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Resistance means the opposition to the flow of current. Electric current is defined as the movement of free electrons; meaning resistance is the opposite of free electrons.
Ohm's law shows the relationship between the voltage across a given piece of metal and the current flowing through it. The law states:
For a metal conductor at constant temperature, the current flowing through it is directly proportional to the voltage across it.
It is also relevant to know of Ohm's Law, which states that the current through a metallic conductor (e.g. wire) at a constant temperature is proportional to the potential difference (voltage). Therefore V¸ I is constant. This means that the resistance of a metallic conductor is constant providing that the temperature also remains constant. Furthermore, the resistance of a metal increases as its temperature increases. This is because at higher temperatures, the particles of the conductor are moving around more quickly, thus increasing the likelihood of collisions with the free electrons.
A resistor is used to slow down the amount of current flowing through a circuit. The resistance of a substance varies with its length. The longer conductor, it more resistance it will have. Also more resistors will mean the current is slowed down repetitively meaning the current is slowed down even more. Such as the image below shows the current displayed as cars passing through tolls, this relates to electrons passing through resistors. When there are three resistors parallel to each other this is more space for each electron to pass meaning the resistors won't be working as effectively.
In a long piece of wire there are more atoms so there is less room for the electrons to get through. The electrons bump into the atoms and lose some of their energy.
The factors that affect resistance are as follows:
* Type of material.
* Cross section of the wire.
* Wire length.
Energy is given to the atoms so that they vibrate faster and causes more collisions between the electrons and the atoms that are moving into the path of the electrons.
Resistance occurs when the electrons moving along the wire collide with the atoms in the wire.
The flow of electrons is slowed down causing more resistance (and more collisions).
Resistance is a measure of how hard it is to move the electrons through the wire.
· Longer wires have more resistance than shorter wires, as there are more atoms for the electrons to collide with.
· Thinner wires have a larger resistance than thicker wires as it is harder to push through a thinner space.
· Thicker wires have lower resistance than thinner wires and therefore produce less heat.
· The hotter the wire the higher the resistance.
· Twice the length = twice the resistance as there are twice as many atoms to collide with.
· Twice the thickness = half the resistance as the electrons have twice as much room to move through the wire therefore halving the chance of collisions.
The aim of this experiment is to investigate how the length of a piece of wire affects the resistance of a piece of wire.
My main aim in this experiment is to find out how the length of a wire affects the resistance. Therefore the independent variable is: length of the wire, the dependent variable: resistance.
Before actually planning the experiment, I will do some preliminary research to find out about resistance and circuits, and matters related to it, such as the amount of voltage to use sensibly that should lead me to good results at the end of the experiment, and also allows me to form a prediction. I will also need to come up with structured ways that will make this investigation fair and safe.
Ohm's law states that a current flowing through a metal conductor is directly proportional to the voltage across its ends (provided all other conditions are constant). So I know that if we add a variable - in this case length - resistance will change. I expect that the longer the wire, the higher the resistance.
The length of the wire increase so does the amount of atoms in the wire, this will therefore increase the resistance, as there will be more atoms for the flow of electrons to pass through. As a result of this the atoms and electrons will collide more so this is why the resistance will increase. More collisions, as there will be more wire and atoms for the flow of electrons travelling, if the length of the wire doubles, the amount of resistance should double, as there will be double the atoms in the wire, doubling the collisions.
Based on previous knowledge I predict that the resistance of the wire will increase in proportion to the length. Double the length the resistance will double, treble the length and the resistance will be three times as much as there will be three times as many fixed atoms for the moving electrons to bump into as they pass through the wire. The resistance and the length should be directly proportional to each other.
Power pack - to allow a current (flow of charge) to pass through the wire
Wires (x7) - part of the experiment
2 crocodile clips - to enable a circuit to be built, between both ends of the wire
Ammeter - to measure the amount of amps being transmitted
Voltmeter - to measure the amount of volts being transmitted
Metre stick - to measure length of wire
Resistor - to measure the resistance of the wire
Out of these 5 variables (see Investigation Research) the one I will be changing will be the length of the wire; we will change the wire by 10cm each time, starting from 10cm to 70cm, enabling us to draw up a precise conclusion of how the length of a wire affects resistance.
I already know, the length of the wire surely affects the resistance, I predict that as the length of the wire increases the resistance will also increase, giving me a complete positive correlation. This occurs because there are more atoms for the free electrons to pass through so there will be more collisions which therefore increase the resistance of the wire. So I can safely say that the resistance is proportional to the length of the wire.
The experiment is not particularly dangerous but as I will be using electricity, I will need to make sure I don't have wet hands when handling the equipment and I will not increase the current unless needed to, as it could make the wire hot and possibly could burn me.
Therefore special care needs to be taken when dealing with:
Liquids - conducts electricity, therefore dangerous in this experiment
Wires - unprotected in some parts, able to harm you
Voltmeters and Ammeters - electrical equipment
Sockets and Plugs - electrical equipment
Collect all the equipment and check that it works.
Set up the circuit correctly, and work with the current as 0.09 and the power pack being on direct current (d.c).
6.50- Link the circuit up so that the Ammeter and Voltmeter can read the current/PD, travelling through the wire and the power pack is supplying energy to the wire. There should be a resistor between the wire and the power pack.
Without cutting the wire, measure 10cm and repeat this until (70cm, 60cm, 50cm, 40cm, 30cm, 20cm and 10cm) wires are cut from a few metres of wire and put crocodile clips at either end of the measurement.
Look at the ammeter and voltmeter and record the reading of the current and voltage in a table. Also calculate the resistance by dividing the current by the voltage (0.07/0.09 = 0.78).
These were the results collected for my group, although I feel they are not entirely correct, therefore I have spotted outliers (a dash placed after them e.g. 5.77-) and I will be using a more reliable set of results from another group. Outliers were spotted by comparing with the classes average resistance table.
The current in the other group is higher at 1.0 Amps; however this shouldn't affect the reliability of the results too much.
5.6I chose this set of results as they were the only results which had the current near to ours, plus the fact that the results are more reliable (no outliers spotted).
There is a clear rise in resistance as the length of the wire increases. Funnily enough I do not believe the first set of other results are accurate either, as the voltage has an obvious pattern with the resistance. The voltage multiplied by 10 always seems to equal the resistance, therefore there may have been a huge error in this groups readings as the resistance could not have been calculated, so I will need to find other more reliable results.
Therefore I will use the average results of the whole class to back up my predictions.
The results show that there is an increase in voltage and an increase in resistance as the length increases. Hence the longer the wire, the more atoms there will be for the free electrons to pass so therefore there will be more collisions.
To conclude, there is definitely an increase in resistance when the length of the wire is increased.
Averaged Results (Whole Class)
Firstly, it's time to test the theory of, if you double the length of a wire, the resistance should double also, I will be using the average resistance to calculate this, therefore if the results aren't exact, they should still be acceptable in some cases.
Wire length 10cm, has a resistance of 0.68 Ω, therefore doubled wire length 20cm should equal to 1.26 Ω, instead the resistance is 1.16 Ω, although this judgement is accepted as there isn't much of a difference.
Wire length 20cm, has a resistance of 1.16 Ω, therefore doubled wire length 40cm should equal to 2.32 Ω, instead the resistance is 2.91 Ω, there is definitely a clear increase in resistance, but this result is not reliable as it doesn't seem to support other claims.
Wire length 30cm, has a resistance of 2.17 Ω, therefore doubled wire length 60cm should equal to 4.34 Ω, instead the resistance is 4.54 Ω, this judgement supports the claim even with a large range of results, with only a difference of -20 Ω.
Wire length 10cm, has a resistance of 0.68 Ω, therefore trebled wire length 30cm should equal to 2.17 Ω, instead the resistance is 2.04 Ω, this judgement supports the claim even with a large range of results, with only a difference of -13 Ω.
Also the amount of voltage increased too, this is due to more voltage being required to penetrate the increasing length of the wire. There isn't much of a correlation between the amount of voltage increasing, however the voltage changed gradually throughout, by +10V (-5V or +5V at different stages).
If we carried on shortening the wire then eventually the wire will have melted. This proves my prediction to be true, as I predicted that the longer the wire is, the higher the resistance will be. As there were fewer atoms present in the wire to collide because the wire was short, there was only a small chance of the atoms colliding so the resistance of the wire was low.
I believe I followed my method very closely while performing the experiment; however certain precautions can have been taken to assure this was a safer experiment. Firstly the wire could have been straightened before being measured to a millimetre rather than to the centimetre, a wire that is damaged in places would have led to the wire being longer than what is recorded in a table as the ruler is measuring extra length. This additional length would have triggered extra resistance and there were several readings with added resistance in my results.
The temperature of the wire was a factor I could not fix as it would have been virtually impossible to control the wire temperature due to the resistance heating it, the room temperature would also have been hard to fix. On a hot sunny day it is likely that my resistance readings would have been higher than my readings on a cold snowy day.
The current could have been more specifically measured using a digital power pack would have decreased human error such as seeing the amount of current from different points of view and changing it each time.
The wire's thickness could have been measured at more points along the wire to make sure the wire's thickness was the same all along. A thicker wire would lead to lower resistance and a lower resistance on a result would affect my calculated average.
The already existing resistance in the crocodile clips could have been reduced by sanding off any dirt on the clips and cleaning them. Doing the same to the connectors at either end could have reduced the resistance in the leads.
The experiment was not needed to be repeated as data was collected from other students from the class, therefore allowing us to collect a wider range of results.
To extend the enquiry of factors that affect the resistance of the wire we can also investigate changing the following factors: cross sectional area of the wire, material the wire is made from, current passed through the wire, temperature of the wire.