Science 7–10 · Year 9
Solar cell output against tilt angle and distance from the lamp
Physical sciences — Energy, content group Sources of energy (NSW Stage 5 focus area; ACARA's Year 9 energy description, AC9S9U05, is about efficiency, which this investigation does not measure, so only Year 9 inquiry codes are cited)
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The idea
A solar cell transforms light energy into electrical energy, so its output depends on how much light energy lands on it each second, which falls as the cell tilts away from the light and as the distance from the source grows.
What you need
- small silicon solar cell (about 2 V, 100 mA in full sun), 1
- 10 ohm load resistor, 1
- digital multimeters for current and voltage, 2
- desk lamp with an 800 lumen LED globe on a stand, 1
- protractor and a hinged mounting board, 1
- metre rule, 1
- blackout blinds or a darkened corner
How to do it
- Connect the cell across the 10 ohm resistor with the ammeter in series and the voltmeter across the resistor. Darken the room.
- Angle: place the cell 30 cm from the lamp facing it directly (0 degrees). Record V and I; calculate power P = V x I. Tilt to 15, 30, 45, 60 and 75 degrees and repeat.
- Plot current against the cosine of the angle, and power against angle.
- Distance: return the cell to 0 degrees and record I and V at 20, 30, 40, 50 and 60 cm from the lamp.
- Plot current against distance and power against distance, and describe how quickly each falls.
- Take the cell outside in sunlight (not looking at the sun) and record its current and power facing the sun and facing 45 degrees away.
What you should see
At lamp brightness the voltage across the 10 ohm load is small compared with the cell's open-circuit voltage, so the cell behaves close to short circuit and its current is proportional to the light falling on it. Current therefore falls with tilt in proportion to the cosine of the angle: at 60 degrees it is close to half the 0 degree value (cos 60 = 0.500), and near zero at 90 degrees. Power into the fixed resistor is I^2 R, so it falls with the square: about a quarter at 60 degrees. Doubling the distance from a small lamp cuts the current to about a quarter and the power to about a sixteenth, for distances well beyond the lamp's size. In sunlight a cell rated at 100 mA in full sun can drive at most about 0.10 A through 10 ohm, that is 1.0 V and 100 mW, and at 45 degrees the current is about 71 per cent of that (cos 45 = 0.707) and the power about half.
What changes
- What you change
- tilt angle (degrees) or distance from the lamp (cm)
- What you measure
- current through the load (mA) and the power delivered to it (mW)
- What you keep the same
- same lamp and load
- room darkened
- cell centred on the lamp's axis
- lamp warmed up before readings
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A solar cell stores energy.
- Tilting the cell does not matter as long as light reaches it.
- Doubling the distance halves the output.
Safety card
Hazards
- warm lamp head and globe
- eye strain from looking at the lamp or sun
Controls
- never look directly at the lamp or the sun
- handle the lamp by its stand
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-EGY-01SC5-WS-04SC5-WS-05SC5-WS-06
- Australian Curriculum v9AC9S9I02AC9S9I04AC9S9I05
Sources
The pages the author read to write this activity.