If resistance in a circuit increases, electric current decreases. A variable resistor can be used to alter the current in a circuit. In this experiment, I will vary the length of a wire (of a particular material) connected to a circuit.
Safety :
There are only a few safety precautions involved in this experiment. The main problem is that a wire could possibly overheat (due to a high resistance) during the experiment. Therefore, I should ensure that no-one and nothing is burnt. To prevent over-heating of wires, I have decided to avoid leaving the circuit on for a long time. Instead, I will turn off the poer pack during taking readings.
Method :
- Firstly, I will set up a circuit similar to the diagram above.
- I will connect wires using crocodile clips and and will also use a metre ruler to measure the length of my wire.
- Once I have turned the power pack on and I have ensured that my circuit works, I will begin to take readings from the ammeter and voltemeter at 10cm and carry on doing so with 10cm intervals (until 80cm.) (I will begin recording results from 10cm as 0cm has zero resistance.)
- My preliminary results will show me at what current the ammeter should be set at before I start my experiment.
- Once I have taken 3 sets of results, I will calculate the resistance by using the following equation :
R = V / I
Apparatus and materials :
- Approximately five or six wires
- A variable power/battery pack
- Ammeter
- Voltmeter
- Crocodile clips
- Wire board
- Metre rule
Prediction:
I predict that the longer the piece of wire, the greater the resistance will be. This is due to the idea of free electrons being resisted by atoms in the wire. In a longer piece of wire, there are more atoms for the electrons to collide with and therefore, more resistance. The relationship between the wire length and resistance should be directly proportional. This is because in a wire twice the length of another wire, there would be the double the amount of atoms causing the resistance. An example of this would be in a 20cm wire. The electrons would have to travel double to distance if it has to go through a 10cm wire. This would in turn double the amount of atoms that the electrons would collide with and then resistance would double. My graph should consequently show that the resistance is proportional to the length. Also, if the length is doubled then the resistance should also double.
Fair testing:
In order to carry out a fair experiment, I must explore and understand the other factors that will vary it.
When temperature is increased atoms in the material vibrate much more, meaning it is more difficult for electrons to pass on current. If the wire is heated up the atoms in the wire will start to vibrate because of their increase in energy. This causes more collisions between the electrons and the atoms as
the atoms are moving into the path of the electrons. This increase in collisions means that there will be an increase in resistance. Therefore resistance is greater. As I have to ensure the temperature is constant (Ohm’s Law), I will make certain that I do not leave the energy supply on for long periods of time. For extra accuracy, I will use the thickest wire to avoid over-heating, which is 28s.w.g.
Thickness also varies the resistance. In a thick wire, there is less resistance because there is more surface area for the free electrons to move and avoid collisions with atoms. Narrow and thin wires restrict the surface area in which electrons can move, therefore the resistance increases. I will keep the thickness of my wire as 28s.w.g to avoid dramatic change in resistance.
The type of wire also varies the resistance. Different metals have different atomic structures, and this means that they have different resistances. The more atoms metals have, the higher the resistance. To prevent affecting the constant resistance throughout each of my experiments, I will only use one material.
The equipment I use must also not change throughout the experiment, as different batteries may have different voltages.
Results:
I am planning to take 3 sets of 8 results for a reasonable and suitable average. This should help me improve the accuracy and reliability of my graphs, which is very important. Also, as the voltmeter and ammeter read to 2 decimal places, I will also record results to 2 decimals places as it is more accurate.
Preliminary results:
I have taken preliminary results to certify that my experiment will be suitable. I used constantan wire (28s.w.g) as it is what my school had provided. According to my method, I took 3 sets of 8 results from 10cm to 80cm.
I calculated the averages above by adding the 3 resistances of each length, and dividing the total by 3.
While carrying out my experiment, I experienced certain problems that restricted:
- In my actual experiment, I will ensure that all my crocodile clips are strong and grip onto the wires well, as in my preliminaries I found that they were not all strong enough.
- I found that although I used 28s.w.g constantan which is on the outside of the wire board, the crocodile clips managed to touch the other wires. I must set up my experiment so that the wires do not touch.
- For future reference, I discovered that the wires on the board are not straight and there are kinks that I cannot fix. This may restrict my experiment.
I also drew a graph to show whether my experiment is dependable, and whether there are any anomalous results. This also showed whether my experiment needed development or not. In my graph, I plotted the length of the wire against the average resistance of the wire.
This graph shows a positive correlation, showing that my experiment will be suitable. In spite of this, there is a single anomalous result. I believe this is because of many possible reasons. For example, the equipment (crocodile clips in particular) may have been flawed or damaged. The temperature may also have affected the experiment, by over-heating slightly. Also, I may have made a error while taking my readings. For these reasons, I will ensure that I use suitable equipment and record accurate results.
The graph also supports my prediction. In spite of the anomalous result, my graph showed a positive correlation, so I feel my experiment will go well. I also feel that changing my material to another wire that my school provides, nichrome. Hopefully, this will help me to prove my prediction once again.