Variables
Input:
- Length of wire.
- Material of wire.
- Width of wire.
- Starting temperature of wire.
Output:
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The resistance of the wire.
- Current in circuit.
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Temperature of wire.
The length of wire will be varied; the other input variables will be kept constant. The Resistance of the wire will be measured.
Predictions
I predict that the longer the wire, the greater the resistance. This is because the longer the wire, the more times the free electrons will collide with other free electrons, the particles making up the metal, and any impurities in the metal. Therefore, more energy is going to be lost in these collisions (as heat).
Furthermore, doubling the length of the wire will result in double the resistance. This is because by doubling the length of the wire one is also doubling the collisions that will occur, thus doubling the amount of energy lost in these collisions.
Method
- One metre length of 0.4mm-diameter “constantan” (a metal alloy) wire is fixed to a metre rule.
- The first crocodile clip is clipped to the wire at the 0cm position on the metre rule.
- The second crocodile clip is clipped to the relevant position depending on the required length of wire.
- The power supply is turned on. The voltage and current are then read off the ammeter and voltmeter, and recorded.
- The power supply is then turned off and the second crocodile clip is moved to the next position.
- I will set up the circuit then I will carry out the following experiment.
- Measure 5cm of wire
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Take the amp and voltage reading and work out the resistance by the formula R=V/I
- Then move the variable resistor to a different position and take down the reading again and use the formula.
- Move the resistor once again and take down the reading using the formula.
- If the resistance in the wire is the same you should after each reading after moving the resistor get an answer exactly the same or extremely close.
When the above steps are completed for each length I will then repeat the entire investigation for accuracy and reliability, as the answers should be the same. I will do exactly the to the four other times but just change the length of wire 10,15,20,25cm.
The following circuit will be constructed to perform the investigation:
A resistor is used to control or reduce the current in a circuit. The simplest form of resistor is a thin wire placed in a circuit. The greater the resistance of the wire, the smaller the current flow. A variable resistor enables different lengths of wire to be added into a circuit, so that current can be controlled.
Using the circuit as the one above, an important general relationship can be seen. The variable resistor is used to control the current in the circuit and the voltmeter measures p.d. across the resistor varies. Provided the temperature does not change significantly, the results give a graph like the one below. This means that the current is proportional to the p.d. The relationship is called ohm’s law. The relationship (Ohm’s law only applies if the temperature is constant, and that it does not apply to all electrical components
Apparatus.
- Constantine wire (5-25cm)
- Variable
- Amp meter
- Volt meter
- Power pack
- Crocodile clips
Preliminary results
In order to decide upon the voltage and lengths of wire to use in the final experiment, the following preliminary work was carried out, also to make sue my final experiment goes to plan.
Wire 22, 24, 28 SWG thickness
For wire 24 swg thickness, I chose to investigate 10 cm.
After performing the preliminary results, it provided results from 5cm up to 25cm and the higher voltage provided no additional ease of measurement.
Furthermore, it was also decided to allow the wire to cool between experiments as considerable heat was noticed at lower lengths and, as mentioned above, an increase in temperature results in an increase in resistance. By allowing the wire to cool between experiments a fair test could be assured
A table to show the results to show how the resistance of wire by the length of wire.
Safety
In order to perform a safe experiment, a low voltage was chosen so that overheating was minimilised. Furthermore, lengths lower than 10cm were not tried, which also helped to avoid overheating.
Analysis of my graph to shoe hoe the resistance of a wire is affected by the length of the wire.
As predicted the graph shows us that the longer the wire the greater the resistance. For example the length of 0.5 Ώ, doubling the resistance gives us the length of wire 10cm resistance, because the resistance is 0.96 Ώ. It is not exactly the same, but extremely close.
The line of best fit is going up steadily, which means that resistance is steadily increasing, not suddenly but at different times. As the potential difference between the ends of a conductor is increased the current passing through it increases. If the temperature of the conductor does not alter, the current which flows is proportional difference applied (ohm’s law). Shows this affect.
The gradient of this graph has a constant value, obtained by driving the potential difference at any point by the current. The value of this constant gradient is known as the resistance (R) of the conductor.
A good conductor is one with a low resistance, a poor one has a high resistance. In some conductors the current is not proportional to the potential difference between its ends. In the wire V/I is found to increase with temperature, that is resistance is not constant but increases as the temperature of the wire increases.
I found out that voltage is proportional to current, other wise the resistance would not stay constant in the in the wire. This was because current increased but as an example of a higher dependent resister (LDR) depends on the temperature surrounding it. However the Constantine wire we were using in our experiment did not change its resistant at all. The only things that affected its resistance was size. As my prediction stated the longer the wire the greater the resistance.
Evaluation
My experiment was successful, because all the appliances worked, and all results were correct. We have no anomalous results because they all fit the pattern perfectly.
There were no problems as the same wire was used for each set of results so it is known that the results for each wire are correct.
My prediction was accurate (‘the longer the wire, the greater the resistance’) as it shows from my graph. As the resistance increases the results on my graph shoe the movement by the steep line. This was proved when we moved the slider of the variable resistor and all the readings were fairly similar. The difference was so little that we counted it as negligible.
