J.N.
INTERNATIONAL
SCHOOL
Physics Investigatory Project
NAME:
CLASS:
ROLL NO:
SESSION:
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CERTIFICATE
This is to certify that
_________________________________, a
student of class XII of J.N. International
School has successfully completed the
project title “To study the variations,
in current flowing in a circuit
containing a LDR, because of a
variation.” under the supervision and
guidance of _________________________________
in the partial fulfilment of the physics
practical assessment conducted
during the academic year 2024-2025 by
AISSCE, New Delhi
Teacher’s signature
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ACKNOWLEDGEMENT
I would like to express my immense gratitude to
my teacher ________________________ for the help
and guidance he provided for completing this
project.
I also thank my parents who gave their ideas and
inputs in making this project. Most of all I thank
our school management, for providing us the
facilities and opportunity to do this project.
Lastly, I would like to thanks my friend who have
done this project along with me. Their support
made this project fruitful.
THANK YOU!
INDEX
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CONTENT PAGE NO
Certificate 2
Acknowledgement 3
Topic 5
Introduction 6
Advantages 7
Experiment 8
Theory 9-12
Procedure 13
Observation 14-15
Results 15
Source of error 16
Bibliography 16
TOPIC
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“To study the variations, in current flowing
in a circuit containing an LDR, because of a
variation: -
a)In the power of the incandescent lamp, used to
‘illuminate’ the LDR. (Keeping all the lamps at a
fixed distance).
b)In the distance of an incandescent lamp, (of fixed
power), used to ‘illuminate’ the LDR.”
INTRODUCTION
The general-purpose photoconductive cell is also
known as LDR – light dependent resistor. It is a type of
semiconductor and its conductivity changes with
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proportional change in the intensity of light.
There are two common types of materials used to
manufacture t h e p h o t o c o n d u c t i v e c e l l s .
They are Cadmium Sulphide (CDs) and Cadmium
Selenide (CDs).
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 glass sheet to protect it from
moisture and dust and allows only light to fall on it.
ADVANTAGES
Lead sulfide (PBS) and indium antimonide (Ins)
LDRs are u s e d for the mid
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infrared s p e c t r a l region. Greco
photoconductors are among the best far- infrared
detectors available, and are used for infrared
astronomy and infrared spectroscopy.
Analog Applications
· Camera Exposure Control
· Auto Slide Focus - dual cell
· Photocopy Machines - density of toner
· Colorimetric Test Equipment
· Densitometer
· Electronic Scales - dual cell
· Automatic Gain Control – modulated light source
· Automated Rear View Mirror
Digital Applications
· Automatic Headlight Dimmer
· Night Light Control
· Oil Burner Flame Out
· Street Light Control
· Position Sensor
*LDR has a disadvantage that when its temperature
changes, its resistance changes drastically for a particular
light intensity.
EXPERIMENT
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AIM: To study the variations, in current flowing
in a circuit containing an LDR, because of a
variation: -
(a) In the power of the incandescent lamp, used to
‘illuminate’ the LDR. (Keeping all the lamps at a fixed
distance).
(b) In the distance of an incandescent lamp, (of
fixed power), used to ‘illuminate’ the LDR.
APPARATUS REQUIRED:
Light Dependent Resistor (LDR)
Connecting Wires
Source of different power rating (bulbs)
Bulb Holder
Metre scale
Multi Meter
Battery
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THEORY
1.) LDR and its characteristics
When light is incident on it, a photon is absorbed
and thereby it excites an electron from valence
band into conduction band. Due to such new
electrons coming up in conduction band area, the
electrical resistance of the device decreases.
Thus, the LDR or photo-conductive transducer has
the resistance which is the inverse function of
radiation intensity.
λ0 = threshold wavelength, in meters
e = charge on one electron, in Coulombs We = work function of the metal used,
in Ev
Here we must note that any radiation with
wavelength greater than the value obtained in
above equation CANNOT PRODUCE any change
in the resistance of this device.
The band gap energy of Cadmium Sulphide is
2.42eV and for Cadmium Selenide it is1.74eV. Due to
such large energy gaps, both the materials
have extremely high resistivity at room
temperature.
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Characteristics of photoconductive cells
Now when the device is kept in darkness, its
resistance is called as dark resistance. This
resistance is typically of the order of 1013 ohms.
When light falls on it, its resistance decreases up
to several kilo ohms or even hundreds of ohms,
depending on the intensity of light, falling on it.
The spectral response characteristics of two
commercial cells were compared in our laboratory.
And we found that there is almost no response to
the radiation of a wavelength which was shorter
than 300nm. It was very interesting to note that the
Cadmium Sulphone cell has a peak response
nearer or within the green color of the spectrum
within a range of 520nm. Thus, it can be used
nearer to the infra-red region up to 750nm. It was
found that the maximum response of Cadmium
Sulphoselenide is in the yellow-orange range at
615nm and also it can be used in the infra-red
region up to about 970nm.
Sensitivity
The sensitivity of a photo detector is the
relationship between the light falling on the device
and the resulting output signal. In the case of a
photocell, one is dealing with the relationship
between the incident light and the corresponding
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resistance.
Spectral Response
Like the human eye, the relative sensitivity of a
photoconductive cell is dependent on the
wavelength
(color) of the
incident light.
Each
photoconductor
material type has
its own unique
spectral response
curve or plot of
the relative
response of the
photocell versus
wavelength of light.
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2.) luminous flux variation:
Considering the source to be a point radiating in all directions;
consider a steradian (or even a simple sphere), take a small
element dA on the steradian at a distance ‘r’ from the source.
It comprises a small part of the energy radiated (dEr).
Now, go further to a distance ‘R’ (R>r) from the source,
consider the same area element dA, it comprises a much
smaller part of energy radiated (dER).
[dEr > dER].
It varies inversely as the square of the distance.
PROCEDURE
Choose a specific position for the source and mount it
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using a holder, make sure it is stable.
Select the bulb with the lowest power rating and
connect it to the holder as shown in the figure.
Connect the LDR, battery(6V) and the multimeter in
series.
Set the multimeter to ohm section and select suitable
range and measure the resistance with a bulb on.
Similarly switch to current section and move to micro
ampere in the multimeter. This gives the value of the
current.
Repeat these steps with different power sources at
different distances and note down observations.
Serial DISTANCE FROM RESISTANCE CURRENT
No SOURCE (Kilo ohm) (micro ampere)
(cm)
1. 50 142.5 40
2. 40 69 80
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3. 30 41 150
4. 20 21 300
OBSERVATIONS
The experiment has been conducted by using
various sources with different power ratings.
Voltage of the battery = 6 V
1.) 15 watts (yellow) (wavelength = 570nm)
Seria DISTANCE RESISTANC CURRENT
l FROM SOURCE E (micro
2.)
No (cm) (Kilo ohm) ampere) 15
1 50 51 120
. watts
2 40 35 170
.
3 30 22 270
.
4 20 11 540
.
(incandescent) (mean wavelength = 610nm
4.) 20
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3) 40watts (CFL) (white light)
Serial DISTANCE FROM RESISTANCE CURRENT
No SOURCE (Kilo ohm) (micro ampere)
(cm)
1. 50 15.5 380
2. 40 10 600
3. 30 6 1000
4. 20 3 2000
RESULTS
The LDR resistance decreases with
increase in intensity of light and hence
there is an increase in the flow of current.
There is an increase in the current as the
distance from the source decreases.
The intensity decreases as the distance
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from the source increases
The error lies within the experimental limit.
SOURCE OF ERROR
The LDR may not be perpendicular to the source.
Connections may be faulty.
The experiment should be conducted in
a dark room.
Measurements should be taken accurately.
BIBLIOGRAPHY
NCERT physics class XII
www.wikipedia.com/
www.electronics2000.co.uk/links/education-hobby/
www.ecelab.com/
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