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Using a Light Source, LDR and Tracing Paper to Model Absorption of Gamma Rays by Different Thickness of Material – Physics IAA

Now, before I say anything, this experiment doesn’t use actual gamma rays as they are dangerous and not allowed at school. Instead we will be investigating how the thickness of tracing paper affects the current produced from LDR (Light Dependant Resistor). This experiment is meant to mimic what will happen if we used gamma rays (the light ) and a different material (the tracing paper).

Firstly, let’s identify what the independent and dependant variables are:
Independent Variable – Thickness of tracing paper
Dependant Variable – Current in the circuit

Independent variable causes a change in Dependent Variable and it isn’t possible that Dependent Variable could cause a change in Independent Variable. So, the thickness of tracing paper causes a change in the current of the circuit.

Controlled Variables
Background light (meant to mimic background radiation), distance LDR is to light bulb, voltage, shroud, LDR, bulb, thickness and type of each individual tracing paper, ammeter, circuit and distance tracing paper is to LDR and bulb.

  • LDR& circuit board
  • Power supply
  • Wire leads
  • Milli-ammeter
  • Lamp & holder
  • Clamp stand, boss and clamp
  • Small squares of tracing paper (roughly 5cm squared) 

Set up
Circuit set up for Physics IAA experiment
1) Build a complete circuit as shown in the picture and set the power pack to 6V DC (direct current). Make sure the lightbulb and LDR are always the same length to each other throughout the whole experiment.
2) Test the lightbulb to ensure it works.
3) Connect the LDR with its black paper tube or shroud in series with a milli-ammeter and then connect the other ends of the leads across the same two terminals on the power supply. 
4) Turn the power supply on and turn voltage to 6V DC and check you get a reading for the current through the LDR using the milli-ammeter. 
5) Place your finger over the opening to the black tube or shroud connected to the LDR and see if the current reduces. If it does you are now ready to do the experiment.

1) From the circuit produced in the set up, place the light bulb on a clamp as shown, 1cm directly above the black tube (shroud) that is connected to the LDR. 
2) Record the reading on the ammeter to 3 decimal places. If the current changes alot, pick the mode number (number that appears the most amount of times for the longest period of time).
3) Repeat steps 1 & 2 but this time place one sheet of tracing paper over the black tube (shroud). 
4) Repeat steps 1-3 ten times.
5) Repeat the experiment 3 times.
6) Ignore any anomalous results and repeat to compensate for this.

Your graph should look something like this:

Pattern from the Graph
From the graph, it is clear that as the number of sheets increase, the current decreases. The whole graph shows negative correlation, with the line starting extremely steep and then eventually levelling out. This shows that after ‘X’ amount of tracing paper, any more sheets of tracing paper won’t affect the current making the number of sheets a limiting factor.

The method was and could have been made more valid by (The point/Explanation for the point)
Keeping same amount of background light so the LDR won’t cause an increase in current due change in background light. (Or turning off the lights in the room off to completely eliminate background light and to stop  the flicker from flurescent lights from affecting current levels)
Same light bulb so the same light intensity is trying to hit the LDR.
Same thickness of each individual tracing paper as different thickness will cause a change in light reaching LDR and therefore change in current.
Same circuit so the current of voltage around the circuit won’t differ due to the circuit being different.
Same voltage so bulb emits same intensity of light.

The method was made reliable by
Using ten separate layers of tracing paper
Measure current to 3 decimal places
Use shroud to reduce background radiation
Repeating the experiment and averaging the results
Ignoring any anomalous data and repeat to compensate

(this is where you should use your results and scientific knowledge)
Ultimately, the thicker the tracing paper, the lower the current is. This is due to the tracing paper blocking some light from reaching the LDR (just like metal can block some gamma rays reaching its destination). This caused the LDR to absorb less light and therefore produce a lower current in the circuit. The graph starts off steep showing the thickness of tracing paper affects the current the most in the first few layers, then starts to level out towards the end. This just proves that the first few layers of tracing paper block out the most amount of light from reaching the LDR, while the layers after that have not as big an impact. 



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