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Investigate the resistance of different wires and how at different lengths the voltage increases and decreases.

Extracts from this document...

Introduction

image00.pngimage01.png



The resistance of wireimage10.pngimage10.png

Planning section

In the experiment I will be investigating the resistance of different wires and how at different lengths the voltage increases and decreases.

Variables and fair test

Key factors

Reasons given for controlling

How I controlled the variable

The current must remain the same.

If the current isn’t the same the results will be affected and they are most likely to decrease if the current is increased and if the current differs for each of set of results it wouldn’t be a fair test.

We also need to keep the current the same during the experiment because we are not measuring the current. We are investigating the resistance. This is a controlled variable

I controlled this variable by using a variable resister to keep the current on the same setting.

I will keep the variable resistor at 0.3 amps.

To supply the current, we will use a power pack instead of batteries because power packs are more accurate and we want the current to be at a certain measurement.

Each type of wire used must be the same SWG.

(width of wire)

If the SWG was higher or lower for each wire the resistance would change which means that this wouldn’t be a fair test and the results wouldn’t be accurate because This will cause resistance to decrease because of the increase in space in the wire. This increase in space means that there is more space for the electrons to flow freely because there would be fewer collisions with atoms which increases the resistance.

I controlled this variable by making sure that I used the same SWG and I did this by checking the label and I will be keeping it at 24 SWG.

Each length of wire must be accurate.

If each length of wire was one or two cm’s or mm’s away from the required length it would mean that the resistance would differ, not only would this effect the accuracy of the results but it wouldn’t be a fair test.

The longer the wire, the longer it will take electrons to get to the end of the wire. This is because there will be more collisions between electrons and atoms.

This is an independent variable.

I would control this by making sure that the wire was of a correct length by measuring the wire when it is completely strait with a 1m stick. I will be ensuring that the wire is completely strait by pulling it tight and then holding it position while measuring it.

Temperature

This isn’t a controlled variable as it very hard to measure the temperature within a wire. The temperature does affect the resistance because if the wire is heated the atoms will move as they have been given the activation energy and enough energy to move this will result in more collisions between the atoms and electrons which will then cause- the resistance to rise

To rise.

I cannot control this variable as it is very difficult and as I don’t have the correct the equipment to do so but I can minimize the heating affect by doing preliminary results to show to show a good range of lengths we can use.

Resistance

This is a dependant variable because this is what the investigation is about. This is not controlled.

I am going to measure the resistance by using a voltmeter and ammeter. We work out the resistance by the formula:  

Resistance (ohms, Ω) = Voltage (volts, V) /

                                       Amps (A)

I will calculate this by using a calculator so I avoid any possible errors.

The material of each wire

If the material being used contains atoms with a large number of electrons on the outer shells, then this means there are more electrons available. So, in theory, if the material has a large number of atoms, there should be less resistance, because of the higher number of electrons .If the atoms in the wire are closely packed, then this will cause an increase in resistance, due to frequent collisions. So trying four different types of wire is getting a wide variety of results.

I am going to use four different types of wire for this investigation and they are

  • Copper
  • Manganin
  • Constatan
  • nichrome

I intend to keep all the variables the same except for the one that I am testing which is the length to see how it affects the resistance and I intend to measure this accurately  

Scientific knowledge

Copper

In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of charge through them. "Conductor" implies that the outer electrons of the atoms are loosely bound and free to move through the material. Most atoms hold on to their electrons tightly and are insulators. In copper, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons which propagates, "domino fashion" through the conductor. image11.png

Simply stated, most metals are good electrical conductors, most non-metals are not. Metals are also generally good heat conductors while non-metals are not.

Copper

Metal

Conductivity

copper

640

silver

670

aluminium

410

mild steel

70

Table 1. Electrical conductivity. The values are a measure of how much current will flow in a standard sample when it is attached to 1 volt.

image02.pngElectrical Conductivity (sigma)

59610000 /ohm.m

How copper conducts electricity

Copper is a metal. It is made up of Copper atoms closely packed together.image12.png

If we could look closely enough, we would see that there are electrons moving about between the copper atoms. Each copper atom has lost one electron and become a positive ion. So copper is a lattice of positive copper ions with free electrons moving between them. (The electrons are a bit like the particles of a gas that is free to move within the edges of the wire).

image13.png

The electrons can move freely through the metal. For this reason, they are known as free electrons. They are also known as conduction electrons, because they help copper to be a good conductor of heat and electricity.

The copper ions are vibrating. Notice that they vibrate around the same place whereas the electrons can move through the lattice. This is very important when we connect the wire to a battery.

image14.png

Conducting electricity through copper

image15.png

We can connect a copper wire to a battery and a switch. Normally, the free electrons move about randomly in the metal.

When we close the switch, an electric current flows. Now the free electrons flow through the wire (in picture 3, they are moving from left to right - although they still move randomly as well).

Electrons have a negative charge. They are attracted to the positive end of the battery. The free electrons move through the copper, flowing from the negative to positive terminal of the battery (note that they flow in the opposite direction to conventional current; this is because they have a negative charge).

The copper ions in the wire vibrate. Sometimes an ion blocks the path of a moving electron. The electron collides with the ion and bounces off it. This slows down the electron. Some of its energy has been transferred to the ion, which vibrates faster.

Magnanin

Manganin is an alloy which mainly consists of magnesium and it has a quite a low resistance.

        On its own magnesium has a low resistance. Its resistance is shown below.

Electrical Conductivity (sigma)

22610000 /ohm’s

Constantan and Nichrome

Constantan tan is also an alloy and Nichrome is as well.

Nichrome mainly consists of nickel and nickel has quite a low resistance and its conductivity is shown below.

image02.pngElectrical Conductivity (sigma)

14310000 /ohm.m

...read more.

Middle

Intended method

In this investigation a simple circuit will be set up to read the voltage and current when the length of the wire changes. The length will range from 20cm - 100cm (1m) with intervals of 20cm. The length of the wire will be changed by moving the crocodile clip across the wire on a ruler. We decided that the best thickness of wire to use would be 24swg. This is because a thicker wire would cause too much heat, and the resistance of a thinner wire would be high and difficult to measure. The reason for this is that a thicker wire has less resistance, because there is more room for the electrons to travel through it.

We did preliminary tests to ensure that the wire did not get too hot, and how many cells to use in the circuit. The amount of energy supplied was important, as it would determine how hot the wire got. If the wire got too hot, energy would be given off as heat, and the resistance would be increased. We did a test that used the same piece of wire at different lengths with two different settings on the power pack (2 and 6). The results showed that with at 6, the resistance was higher due to the wire being hotter. For this reason I will use a setting of 2 to keep it a fair test.

The circuit should be set up as in the circuit diagram. It is important that the voltmeter is set up in parallel and the ammeter in series. The readings from the ammeter and voltmeter will be used to work out the resistance. This can be done using the formula: V=I/R
Plan (Method)

1)  Collect all apparatus –I will need four metal wires - copper, Manganin, constantan and nichrome.

...read more.

Conclusion

Improvements that could be made to the experiment

If I was to improve my experiment I would have to this by keep the wire at a more accurate constant temperature so the shorter wires would not get so hot and give inaccurate readings. This could be done by covering the wire in a plastic coat so it would keep it cool, or you could use a condenser. Another option is to use longer wire lengths so the wires don’t get hot easily and quickly.

To speed up the pace of the experiment, an ohmmeter could be used to directly calculate the resistance. Out experiment was too slow and took a lot of time. Instead of reading the voltage and calculating the resistance, an ohmmeter could be used. This would speed up the method and give more accurate readings.

Does the evidence support a definite conclusion?

My graphs and my secondary sources graphs do not match my fully match my theory. Although my results were reasonably accurate they aren’t accurate enough to support a reasonable conclusion.

I think that my results were mainly affected by the heating affect and if this didn’t occur there wouldn’t have had an anomalous results and it would support a definite conclusion.

My results were not 100% accurate; I know this because not all of my results fall on my line of best fit. If my results matched the theory 100%, all the points would be in a straight line and there would be no points outside of the line of best fit.

The results for the secondary source of data were not 100% accurate either as there were quite a few points that fell outside the line of best fit.

The reason why my results do not support a definite conclusion is because of not being able to control the temperature variable. This had a big affect on my results.

...read more.

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