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    • Thread Starter

    hi guys,
    im doing advanced higher physics and my investigation is on Youngs Modulus. so, i have done the experiment, adding weights and measuring the increase in length. But, the part i am stuck at is this;
    when i was doing the experiment, first i measured the original lenght of the wire then i added 1Kg, i measured the increase, then i removed the weight and measured to see if the wire returned to its original length.
    now i was able to add up to 3Kg, one by one, untill the wire was deformed. so i am stuck in the calculation part, when i am entering the valuse to delta (l). which value do i use, the 1Kg one or the other one. Also, for instance, if i use the the l value for 2kg, then do i use the 2kg to get the force?
    thanks for any help, much appreciated.

    Also, if there is any other way of determining youngs modulus other than hanging wires, could please inform me as i need to do other experiments. I must be able to carry it out in a Physics classroom, so i cant do anything fancy.
    thanks alot.

    It's usual to draw a graph of the extension of the wire (difference between extended length and original length) against the force (the weights of the masses).
    The slope of the graph will give you the information you need to find the Young's Modulus.
    You also need to measure the diameter of the wire in order to find its cross sectional area.
    Use E=stress/strain

    You need about 6 or 7 points on the graph. It sounds li8ke you only have 2 or 3 but I may be misunderstanding your post.
    • Thread Starter

    thanks for your reply.
    I didnt know you have to draw a grapgh to obtain the value of E. Could you please explain how i would do that.
    The first wire i tried, i got up to 3Kg and then when i took it off, the wire did not go back to its original lenght but it wasnt far off. so im not sure if i should consider those values.

    You need to use a number of different weights, say 7 or 8. If the wire can take 2kg then go from 0 to 2kg in steps of 200g. That would give 10 readings and is plenty.
    Measure the extension for each weight, and let the wire go back to its original length each time.
    Don't take any values once the wire stops returning to its original length.
    You need to measure the diameter of the wire at a number of places, take the average, and use this to calculate the cross section area.
    Tabulate your results and draw a graph of stress (y axis) against strain.
    The gradient is stress/strain which equals the Young's Modulus for the wire.
    Stress = force F (weight in Newtons) / cross section area A
    Strain = extension e / original length l
    You can also directly plot force against extension and find the slope. In this case the slope is EA/l (You would need to prove this)
    It's not good lab practice just to take one reading if it's possible (as it is here) to take a number.

    What equipment are you using to do this, by the way?
    • Thread Starter

    Thanks for explaining, i understand now.
    I am using a travelling microscope to measure the change in lenght. I have a wire with a piece of paper attached to it and on the paper there is a line, so i measure the change in position of the line.
    For all of the different wires i have done the experiment, after about 3 weights being added, the wire does not return exactly to its original position but it is very close, do i still consider these values otherwise i will not have meny readings to plot.
    Thanks for all of your help, much apprecited!

    Just make sure you do not stretch the wire beyond its elastic limit.
    Another thing that is worth keeping an eye on is temperature.
    Measure the room temperature during the experiment. The danger is that if this goes up or down too much, the wire will of course expand or contract due to its getting hotter or cooler. This thermal expansion will interfere with your readings.
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