using StatPlot2)