2) Diameter of wire the thinner the wire, the larger the resistance. This is because the thinner the wire is, the fewer paths there are for electrons in the wire therefore there are more collisions with metal ions and the harder it is for current to flow. This results in the energy not being able to spread out as much and more collisions between electrons and metal ions, so the resistance will be higher. If the diameter of a wire is thicker the electrons can move more easily and there are fewer collisions between electrons and metal ions therefore less resistance.
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. If the atoms have a high number of electrons in the outer shells in theory there should be less resistance. If atoms are closely packed collision occurrence will be higher as will the resistance. 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 more than one material. If different materials are used throughout the investigation, it will affect the results.
All these factors must be kept constant to make the investigation fair. The same apparatus must be used throughout the investigation.
Preliminary work
Whilst doing preliminary work I will investigate the following:
- Find out what kind of resistance levels to expect for different lengths.
- Find out what diameter 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 what 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 lengths 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.
Preliminary Method
The aim of preliminary work is to find apparatus, make adjustments and investigate other factors listed above. 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 length that can be obtained will be used as well as one or two in the middle. The two materials copper and nichrome will be tested. The apparatus will be set up as shown in the diagram. The 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 the equation Resistance= voltage / current. Therefore I will use a voltmeter and ammeter to measure current and voltage. I will keep the current constant throughout the experiment using a variable resistor.
Three materials will be tested and constantan, copper and nichrome. The investigation will be carried out by measuring the resistance of different materials and 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 to be investigated will be decided whilst doing the preliminary work. I will find resistances of three different materials in order to decide which material will be used.
Prediction
My hypothesis is that as the length increases the resistance increases. The longer the wire, the larger the resistance. This is because there are more metal ions so there is more chance that the electrons would collide with one of the ions therefore there is more resistance. Electrons have a longer distance to travel so there are more collisions. I predict that if the length doubles the resistance will also double this is because length and resistance are directly proportional.
The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made. Resistivity describes a material's ability to transmit electrical current that is independent of the geometrical factors. The resistance of a wire can be expressed as:
L=length
A=cross section area
In the case of a wire, resistivity is defined as the resistance in the wire, times the cross-sectional area of the wire, divided by the length of the wire. The units of resistivity are Ω meters.
I can predict the resistance of 1m of nichrome using this equation
Resistivity = 103 x 10 Ωm
Length = 1m
Cross section area = 1.11 x 10
R= 103 x 10 X (1/ 0.111 x 10 )
R = 9.28 Ω
Safety
Whilst doing the investigation, it is important to take safety into consideration. 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 recognize 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 results so when the mean is taken, an accurate table is drawn up and if one result is anonymous the other two results would point out the anonymous result.
Apparatus
The following apparatus will be used:
A Voltmeter, Ammeter, crocodile clips, six leads, nichrome wire 1 metre 28 S.W.G, Scissors, meter rule, power pack, rheostat, masking tape.
Method
The apparatus was set up as shown in the diagram. A piece of nichrome wire of length 1m was cut and placed on the ruler the ruler had terminal blocks at either end to connect the nichrome wire to the circuit. A power pack was used to supply the voltage. The current was kept constant at 0.3 amps using a variable resistor. A voltmeter was used to calculate voltage and an ammeter to calculate current from this resistance can be calculated using the equation Resistance= voltage / current. There were three repeats taken for each value and then an average was calculated.
Diagram
Analysis
As the length of the wire increases so does the resistance, this happens because when the length of the wire increased so did the number of metal ions therefore more collisions between electrons and metal ions would have been occurring. I also found that as the length of the wire doubled so did the resistance.
My results support my prediction made in the plan. The resistance I calculated using the equation however was wrong this could be for a number of reasons.
Evaluation
The procedure I used was very efficient and enabled me to collect a wide range of data easily and quickly. When looking at my results and my graph it is apparent there are no anomalies. When plotted on my graph the results form a clear straight line with no outlying results proving that my results are accurate and reliable.
I took three readings of resistance for each length I experimented to ensure I had accurate results, this method worked well and has proven to give me accurate and reliable results. The evidence I have gathered is sufficient to support my conclusion as my evidence is reliable and accurate and I have a wide range of readings.
To provide additional evidence to support my conclusion I could repeat the experiment using constantin wire or manganin wire. This would enable me to experiment whether only nichrome behaves as I have found or whether a wider range of materials do so.
