Required Practical 4: Determination of Young's Modulus

Give a brief summary of this practical:

• Aiming to find the Young’s modulus of a wire

• .. by loading the test wire with mass at set amounts

• & reading the measurement off the vernier scale (or ruler)

• ..to then find the change in length

• This experiment can then be repeated again except with unloading the wire instead (removing the masses at set amounts)

What are two main methods of this type of experiment?

• Using a ruler to measure & thus the change in length or..

• .. a vernier scale

What do you need to do before the experiment is conducted?

• Measure the initial length of the test wire, l, with a metre ruler

• … ensure both wires are initially taught.

How can you ensure that the comparison wire & test wire (for the vernier scale apparatus) are taught?

• Attach an initial (same) amount of mass on both wires e.g. 1kg.

How do you calculate the change in extension?

• Subtracting the current vernier scale reading

• .. from the initial one (when no mass was applied)

• (vice versa with ruler method).

When repeating the experiment when unloading the masses, what does it mean if the extension values for unloading are greater than the loading ones?

• The elastic limit for the wire might have been exceeded.

What is one type of graphical analysis that you can do for this experiment?

• Plot a stress against strain graph

• .. where the gradient = Young’s modulus.

What is an advantage of plotting a stress strain graph for this experiment?

• You can plot up to the limit of proportionality

• .. & finding additional values such as the elastic limit.

What is another type of graphical analysis for this experiment?

• Stress/ strain = E

• .. which also means: F/a / change in length/ l

• .. this also means fl/ a X change in length = E

• .. when rearranging this equation & making F the y value & change in length the x value we get:

• F = (EA/length) change in length

• .. so the gradient of a force against change in length graph will be EA/length..

• which can rearrange to find the Young’s modulus of the material i.e. Gradient x length/ cross sectional area

How can you reduce the percentage uncertainty of this experiment?

• Measure the diameter of the wire

• .. 3 X

• .. & find an average