assignment Flashcards
title
Investigating the resistance of an LDR based on variations in light intensity.
aim
To investigate how variations in light intensity affect the resistance of a Light Dependent Resistor (LDR).
underlying physics
physics principles
When the light intensity goes up, the resistance goes down
LDRs function through photoconductivity
LDRs are non ohmic conductors
Their relationship between resistance and light intensity is non linear
light up res down points
-DEFINITION
a device across which resistance decreases with increasing light intensity.
-EXAMPLE
higher resistance, such as several megohms in the dark much lower resistance in light such as only a few hundred ohms.
-RELATES to my experiment as
increasing the distance of the light source from the LDR, the light intensity will decrease.
can expect the resistance of LDR to increase when the light source is further away.
Photoconductivity
LDRs are made of photosensitive semiconductor materials, and they work through photoconductivity.
Photoconductivity is the phenomenon where the electrical conductivity of a material increases upon absorbing photons. Photons are the smallest possible particles of any kind of electromagnetic radiation.
When the photons hit the LDR, they will ‘excite’ the electrons in the semiconductor material.
What happens is that the energy from the photons can give some of the electrons enough energy to move from the valence band to the conduction band.
When the electron moves, it leaves behind a ‘hole’ in the valence band.
The now free electron and the hole can both carry current, meaning that the conductivity of the material has increased.
The principle of ‘light up resistance down’ can be explained through photoconductivity.
When there is more light (light up), there will be more photons being absorbed by the LDR. This increase of photons means that the conductivity increases, causing resistance to decrease (resistance down).
The resistance decreases as it is easier for the electric charges to flow through the LDR when the conductivity is higher.
Non Ohmic Conduction
LDRs are also non ohmic conductors
do not follow Ohm’s law as the electric current across the LDR is not proportional to voltage.
The current is not directly proportional to the voltage, as an LDR’s resistance changes based on light intensity and not voltage.
Overall when current and voltage are plotted against each other, a curve is shown.
Non Linear Relationship
LDRs have a non linear relationship between light intensity and resistance.
A slight change in light intensity can cause a large change in resistance of the LDR.
For example, in a graph plotting light intensity against resistance, a curve will be created, illustrating how a small change in light intensity can result in a significant change in resistance.
This means that I can expect the graph from my results to have a curve, when resistance and light intensity are plotted together.
underlying physics conclusion
In conclusion, these main National 5 level principles of LDRs will help me to effectively understand and analyse the results of my experiment.
Data Collection and Handling intro
In this experiment, I am changing the distance of the light source from the LDR. I am measuring the resistance of an LDR and light intensity of the source. The position of the LDR is staying the same.
method
An Ohmmeter was used to measure the resistance of a LDR when the light source was at varying distances.
A light intensity meter was used to measure the intensity of the light source from the same position and distance from the light source as the LDR.
The results were repeated 5 times to ensure reliability
a range of 10 to 60 cm was used as the distance of the light source from the LDR.
analysis
Overall the graphs both follow similar curves and show a clear inverse relationship between light intensity and resistance, as when it gets brighter, the resistance decreases
indicating that as light intensity increases, resistance decreases.
Although the graph from the internet has a smaller range of values, its curve still confirms this principle.
The larger range of values in my graph allows for a more detailed examination on the behaviour of an LDR and how its resistance changes with different light levels.
evaluation points
ambient light
taping down the LDR
human error
ambient light evaluation
ambient light in the experimental area could affect the results due to there being inconsistent amounts of light on the LDR at different times
leading to variable and inconsistent measurements.
My experiment was performed in a darkened room however could further be improved with shielding around the light source and the LDR
this would block any other small light sources.
This would ensure that the light intensity and light upon the LDR would remain true to that of the light source only, giving more accurate results.
taping down LDR evaluation
taping down the LDR so it remains in one place,
as any small change in position could cause the light intensity upon it to change,
causing inaccurate results.
