Onsequently the dislocation is pinned - Sample Essay
In this physics coursework, I have been asked to investigate young’s modulus behind Constantan and Copper. I will produce the stress strain graphs showing the young’s modulus for both the metals. I will after when constructing the graphs, I will discuss the physics behind my results and I will compare the young’s modulus for both materials. Aim: To draw the stress-strain graphs for a metal and an alloy. To work out the figures for young’s modulus for both metals. To discuss the physics behind my results. Plan:
For this particular investigation, I have been issued with the results and have been asked to analyse the results finding the young’s modulus for two materials, which are: Copper and Constantan. From the results given, I will construct the necessary graphs, analyse, and compare the materials so I can successfully compare the young’s modulus of the given materials. I have been provided with the following information: the force applied to the material, three results for the extension, which will help me gain accuracy when averaging this out.
I will produce a graph showing the stress-strain from the data, which has been provided. This will present me the young’s modulus of the metal (alloy), and subsequently I will create a second graph, which will show Force-extension, and I will average out the extension. In this graph, I will present the metal and the alloy on the same graph paper so that I can find any patterns, which may arise. I will hand draw my graphs to great accuracy; therefore the results and judgment I make will be based on how accurate my graphs are drawn out.
I will be using Microsoft Excel to plot all of the data I have been issued with; I will use this program to the best of its ability, as I will be using formulae to work out average of extension along with conversion for the units and the young’s modulus of the materials. By using this program, I will be able to achieve very precise results. Method: I will firstly collect the various equipment I will need which includes: A G clamp, wooden blocks, pulley, mass hangers with slotted weights, copper and constantan wire samples and a ruler. I will measure using a micrometer, the diameter of the wires and the length with a meter ruler.
I will measure the length of the wires in metres and measure off very accurately to one decimal place. I will work out the cross sectional area of the wire by simply halving the diameter and then working out the area of a complete circle. This will give me the cross sectional area figure. Subsequently I will set up the equipment and I will use cello tape, stick down the meter stick so that the extension can be shown using a marker pen. I will then after setting up my apparatus add the load every 10N (Newton’s), until the wire completely breaks.
I will measure the extension from the initial point plotted on the meter ruler to the new extension. I will then write this down and record this in the form of a table. Here is a diagram of the equipment I used, showing the set up of the experiment conducted: (This picture is taken from Advancing Physics CD) This diagram shows the G clamp, which holds the bloc in place, which exerts pressure onto the wire and keeps it in place. The cardboard bridges ply an important role, making sure that when the wire breaks it does not cause any harm to the people.
The pulley, which I had used, lets the wire slot into place and lets the wire freely move allowing no added pressure onto the wire, so the wire only receives pressure form the weights which are added on. Before attaching any weights, the first thing I would do is beside the metre stick, I would mark off using the marker the initial point so from when I add the weights I can see the increase in the extensions and finally when it completely breaks I will mark beside the metre ruler and thus work out young’s modulus.
I will be using the following formulae’s: Area: pi r^2 Stress formula: Stress (Nm-2) Strain: Prediction: I predict from general knowledge that constantan will be able to hold more weights then the copper wire as I believe that the constantan wire will have a higher young’s modulus simply because of the fact that constantan is an alloy and copper is a metal. Constantan is made up of 60% of copper and 40% of nickel. Alloys alone are very strong since constantan is the oldest, and still the most widely used alloy.
The addition of nickel to an alloy makes increases the alloys strength without diminishing its excellent ductility, toughness and corrosion resistance. Nickel increases the ductility of steel while allowing it to maintain its strength. When large quantities of nickel are added (25-35%), the steels not only become tough but also develop high resistance to corrosion and shock. There is a large percentage of Nickel, which is contained within constantan and therefore the high strength property it contains lies in its ratio of nickel it has.
An alloy is very different to a metal due to the properties and the difference lies in their atomic arrangement. The property of the other elements is what makes an alloy stronger then a pure metal. Necking does not occur in metals, it is quite hard for it to occur, this is because there is no dislocation of atoms, which occur within the structure to make this, stronger, and hold its atoms together. If this was the case and had occurred then the metals arrangement would be stronger due to the dislocation.
In an alloy, once a dislocation occurs, it is hard to cause atoms to slip as it is a fixed position, therefore other atoms would not replace a lattice of molecules with dislocation in a row. Consequently the dislocation is pinned, the below diagram shows the pure metal which refers to copper in this case, and the alloy which is constantan. We can notice the difference between the structures of the atoms in both materials. The above diagram shows the difference between the pure crystal and the alloy which in this case refers to copper and constantan.
You can observe that the atoms are free to move in the pure crystal, however in the alloy it is harder for the atoms to move past each other and this is due to the pin which strengthens the structure so that there is no dislocation. Relating this to this experiment with the material copper and constantan, we can see that constantan will be able to handle more load as it is a tougher wire. It is evident that it can take on more stress from the weights, and it would require more loads to shatter this wire then the copper wire.
The atoms in copper will just move easily and therefore will stretch apart causing the gap and slowly it will break as the bond will be pulled by the great load of weights which pulls the copper wire apart. The copper is therefore less ductile than the constantan material (alloy). Copper would have a small elastic limit meaning that it is able to be pulled apart more easily, and it would take fewer amounts of weights before it can with stand anymore and consequently shatter. Copper will therefore result in the plastic formation once it breaks as it will not return to its original shape.