RP04: Determination of Young's Modulus

Question 1

Define young’s modulus

Answer 1

Ratio of tensile stress to tensile strain of a material. (Measure of object’s stiffness)

Question 2

How is stress calculated?

Answer 2

Stress = force/ cross sectional area

Question 3

How is strain calcualted?

Answer 3

Strain = change in length / original length

Question 4

What is the unit of stress?

Answer 4

Pascals or Nm^-2

Question 5

What is the unit of strain?

Answer 5

Unitless since ratio of two lengths

Question 6

What is the unit of young’s modulus?

Answer 6

Pascals of Nm^-

Question 7

How can the cross sectional area of a thin wire be measured?

Answer 7

Wire’s diameter measured in several places along the wire using a micrometer. The average diameter can then be used to calculate a mean cross sectional area.

Question 8

What safety precaution should be taken when stretching thin wires?

Answer 8

Safety goggles should be worn since the wire may snap when under a tensile load and could cause an eye injury.

Question 9

Why should the temperature of the surrounding be kept constant in this experiment?

Answer 9

Metals undergo thermal expansion when there is a temperature increase and this would slightly change the dimensions of the wire.

Question 10

Why should a pre stress be applied to the wire when setting up this experiment?

Answer 10

A pre-stress should be applied so that all bends in the wire are removed and the wire is taught, before measurements are taken.

Question 11

How can the Young’s Modulus be determined from a graph of extension against load?

Answer 11

Gradient of graph is e/F
E=L/(A x gradient)

Question 12

Suggest what has happened if the length of the wire doesn’t return to its original length when unloaded.

Answer 12

If the wire doesn’t return to its original length when unloaded, the load may have exceeded the wire’s elastic limit and consequently the wire has undergone plastic deformation.

Question 13

How can the load applied to on a wire be calculated from the mass added to the end of the wire?

Answer 13

Load = mass x g
F=mg

Question 14

What safety precaution should be taken when using hanging masses?

Answer 14

Never stand with your feet below the hanging masses in case the wire snaps and the masses fall. It is god practice to place a padded bucket below them.

Question 15

Suggest how the extension of the wire may be measured.

Answer 15

A marker, such as a small piece of tape, could be added to the wire. A ruler could then be placed underneath the wire allowing the movement of the marker to be measured.

Question 16

Why is the choice of wire diameter important in obtaining successful results?

Answer 16

If the wire is too thick, the extension will be too small to measure, If the wire is too thin, the wire may begin to deform plastically before a good range of results have been obtained.

Question 17

Suggest why a comparison test wire is used when conducting this experiment using Searle’s apparatus.

Answer 17

A comparison wire is included so that nay changes in the environmental conditions such as a change in temperature are accounted for and won’t skew the results.

Question 18

Why is your choice of wire length important in this experiment?

Answer 18

The extension of the wire depends on the wire’s length since x=FL/AE
This means that the length needs to be sufficiently long enough for the extensions to be easily measurable.

Question 19

Suggest how the wire may be fixed in place when carrying out this experiment.

Answer 19

The wire can be clamped tightly between two blocks of wood at one end. These blocks can then be clamped to the bench.

Question 20

Alongside a meter ruler what other tool will help you measure the extension accurately?

Answer 20

A set square can be used to help read the extension accurately.

Question 21

When measuring the diameter of the wire in multiple places, why should you rotate the wire between measurements?

Answer 21

You should measure the wire in different orientations to ensure that the wire is circular across the full length of the wire.