Why is there resistance in wires, and when you differ things like the extent of the wire, why does the resistance alter?

The cause in support of resistance in wire is because of the composition of the atoms such as copper, aluminium, of which a particular wire is made of and the plan of the atoms of these metals. When an electron passes through the wire, the electrons strike these atoms whilst making the trip from one end to the other giving opposition or resistance to the electrons. When this happens electrons stir an electromotive force such as voltage, and in hitting these atoms, also creates heat via friction of the electrons and atoms. When the wire is lengthened, the journey is considerably longer and the resistance changes. Also, the size of the wire changes the amount of resistance. That is one basis why you don't use a #22 gage lamp cord extension to supply extended power to an appliance of heavy duty. You don't have the capacity to supply sufficient electrons capable enough to run the domestic device.

A enhanced way to respond to this question is through an analogy. Visualize the charge that is flowing through a wire to be water flowing through a pipe. The Rate at which the water is flowing is the current in an electrical system, how fast are the electrons moving. Resistance is what restricts the movement of the charge through the wire. The length of a pipe, the cross-section, and the imperfections in the pipe (clogs and such) all impede the flow of the water, and are analogous to how electric current is impaired as it flows through a wire. Things like making the wire longer and thinner make the resistance go up since it is harder to push charge through the wire. So you can see that resistance is inherent in every type of wire. There are cases when some materials can be cooled down to very temperatures where they have effectively zero resistance, but in daily encounters electricity this, effect doesn't really have much bearing.

Electrical resistance results from the collision of the current-carrying charged particles that have fixed particles that make up the structure of conductors. The collisions result in thermal energy (heat) in the conductor.

Current -→ - - - ←-Resistance

The resistance of a wire (or any conductor) is given by the following equation:

R = r l/A

Where:

As you can see from the above equation, thicker wires have lower resistance than thinner wires. This analagous to a thick water pipe vs. a thin water pipe. A thin wire carrying a given current will have a higher current density when compared with a thick wire carrying the same current. Higher current density means a higher probability of collision with stationary atoms and higher resistance.

Each type of wire has a certain amount of resistance per foot -- the longer the wire, the larger the resistance. If the resistance is too high, a lot of the power that flows down the wire will be wasted, the energy lost as heat builds up in the wire. Ultimately, heat build-up limits the current carrying capacity of the wire, as the temperature must not get hot enough to melt the insulation.

Below is a diagram to show the resistance inside a wire.

Factors effecting the resistance in a wire.

There are three factors which effect the resistance in a wire:

The length – the longer the wire the more resistance it gains, and the smaller the wire gets the less resistance it has. This is because the electrons have less length to travel.

The thickness – the thicker the wire is the less resistance it has and the thinner the wire the more resistance it has. This is because in the thicker wire the electrons have more room to move through the wire and in the thinner wire they have less causing more resistance.

The material – if you test different metals for resistance you will find that there is a lot of difference between different ones. Take copper, it has a very low resistance while as constantan has a very high resistance compared to copper as does nichrome and manganin.

Below is a list of apparatus that I will use to conduct the experiment safely.

Ammeter - To be positioned in series with the resistance wire to calculate the current in the circuit.

Voltmeter - To be set up in parallel to the resistance wire to measure the potential difference across the resistance wire.

Variable resistor – To be set in series with the resistance wire in order to change the current flowing through the circuit and to make sure my results are precise.

Meter ruler – To measure the lengths of wire.

Lab pack – To create an electrical surge through the circuit essential to carry out this experiment.

Crocodile clips – To connect the wire Iam testing with the other wires from the circuit.

Wires – To run from the lab pack to the wire witch Iam conducting the investigation on.

Below is a diagram of what the circuit will look like.

Variables:

There are three variables which I can select from to perform my experiment; I can either use all four wires ‘ copper, constantan, manganin and nichrome ‘ at a unchanging length and compare the resistance of each material or I can differ the thickness of one type of wire to see what effect this will have on the resistance or I can vary the length of a type of wire to see what result this will have on the resistance.

I am going to carry out the latter variable. I am going to work with constantan as my resistance wire and I am going to vary the length of the wire and observe what happens to the resistance. I am going to set the results out in a table and evaluate the results for each length using a graph.

The variables in this are that I will select lengths 100cm, 80cm, 60cm, 40cm and 20cm. I have chosen these numbers because if I were to choose a lower value of number then the variation in the readings would not be substantial enough to record on a graph, if I were to pick a higher value of numbers then that amount of wire would be to hard to get hold of and it would be less practical.

Fair testing:

In order to make this experiment fair I will do the following procedures:

Repeat the measurements three times to make sure that my results are reliable.

Take my readings quickly and accurately to ensure that the temperature of the wire dose not change as heat is a variable which affects the resistance of wire.

Make sure I test each piece of wire for the same amount of time and give them the same atmosphere.

Prediction

My prediction is that the longer the measurement lengthwise of wire, the greater the resistance will be, I assume this for ...

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Fair testing:

In order to make this experiment fair I will do the following procedures:

Repeat the measurements three times to make sure that my results are reliable.

Take my readings quickly and accurately to ensure that the temperature of the wire dose not change as heat is a variable which affects the resistance of wire.

Make sure I test each piece of wire for the same amount of time and give them the same atmosphere.

Prediction

My prediction is that the longer the measurement lengthwise of wire, the greater the resistance will be, I assume this for the reason that the current will be higher throughout a longer piece of wire and resistance opposes current therefore there will be a high resistance. Furthermore if the current is higher (stronger) there will be a larger resistance since resistance opposes current.

The resistance of a wire depends on certain factors. Some of these variables are listed below:

· Length of wire

· Diameter or thickness of wire

· Temperature at which wire is kept

· The material of which wire is made out of.

· The potential difference or voltage.

· Humidity

· Cross sectional area.

· Voltage across circuit

All these factors will have to be kept constant except the diameter of the wire whilst doing the experiment to ensure that the investigation is a fair test.

I have decided to investigate how the diameter of a wire affects its resistance because other factors such as temperature are hard to control or vary. There is not a large enough range of materials to investigate how materials affect the resistance of a wire. The way in which the diameter of a wire affects the resistance is an efficient experiment to do. A graph can be plotted easily, there is a large range of results and the results can be recorded easily.

Variables

The investigation is to investigate the resistance when the diameter has changed. In order for the investigation to be a fair test, all other factors or variables should be kept constant. The investigation has to be done in a fair manor otherwise the results would be inaccurate and the conclusion would be incorrect. The variables, which must be kept constant, are:

1) Temperature When the temperature of a metal increases the resistance of that metal increases. This is because when the temperature increases the atoms of the metal vibrate more vigoursly because of the increase in energy. This means that the electrons have more difficulty getting through the wire as they collide with the atoms which are in their pathway. This increases the amount of collisions therefore there is more resistance. However it is hard to keep the temperature exactly the same as the room temperature might change from day to day. It is essential to use a low voltage because it means a low current that will not heat up the wires. If a high voltage is used the energy would be in form of heat which would make the experiment unfair. The investigation will be done at room temperature. The temperature cannot be investigated because it is hard to control the range of temperature needed without the correct apparatus.

2) Length of wire The larger the length of the wire, the larger the resistance. This is because there are more atoms from the metal so there is more chance that the electrons would collide with one of the atoms therefore there is more resistance. It is important to keep the length of the wire the same each time otherwise it could not be certain which variable is changing the resistance. The length of wire will be 50cm throughout the investigation. Electrons have a longer distance to travel so there are more collisions .The length of the wire will make a difference to the resistance. This is because when you have a long wire, the electrons have to squeeze together for longer to be able to pass through the wire than they do in order to be able to pass through a short wire. I predict that the longer the wire, the greater the resistance. If I had a 30 cm wire and a 60 cm wire, the 60 cm wire would have a resistance twice that of the 30 cm wire.

3) Type of material Different materials have different resistances because the materials’ atomic structures are different so some metals have low resistances and some have high resistances. Therefore it is important to keep the material the same throughout the experiment unless a different material is used to check if the conclusion or theory works for all materials. If different materials are used throughout the investigation, it will affect the results. For example if sometimes copper is used and sometimes nichrome is used, the results where copper is used will be of a low resistance because of the material and not because of the diameter of the wire. Throughout the experiment Constantan and nichrome will be used. Constantan is used to make sure my conclusion works for all materials. The type of material will affect the amount of free electrons that are able to flow through the wire. The number of free electrons depends on the amount of electrons in the outer shell of the atoms, so if there are more or larger atoms then there must be more electrons available. If the material has a high number of atoms there will be high number of electrons causing a lower resistance because of the increase of the number of electrons. If the particles in the material are tightly packed together, the electrons will have more collisions and therefore more resistance.

All these factors must be kept constant to make the investigation fair. The same apparatus must be used throughout the investigation. It is also important to take three repeats and find the mean so if one result is very inaccurate, the others will average it out.

Research

All materials, solid, liquid or gases are made up of atoms. The atoms themselves consist of a central bit, called the nucleus, made up of particles called protons (which have a +ve electrical charge) and neutrons (which have no charge) Orbiting around the nucleus are electrons which are very tiny and have a -ve electrical charge. One can think of the electrons orbiting in layers like the rings of an onion, and it's the ones in the very outside layer, the outer shell, that are the most important when thinking about conduction.

In metals, the outermost electrons are held only very weakly to the atom and often wander away from it and go to the nearby atom or one a bit further away. These wandering electrons are called conduction electronsand the more of these there are, for a given volume of metal, the better the metal will be as a conductor of electricity. When you connect a battery across a wire, one end becomes +ve and attracts the conduction electrons, which drift towards that end of the wire. But the electrons have obstacles to face because the metal atoms are jiggling about because of their thermal energy and so the electrons collide with them and are knocked all over. It’s this difficulty that the electrons have in moving along the wire that we call resistance.

Resistance involves collisions of the current-carrying charged particles with fixed particles that make up the structure of the conductors. A resistor is a material that makes it hard for electrons to go through a circuit. Without resistance, the amount from even one volt would be infinite. Resistance occurs when electrons travelling along the wire collide with the atoms of the wire.

The unit of resistance is Ohms and the symbol is:

The higher the resistance, the lower the current. If there is high resistance, to get the same current a higher voltage will be needed to provide an extra push for the electricity.

Some metals have less resistance than others. Wires are always made out of copper because copper has a low resistance and therefore it is a good conductor. The length and width of a wire also has an effect. In this investigation I will investigate how the diameter of a wire will affect the resistance in the circuit.

Resistance opposes the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires are either very thin or very long.E.g. The filament aan electric light bulb

Ohm's law

He discovered relationship that the amount of steady current through a large number of materials is directly proportional to the potential difference, or , across the materials. Thus, if the voltage V (in units of volts) between two ends of a wire made from one of these materials is tripled, the I (amperes) also triples; and the quotient V/I remains constant. The quotient V/I for a given piece of material is called its resistance, R, measured in units named ohms. The resistance of materials for which Ohm's law is valid does not change over enormous ranges of voltage and current. Ohm's law may be expressed mathematically as V/I = R. That the resistance, or the ratio of voltage to current, for all or part of an electric circuit at a fixed temperature is generally constant had been established by 1827 as a result of the investigations of the German physicist George Simon Ohm.

Alternate statements of Ohm's law are that the current I in a conductor equals the potential difference V across the conductor divided by the of the conductor, or simply I = V/R, and that the potential difference across a conductor equals the product of the current in the conductor and its resistance, V = IR. In a circuit in which the potential difference, or voltage, is constant, the current may be decreased by adding more resistance or increased by removing some resistance. Ohm's law may also be expressed in terms of the , or voltage, E, of the source of electric energy, such as a battery. For example, I = E/R.

With modifications, Ohm's law also applies to alternating-current circuits, in which the relation between the voltage and the current is more complicated than for direct currents. Precisely because the current is varying, besides resistance, other forms of opposition to the current arise, called reactance. The combination of resistance and reactance is called , Z. When the impedance, equivalent to the ratio of voltage to current, in an alternating current circuit is constant, a common occurrence, and Ohm’s law is applicable. For example, V/I = Z.

With further modifications Ohm's law has been extended to the constant ratio of the magneto motive force to the magnetic flux in a .

Resistance values in electronic circuits vary from a few ohms, W, to values in kilohms, kW, (thousands of ohms) and megohms, MW, (millions of ohms). Electronic components designed to have particular resistance values are called resistors.

Prediction

My hypothesis is that the thinner the wire, the higher the resistance. The thicker the wire, the lower the resistance.

This is because the thinner the wire is, the less paths there are for electrons in the wire therefore the harder it is for current to flow. This results in the energy not being able to spread out as much, so the resistance will be higher. This theory is similar to that of a door, where people represent the electrons and the door represents the diameter of the wire. If the door is wider, more people can get through, more easily. If the door is small, people have more difficulty and not as many people can walk in or out of the door. It is the same principal for electrons. If the diameter of a wire is thicker more electrons can go through the wire, therefore less resistance. The atoms from the metals cannot stop or collide with as many electrons because the diameter of the wire is larger. This can be seen in the diagram on the next page.

From my preliminary work, I can see that if the area of the wire doubles, so does the number of possible routes for the current to flow down, therefore the flow of electrons are twice as spread out, so resistance might halve,

i.e. Resistance 1/Area.

This can be explained using the formula

R = V/I

Where R=Resistance, I= Current, V=Voltage

When there is twice the current, and the voltage is the same, R (resistance) will halve. I did some research and in a book called ‘Ordinary Level Physics’ By A. F. Abbott, it says ‘that doubling the area will therefore halve the resistance’- in other words the resistance of a wire is inversely proportional to its area, or R 1/A, but we are measuring diameter so the following calculations have to be made:

Safety

Whilst doing the investigation, it is important to keep safety into consideration. The scissors should only be used for cutting the wire to the appropriate length and for no other reason. Before using the power pack, the pointer should point at 0 volts. It is important to be careful while using the power supply. While handling live wires, it is essential to be careful. The voltage should be kept low because of the safety factor and the wires heating up.

Reliability

To make the experiment reliable, all apparatus must be checked to see if it is functioning properly and is giving a true reading. This will partly avoid systematic error. Another way to make the experiment reliable is to use two methods: to do the investigation in two different ways to measure the resistance when the diameters are changed. If one method contains systematic error or is very inaccurate, the other method will be used to recognise that.

Precision

I will take as many different results as possible so that there a wide range of results and that I am able to arrive at a good conclusion. To increase the accuracy of the experiment I will do repeats for all the experiments so when the mean is taken, an accurate table is drawn up and if one result is anonymous the other two results would contrast the anonymous result.

Preliminary work

Whilst doing preliminary work I will investigate the following:

· Get some sort of idea whether there is a connection between the diameter of the wire and the resistance within the circuit.

· Find out what kind of figures the resistance are for different diameters.

· Write the results of the preliminary work in a table below.

· Find out what length of wire should be used throughout the experiment.

· Find the appropriate apparatus and use them.

· Investigate ways of making the experiment fairer and more reliable.

· Think about two methods of doing the investigation to make it more reliable and refine methods of doing the investigation.

· Think about what materials (two: to make sure my conclusion works for all materials) should be used.

· Investigate the range of results, which will be appropriate and can be plotted in a graph to show some sort of connection.

· Think about what diameters or gauges should be used and state why they are being used.

· The verification of techniques is important whilst doing preliminary work and to decide adjustments, which will have to be made whilst having precision and reliability in mind.

· Think about the appropriate length of the wire whilst doing the investigation. This can be found whilst doing preliminary work. The appropriate length will be one, which is easy to work with and has more precision.

Preliminary Method

The aim of preliminary work is to find apparatus, make adjustments and investigate other factors listed above. With this in mind it is important to collect sufficient data but it is important not too contain too much data. The preliminary work is not designed for collecting data but for investigating the factors above. It is for this reason that the extremes will be taken. This means the highest and lowest diameter that can be obtained will be used as well as one or two in the middle. The three materials, which are available in more than three diameters, will be tested and in the conclusion, the best two materials will be stated. It is also important to find the appropriate length so a 50cm and 100cm length of wire will be tested and again in the conclusion, the most appropriate length of wire will be stated.

The apparatus will be set up as shown in the diagram. A piece of wire will be cut of 50 cm and 100cm (They will both be tested). The 50cm wire will be placed in the middle of the circuit. It will be placed on the ruler and stuck down with masking tape. Crocodile clips will be placed on either side of the wire and the resistance will be measured using an ohmmeter.

Three materials will be tested and two materials will be picked out of Constantan, copper and nichrome. The investigation will be carried out of carry out by measuring the resistance of different materials and two different lengths of wire. This will be done using an ohmmeter and the resistances of different diameters of different materials will be measured in ohms. The range of results will be investigated and decide whilst doing the preliminary work. The resistance of different diameters of wire will be measured whilst keeping other variables such as length and material constant. The results will be written down in a results table on the next page. I will find resistances of three different materials in order to decide which material will be used. In the real experiment the second method will also be used by which the resistance can be calculated by finding the potential difference and the current and using the formulae below to calculate the resistance as a test if the method is easy and it works. In the preliminary work, it is best to leave this alone because preliminary work is not designed to collect data.

Resistance= voltage

Current

Preliminary results

All the beneath results were taken whilst a 50cm wire was used.

The following results, were taken from Nichrome:

The following results were taken from copper:

The following results were taken from constantan.

The following results were taken when a 100cm piece of wire was used:

The following results were taken from Nichrome:

The following results were taken from copper wire:

The following results were taken when constantan wire was used:

Preliminary conclusion

There is clear evidence from the preliminary results that:

1) Copper will not be used because the results are very random and because copper has a low resistance. This means copper is liable to be inaccurate and very imprecise. The smaller the resistance the larger the percentage error which can occur and since copper has a very low resistance, it will have a very large percentage error making the results inaccurate. The other two materials show a greater consistency and show there are patterns and trends. They also have high resistances, which not only are easier to work with but also have a small percentage error and it is easy to observe changes. This is because the other metals have a high resistance. Other metals could not be used because it is not widely available.

2) The length, which is better to use, is the 50cm wire because although the 100cm has higher resistances and therefore less percentage error, the 50cm wire is easier to handle, measure and cut. In other ways the 50cm wire is more accurate, for example during the preliminary experiment the wire, especially the thicker and longer wires could not placed straight on the ruler and so the length could not be measured properly. The longer the wires, the more bent the wires were and the more inaccurate the results would be. This is the reason for using pieces of metal wire, which are 50cm long, in the method.

3) The range, which should be used in the real experiment, is from 24 gauge to 38 gauge in 2 gauge intervals. This means that 24, 26, 28, 30, 32, 34, 36 and 38 gauge will be used. This range of 8 values is going to be used because the range is sufficient to plot a graph, find trends and patterns and those gauges are easy to obtain. It will also be used because those values have high resistances, which means there will be less percentage error and the results will be more accurate. It is important to take as many different measurements as possible so that there can be a good spread of results so that I am able to arrive at a good conclusion.

During the real experiment only the 50cm wire will be used, constantan and Nichrome will be used and data will be collected from the gauges between 24 and 38 inclusive in 2 gauge intervals. This should enable me to get enough data and to test if different lengths and materials work for my conclusion. The method will be accurate as repeats will be taken and the following materials and lengths have been chosen in preliminary work. The method will be reliable because two techniques will be used.

Apparatus

The following apparatus will be used for the first method (ohmmeter):

An ohmmeter, two leads, Constantan wire-24 ,26 ,28 ,30 ,32 ,34, 36, 38 gauge, nichrome wire- 24, 26, 28, 30, 32, 34, 36, 38 gauge, scissors, meter rule, crocodile clips, Masking tape

The following apparatus will be used for the second method (voltage/current):

A Voltmeter, Ammeter, crocodile clips, six leads, Constantan wire- 24, 26, 28, 30, 32, 34, 36, 38 gauge, nichrome wire- 24, 26, 28, 30, 32, 34, 36, 38 gauge, Scissors, meter rule, power pack, rheostat, masking tape.

Method 1

The apparatus was set up as shown in the diagram. A 50cm piece of Nichrome wire was cut, of gauge 24 using a scissors and was placed on a ruler. Masking tape was used to stick the wire down. Crocodile clips were connected to the wire at each end and the resistance was measured and noted down using an Ohmmeter to measure the resistance. The wire was then taken out and another 50cm nichrome wire was cut but this time gauge 26 was used and its resistance was measured. This method was also used for gauge 28, 30, 32, 34, 36, 38. Once the results were taken, Instead of using Nichrome wire, constantan wire were used so when analysing the data, I can be surer that my theory works for all metals, however I still cannot be certain because I have not tested all metals. With constantan, the resistance was measured again for gauges 24, 26, 28, 30, 32, 34, 36, 38. For each reading, three repeats were taken so a mean could be taken and there would be more precision. The resistance for the leads was measured and this value was deducted from all the data to make a fair test.

Method 2

Another method must be used for reliability. The other method involved calculating the resistance by measuring the voltage and current and dividing it to find the resistance. The apparatus was set up as shown in the diagram. A piece of nichrome wire of length 50cm was cut and placed on the ruler. Again, masking tape was used to stick it down. Crocodile clips were clipped on both sides of the wire and a power pack was used to supply the voltage or the potential difference .The voltage was kept constant at one volt (because the wires would not get heated up at low voltages. If the wire get heated up, the resistance gets altered and it would not be a fair test) and the current was measured using an ammeter. From this the resistance could be deduced. This was down for constantan as well. The nichrome was replaced by constantan and the resistance was measured for the same gauges using this voltage/current method. Two volts was then used instead of one so while analysing the results, I could be sure the method works for any number of volts. The resistance was measured for same gauges, for both constantan and nichrome, but this time for two volts. There were three repeats taken for each value. The resistance of the leads were taken and this value was deducted from all the data.