Physics Investigatory Project
“To study the variations in current flowing in a circuit containing an LDR “
Name:
Class: XII- IPL
School: Sri Chaitanya Techno School
CERTIFICATE
This is to certify that the project report entitled“To study the
variations in current flowing in a circuit containing a LDR
” submitted by ______________________of class XII (IPL).
This project is considered as a part of practical exam of BOARD
2025 Physics conducted by Central Board of Secondary
Education, is a bonafide record of project work carried out under
our guidance and supervision at Sri Chaitanya Techno school.
This project is evaluated on:-
INTERNAL EXAMINER NAME:-
______________
SIGNATURE:-
HEAD OF INSTITUTION NAME:-
______________
SIGNATURE:-
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ACKNOWLEDGEMENT
I express my gratitude towards my guide MR.Ramesh sir
for his extended guidance and support for completion of this
project work.
I want to thank my friends for their contribution and
co-operation.
A special thank to CBSE for the scheme of project
introduction to make students discover their inbuilt abilities.
Last but not least I would like to thank my Parents for their
love and support.
Signature: -
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INDEX
●ABSTRACT
●AIM
●MATERIALS REQUIRED
●THEORY
●PROCEDURE
●OBSERVATION AND DATA RECORDING
●ANALYSIS
●CONCLUSION
●EXTENSION AND APPLICATION
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ABSTRACT:
The general purpose photoconductive cell is also known as LDR – light dependent
resistor.It is a type of semiconductor and its conductivity changes with proportional
change in the intensity of light. There are two common types of materials used to
manufacture the photoconductive cells.They are Cadmium Sulphide (CdS) and
Cadmium Selenide (CdSe). Extrinsic devices have impurities added, which have a
ground state energy closer to the conduction band - since the electrons don't have
as far to jump, lower energy photons (i.e. longer wavelengths and lower
frequencies) are sufficient to trigger the device. Two of its earliest applications
were as part of smoke and fire detection systems and camera light meters. The
structure is covered with a glass sheet to protect it from moisture and dust and
allows only light to fall on it.
AIM:
To study how the current in a circuit with an LDR varies with:
1. The power of an incandescent lamp placed at a fixed distance.
2. The distance of a fixed-power incandescent lamp.
MATERIALS REQUIRED:
1. LDR (Light-Dependent Resistor)
2. Variable power supply
3. Ammeter
4. Voltmeter
5. Resistor (to limit current)
6. Breadboard and connecting wires
7. Incandescent lamps of different powers (e.g., 25W, 40W, 60W)
8. Ruler or measuring tape (for distance measurement)
9. Adjustable lamp holder or stand
10.Multimeter (optional, to verify resistance changes)
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THEORY:
LDR Behavior:
● The resistance of an LDR decreases as the intensity of light falling on it
increases.
● The current in the circuit is inversely proportional to the LDR's resistance.
Dependence on Light Intensity:
● The light intensity from a lamp depends on its power and distance.
● Light intensity (I) varies with the inverse square of the distance (d):
(I ∝ 1/d )
EXPERIMENTAL SETUP:
1. Connect the LDR in series with a resistor to form a voltage divider
circuit.
2. Attach the voltage divider circuit to a variable power supply.
3. Connect an ammeter in series and a voltmeter parallel to the LDR.
4. Place the lamp at a known distance from the LDR using a stand.
5. Ensure no other light sources interfere with the experiment.
PROCEDURE:
PART A: Varying lamp power-
1. Set the lamp at a fixed distance (e.g., 30 cm).
2. Turn on the power supply to the circuit and note the current.
3. Replace the lamp with another of different power (e.g., 25W, 40W,
60W).
4. For each lamp, note the current reading.
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PART B: Varying lamp distance-
1. Use a lamp of fixed power (e.g., 40W).
2. Place the lamp at various distances from the LDR (e.g., 10 cm, 20 cm,
30 cm, 40 cm).
3. For each distance, record the current reading.
OBSERVATION AND DATA RECORDING:
LAMP POWER (W) CURRENT(mA)
25
40
60
Distance(cm) Current(mA)
10
20
30
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ANALYSIS:
1. Plot a graph of current vs. lamp power (Part A).
2. Plot a graph of current vs. distance (Part B).
3. Analyze the trends:
● Expect higher lamp power to result in greater current.
● Expect current to decrease as the distance increases due to reduced light
intensity.
CONCLUSION:
1. State how the power of the lamp affects the current in the LDR circuit.
2. State how the distance of the lamp affects the current.
3. Confirm whether observations align with theoretical expectations.
EXTENSIONS and APPLICATIONS:
1. Investigate how the type of lamp (e.g., LED vs. incandescent) influences the
results.
2. Study the response of different LDRs to the same experimental setup.
3. Real-world applications: LDRs are used in automatic lighting systems, light
meters, and security systems.
