A potential divider is a device which removes a tiny proportion of the input with a sliding contact moving across the surface of a high resistance to supply a controlled amount of output. Equal movements of the sliding contact gives equal changes in the output.
There are three main types of potential dividers:
A chain of resistors- this is when there are many resistors connected in parallel to each other.
Potentiometer with resistive track- this is one resistor with a moving contact which changes the voltage.
Rotary Potentiometer- this is a circular resistor with a moving contact which may be attached to a rod for movement, this is the type of potential divider I used in my experiment.
(Advancing Physics AS Textbook)
Equipment
Rotary Potentiometer
Metal Rod
Measuring Beaker
Voltmeter
Circuit Wires
Float
Power Pack
Water Bath
Method
-Firstly, I will connect up my circuit and make sure that the voltmeter is in parallel with the rotary potentiometer.
-Then I will secure the rod, which is attached to the float, to potentiometer and make sure it will not come apart.
-I will then get the water bath and place the float inside it.
-Using the beaker I will fill up the water bath 200ml at a time, and will take down the voltage on the voltmeter each time.
-After 12 measurements I will empty the water bath and repeat the experiment another 5 times.
The formula for the voltage output is:
Voltage output = Voltage input (R1/R1+R2)
Circuit Diagram
Safety
I will take a number of safety precautions to make sure that this experiment is as safe as possible.
I will check that all of the equipment is in good condition, so that the circuit will not get damaged.
I will clean up any water spills so that it won’t damage circuit wires and so there is no risk of an electric shock.
All glassware will be kept in sensible areas to avoid accidental drops and breakages.
The metal parts of the wires and the crocodile clips will be kept at a safe distance from each other to prevent short-circuiting and wrong voltage readings.
After I have collected a full set of results, I shall turn the power pack off for a while to stop it from over-heating or overloading.
Trial Experiment
The trial experiment is to test out what measurements would be best to use, what voltage the power pack should be on, if it should be AC or DC etc.
Uncertainties may occur such as differences of readings, but these can be eliminated by repeating the experiment several times and taking the averages. This will make the results more reliable.
I have decided to use a DC current, as it will be more accurate, and the power pack will be set at 12 volts, because this gives a good set of results with the voltages being more spread out.
When I measured the power pack on its own in parallel to the voltmeter, it actually gave a reading of 12.25 volts.
Things which have affected my results were wrong levels of water in the beaker due to some left in from the previous reading, so I tried to make it as empty as possible.
When the metal parts of wires accidentally came into contact with each other the results were affected, so I had to spread them out even more and keep them secure.
Sometimes the float would not be attached properly to the metal rod, which gave wrong readings, so made sure that the float was fixed on properly and was firm.
Calibration Experiment
This is the main experiment for this investigation. It is to see how much output is produced by changing the level of the variables, in this case the depth of the water. I will conduct the experiment exactly as mentioned in the method, as it is the easiest and quickest way of doing so. The trial experiment has helped this experiment because I now know exactly what quantities of water to use, what the voltage supply should be from the power pack, and just how to make the experiment as accurate as possible and to decrease the chances of any human error.
To make the results as reliable as possible I will conduct the experiment 6 times, and get 6 sets of full results, and then I will take the averages to make them extremely accurate.
Results
Here is my table of results, and you can see that when I worked out the averages, I left out the anomalies, which are highlighted in red; this would make the average more accurate.
Graph
From my graph you can see a slight curve is produced, this shows that the depth of water is proportional to the voltage produced, in other words, as the depth of water rises, the voltage rises as well.
Using the graph, we can see what the voltage would be at any depth of water level, e.g. at 1200 ml, the voltage is 0.15v, but we can now see the voltage at other depths as well because of the curve, e.g. at 1500 ml, the voltage is 0.285v.
So with the graph we can see any voltage at a given depth, and we can also see any water depth at a given voltage, e.g. at 0.55v, the depth will be 2020 ml.
As I have mentioned before my sensor can be used in transports such as cars, and it can also be used for other types of depth measurements, e.g. for swimming pools or oil refineries.
It can be developed further so it can measure substances other than liquids, such as solids and gases. One way it could be altered to measure gases is if the float was made much lighter so that it would rise with the increase of any dense gas. I say dense gas because this type of gas would start from the bottom and start rising, while lighter gases may just spread around.
Analysis and Conclusion
I calculated the sensitivity of my sensor to be 800 ml of water produces 0.05 volts, so 660 ml of water produces 0.01 volts; I did this using the graph.
I dealt with uncertainties, such as anomalies in the results, by eliminating them when calculating the average, this made it more accurate.
I also had to make sure that all the equipment was in good condition for the experiment to work properly, and I replaced the ones that weren’t. When I used the damaged equipment the voltmeter gave odd readings, so I knew something was wrong.
The resolution is the smallest change it can detect in the quantity it is measuring. The smallest change for my sensor which could be measured was 0.01 volts, and this was when the water depth came up to 660 ml.
The response time was quite fast, as soon as you fill up the water bath a little bit the floats starts to rise up, but only if the float has risen a bit already. The response time then was about 0.1 seconds.
As a result from my experiment, I have found my sensor to be very fit for its actual purpose, it can do what it’s meant to and can do so very accurately.
