Experiment Plan
Prediction: I predict that as the length increases, the resistance will increase. The electrons would collide with the atoms of the wire. If the length of the wire were to be doubled or tripled, the resistance would also be doubled or tripled.
Range of measurements: I will choose the 200mA range. In previous homework, I have seen how sensitive different ranges can be. In the 10mA range, the sensitivity is every 0.01A. I feel that this is not sensitive as the 200mA range, which produces a hundred times sensitivity, 0.0001A.I have chosen the range of 50mA, 100mA
Equipment:
- Three cells (each 1.5V)
- An ammeter on 200mA range
- A voltmeter on 20V range
-
A potential divider (16Ω)
- Nichrome wire - thickness 0.457mm - length 300cm
- Crocodile clips
- Connecting wires (2 long wires)
- Metre rule
Method:
- The circuit would be set up as shown in the circuit diagram above. I will use only one cell and I will not be using a voltmeter for safety reasons.
- The ammeter will then be checked for negative signs, and the circuit will be checked if the polarities and meter ranges is not exceeded.
- The voltmeter is added in, and then rest of the cells.
- A table will be drawn to record the results of the experiment. I will measure the wire to the length needed with a metre rule, and secure the crocodile clips.
- I will then switch on the power supply. I will adjust the potential divider to the desired current.
- I will then record results as I move from one length to the other, and turning off the power supply every time I do move.
- Carry on to record all readings from the different currents. In between the current changes, I will see if the circuit is accurate- if there is any ‘breaks’ in the circuit.
Safety:
I will only use the 4.5 voltage, so the variable resistor will not heat, which will affect the resistance
I will be careful when handling wires.
I will make sure that the power supply is disconnected before removing the wire.
I will make sure my hands are dry when handling electrical equipment
I will put the voltmeter in the circuit last, and I will exceed to more than one cell when I know that the current is safe.
Factors which I must keep constant to ensure a fair test:
The power supply
The equipment
The wire must be of the same thickness
The wire must be of the same material
The surrounding temperature
Analysis
The shape of my graph is a straight line going through most of the plotted points and through the origin. The variable (length) goes directly proportional to the resistance. So as the length increases on the graph, so does the resistance. The other factors were kept constant- material, thickness, and temperature. To prove that both are directly proportional, I will choose two lengths making sure that one is triple than the other. With this, I will read-off the resistance on my graph, and one resistance should be triple the other
Length: 75cm 225cm (triple difference)
Resistance: 5.3Ω 16Ω (triple difference)
In the wire, resistance occurs when the electrons collide with the atoms of the wire, causing the flow of electrons to slow. When increasing the length, it gives a larger surface area of atoms. This would therefore cause more collisions from the electrons with the increased atoms, slowing down the flow even more, consequently resulting in a higher resistance.
The conclusion supports my prediction only for Nichrome with a 0.457 thickness, using 50mA, 100mA, and 150mA at room temperature.
My graph proves that the length and resistance is directly proportional, as I have stated in my prediction. I have proved that if the length happened to be doubled or tripled, the resistance would also be doubled or tripled.
Evaluation
I found the Nichrome wire was very tricky to handle around the block of wood, trying not to make the wires touch.
Most of my readings were on the line of best fit. There are a few slightly off the line, but not enough to be an anomalous result; therefore I believe that my results are accurate. I have no anomalies on my graph or table.
The method throughout the experiment did not seem to be difficult, and I managed to obtain all my results without fail, except for the last. In the last reading, 300cm on the current 150mA, I could not get a high enough reading to get 150mA when adjusting the potential divider. This had resulted in using 4 cells in the circuit, increasing the voltage to be able to obtain the 150mA. However before that I had obtained reliable results consistently.
I would improve the method by using a larger power supply at first, so then the experiment would not have to change part way through it. I would use 4 or 5 cells to begin with, rather than just 3 cells.
The evidence that I have obtained is enough to support a conclusion. The graph is a straight line and it goes through most of the plotted points. There were no anomalies, which supports it to be accurate for the resistance of a 0.457 Nichrome wire, with 200mA range. It is sufficient to authenticate my conclusion.
I have no anomalies to account for into reasoning why the result was obtained.
To change the variable in order to extend the range of evidence, I would perhaps have chosen a longer length of wire to test on. This would have been perhaps 400cm, just to acquire more results of a 25cm gap. I believe there would be sufficient and enough evidence for the conclusion.
To repeat with a different material would be such as copper would be motivating. Copper is used for connecting wires, as it is a good conductor. Thus, the copper would heat as the current is flowing through it, and temperature is one of the four factors that affect resistance. However Nichrome is used in the heating elements of electric fires. To see whether Copper or Nichrome would be suitable, I would set up a series circuit with a 30cm part of both wires with an ammeter and voltmeter with 3 cells. Using only my touch senses I would see which would heat more than the other, as I know that voltage is shared between the components, and it would be a fair test.