Circuit Diagram
Equipment
I am planning to use this equipment:
- Two crocodile clips
- A Ruler
- A digital ammeter
- A digital voltmeter
- A variable low voltage power supply
- A calculator
- Connecting wires
- 100 cm of wire
- Micrometer screw gauge
- Variable resistor
Fair Test
I have decided to test how the length of the wire affects the resistance of the wire and so I shall try to keep all the other factors as constant as possible when carrying out my experiment.
The temperature shall maintain at room temperature throughout and I shall check this by using a thermometer to check that the temperature is still the same and no drastic changes in temperature have occurred. In order to make sure that the thickness of the wire remains the same each time I shall use a micrometer screw gauge to measure the thickness of each wire I use. I shall make sure that this material stays the same for each of my tests.
I shall repeat the experiment at each length three times to make sure that even if one set of results go wrong I should be able to tell which set was wrong. When I repeat the experiment
I have chosen to use the digital ammeter and digital voltmeter instead of an ohmmeter, because they are more accurate than an ohmmeter. I have chosen to use both a voltmeter and an ammeter to check that the readings are correct in case something has gone wrong.
I am using a variable low voltage power supply, because a battery might run out and a low voltage power supply would keep the power at a more consistent level. I chose to use a micrometer to make my measurements of the width of each wire more accurate as it is very difficult to get a precise measurement using only a ruler as the width is so small.
I will use a calculator to work out the resistance of each wire from my results, because it will hopefully help prevent human error occurring when the calculation is done.
Variables
There are several different aspects, which should be considered that might have an effect on the resistance of a wire in a circuit.
Independent variables
The independent variable in this experiment is the length of the wire. This will be changed to see how the length of the wire affects the resistance. Also I will be changing the variable resistor, to vary how much resistance in allowed through.
Dependent variables
The dependent variables that may change are the temperature of the wire and the width of the wire. These may change because I am altering the independent variables
From background knowledge I found out that an increase in temperature meant more resistance. This is because the vibration of the atoms increases with heat making it more difficult for the current of electrons to flow. Therefore it is important that the temperature does not change at any point in my experiment, because otherwise my results will be unreliable.
Other evidence of what affects the resistance came from the physics book “The World of Physics” by John Avison, published by Nelson on page 231 and 247. From this, I learnt that different wires may have a higher resistance because of their dimensions (length and width) and the material the wire is made from all of which effect the resistance.
From my preliminary experiment I found out that metallic conductors have a fixed resistance subject to temperature and other variables remaining constant. Different materials will alter the resistance because in different metals the amount of electrons for each atom is different, those having more electrons will have stronger forces of attraction (bonds) holding the atom together. With these stronger bonds more energy is needed to allow the sea of outer electrons to move as freely. This is something that affects the current of electrons so causes more or less resistance.
The width of the wire affects the resistance because if the wire is very narrow fewer electrons will be able to get through that wire at one time and so will affect the current.
Risk Assessment
To make sure my experiment is as safe as possible at all times I must evaluate all the risks.
- One possible risk is that if I put too much current through the wire the wire will become hot and I could burn my finger from the heat. Also it may affect the resistance of the wire or blow a fuse in the power supply.
- Another risk is that if my resistance wire is too short a short circuit or similar danger may occur; therefore my smallest length of wire shall be 10 cm in order to eliminate this risk.
Planned Method
- Measure the wires width at different parts of the wire to check that it has a constant width
- Set up the above circuit connecting the wire into the circuit using crocodile clips at 10 cms apart using a ruler.
- Turn on the voltmeter and the ammeter and make sure that both have a positive reading.
- Making sure that variable resistor is at the begging take the first reading on the ammeter and the volt meter and record these results
- Now move the variable resistor along and take the new readings
- Continue to move the variable resistor until you get at least ten readings
- Next move the crocodile clips 20 cms apart and start instruction 4.
- Repeat all of this adding an extra 10 cms each time
9. For each result, use a calculator and Ohms law to find out the resistance and record this
Ohm’s law that resistance = voltage, work out the resistance current
Results Table and Graph
Results
10cms
20cms
30cms
40cms
Resistance Calculation
Examples-
First set of equipment: Length of wire= 10cm, Voltmeter reading= 0.15V, Ammeter reading= 1.40A
Formula: Voltage= Resistance 0.15V = 0.11 Ohms (2d.p)
Current 1.40A
Therefore the resistance of the wire was 0.11 Ohms, with the wire length at 10cm and using the first set of equipment.
Second set of equipment: Length of wire= 60cm, Voltmeter reading= 0.86V, Ammeter reading= 0.46A
Formula: Voltage= Resistance 0.86V = 1.87 Ohms (2d.p)
Current 0.46A
Therefore the resistance of the wire was 1.87 Ohms, with the wire length at 60cm and using the second set of equipment.
Third set of equipment: Length of wire= 90cm, Voltmeter reading= 0.94V, Ammeter reading= 0.13A
Formula: Voltage= Resistance 0.94V = 7.62 Ohms (2d.p)
Current 0.13A
Therefore the resistance of the wire was 7.62 Ohms, with the wire length at 90cm and using the third set of equipment.