List Of Required Equipment:
- Wires
- 3x 123Ω resistors
- 1metre length of very fine wire
- μamp meter
- μvolt meter
- 12volt battery pack
- pulley
- weights (up to 700 grams)
Why did I choose the Wheatstone Bridge?
Mainly I believe out of all the circuits I have researched, Wheatstone Bridge and the Potential Divider Circuit, I found the Potential Divider circuit was not sensitive enough. The Potential Divider circuit couldn’t detect the strain on the fine wire well enough to predict the effects. With any sensor system u need a big enough signal given out so an instrument which u have can record it. There are two problems
- The signal has to be the right size for the detector to detect it
- Is the resistance too big for the current being passed into the circuit?
Point 2 was the case in the Potential Divider circuit, and therefore I chose the Wheatstone Bridge circuit.
Method:
Firstly I set up the equipment as shown below in my diagram. I have placed three resistors in my experiment to see if a differential voltage will be present across the wire. The three 123ohm resistors connected across the circuit as shown below. As my theory shows the resistance of the wire must be the 123ohms also, but as we didn’t have sufficient equipment we could only achieve 122.8ohms. To show relationship between stress and strain this was accurate enough to obtain good results. The Resistance of R3 and R4 shouldn’t change therefore showing me a change in resistance across the wire as it stretches. I have connected a 12volt-battery pack providing enough voltage to obtain accurate results, rather than a 6volt, which was too weak to obtain accurate results. I’m using 1m of very fine wire so I can measure the strain on the wire more accurately. I found the best way to attach the fine wire to the circuit was to use crocodile clips rather than rapping it round. I have placed a voltmeter and ampmeter across the circuit to measure the voltage and amperage change of the fine wire. I will adjust the voltmeter and ampmeter to 0 when the weight is 0 to show how these variables change to increase in strain.
To insure we connected the circuit up correctly we used an A1 sheet of paper to the worktop and drew the circuit on it. This helped us with construct the circuit with ease and also mark how the wire stretched when weight was added.
Diagram:
Risk Assessment/Safety:
There are very few risks and safety aspects involved, but I must be sure to wear safety glasses in case the wire snaps. I must also be sure to connect the circuit up correctly. I must check the wires can withstand the amount of voltage being passed through them, so they do not catch alight.
Variable Table:
Preliminary Results:
These are my preliminary results that helped me to understand the physics and approach needed to make my experiment successful. In exp1, 3 and 4 I used the same wire and exp2 was a fresh wire. Clearly u can see the set of results obtained in exp2 were different to the other sets of data. After a wire stretches the wire Changes State and becomes a harder material, meaning it will stretch less the next time u apply a strain. Therefore in my real experiment I used fresh wire for each experiment.
Trend In Graphs:
From the graphs u can see a general trend formed in exp2, as weight is applied the voltage across the rest of the circuit decreases, therefore increasing across the wire. In the other three experiments u can see weight has little or no effect on voltage, thus showing the wire has already been stretched. This can be seen in ‘Chart 1’.
Results
I have used ‘Ohm’s law’ which is a formula, to get a resistance for my results. R= V/I
The anomalies are in red.
I have also carried out a further experiment to find out how there is a general increase in voltage and resistance across a wire as it becomes longer. These results support the prediction I have made before hand. I predicted that the longer the wire the greater the resistance will be and then more energy will be needed to push the electrons through. Now I can carry out my experiment, knowing current and voltage will increase as strain is applied on the wire.
Results:
1st Set…
This is my first set of results, which I believe to be the most accurate. As u can see there is a clear trend in the increase of current and voltage. There are no anomalies in this experiment.
2nd Set…
Observations:
There was only one major observation and that was to see how the wire stretched as more weight was applied. As more weight was applied the wire slowly stretched until its deformation point. The deformation point is where the wire is stretched or re-shaped so much it cannot return to its original state.
Accuracy of Results:
The accuracy that I achieved was very high even though I had very little time. Also I got lots of results to make a good analysis. The level of accuracy in this investigation was as high as I could get. The only thing I could of improved was to use more weights and measure every 50grams however this may have taken too long to achieve. I got someone else to check the reading on the meters to be sure I was reading them correctly. Before taking each reading I would also check the connections were crisp between the wire and the rest of the circuit.
I could change one or two thing during this experiment. A few examples of these could be the thickness of the wire, the type of wire for example copper and gold.
We made sure to limit out systematic error by valuing all the results a 0 for 0 weight. This way only my own human judgement can effect the quality of the results. A random error could of included, when I was reading the results someone knocked or a change in the gravitational field, effected the weight. Also a slight wind could of pushed on the weights for that precise second, tiny chance, but possible!
Analysis:
First Experiment:
The first experiment clearly went much better then the second as I have a significant and clear trend between the strain, current and voltage. From ‘Chart 2’ u can clearly see that as u stretch the wire the current and voltage increase. The voltage only grows very slowly until the wire is deformed, then the electrons need much more energy to move across the wire (large increase in resistance). The sharp slope proves this in ‘Chart 2’. Chart 3 and Chart 4 show this in much more detail. In ‘Chart 3’ u can see where the wire snaps because the stain is too much for it to withstand. Therefore from the first experiment we can see that my theory and hypothesis is proven.
Chart 8 shows the general pattern between voltage and current and how resistance increases. Between point 6 and 7 on the X-axis the wire has deformed and resistance has greatly increased. The wire has stretched 45mm, which also increase resistance.
Second Experiment:
There is one major anomaly in experiment 2 and that is when 100grams were placed on. The reading of power/voltage and current are much too high. On Chart 5 and 6 I have draw a pencil line which I believe should have been the results gained.
The general trend again shows that as u stretch the wire the resistance across it increases. Chart 7 shows the general pattern between voltage and current and how resistance increases.
Evaluation:
A strain gauge can predict when a wire or something with applied stress or stain will deform or even break. When the voltage and current increases in a wire it must mean resistance has also increased.
Improvements:
If I were to do the experiment again, I could make some improvements to make the level of accuracy better. For example, I could make sure that the joints/connections were jointed properly and would have to make sure I use the same equipment every time. I could also get a machine to measure the length of the fine wire.
I would also take more than two sets of results, which would eliminate any factors of anomalies and would therefore make the experiment a lot more reliable.