Materials Flashcards
What is stress?
The force per unit area applied to a material. F/a
What is strain?
The change in length over the original length.
State the steps in the method for an experiment to determine the Young Modulus of a wire (the one we didn’t do).
Hang two wires from a beam attached to the ceiling. Hang the main scale from the comparison wire and the vernier scale from the test wire. Measure the initial length of the test wire. Hang a 1kg mass from both wires to ensure they are taught. Take the initial scale reading using the vernier scale to read to 0.1mm. Add an additional 1kg mass to the test wire and take the new scale reading using the vernier and calculate the extension by subtracting the two scale readings. Repeat the process up to 8kg. Remove 1kg and take a scale reading and repeat until there is 1kg left. If the extension from unloading is greater then loading then the elastic limit may have been exceeded. If the extension values are similar, calculate a mean extension. Measure the diameter of the wire using a micrometer in several places. Plot a graph of extension in metres against load in Newtons. The Young Modulus of the wire is the gradient of the graph. Estimate the uncertainty in l, A, e and m to estimate the overall uncertainty in the value obtained for Youngs Modulus.
State two safety considerations in the experiment to determine the Young Modulus of a wire.
Goggles should be worn and a sand tray should be placed underneath the masses in case the wire breaks.
In the Young Modulus experiment, how should the wire be physically before starting the experiment?
Free from kinks.
Why is it important for the wire to be free from kinks in the Young Modulus experiment?
So the diameter can be measured more accurately and so that the extension is more accurate; if there are kinks then any measured extension will partly be due to the straightening out of kinks.
In the Young Modulus experiment, what is the function of the comparison wire?
To compensate for sagging of the beam and thermal expansion effects and to provide a reference point against which to measure the extension of the loaded test wire.
When stretching the wire horizontally in the Young Modulus experiment, how is the extension of the wire measured?
Using a high precision ruler underneath and a marker attached to the wire. The marker can be Tippex or a small dressmaker pin if a proper one isn’t available.
When stretching the wire horizontally in the Young Modulus experiment, how is the wire fixed in place?
One end is placed between two wooden blocks and clamped to the end of the table and the other end is fed through a pulley with the masses hanging from it.
Why must the temperature be kept constant during the young modulus experiment?
To avoid thermal expansion of the wire changing the dimensions.
What has a wire undergone if it exceeds its elastic limit?
Plastic deformation.
Other than subtracting the original length from the extended length, how can you measure the extension of a wire?
Using a marker on the wire such as tape and placing a ruler underneath it so that the marker moves when the wire extends.
What is the comparison wire for when using Searle’s apparatus?
So that any changes in environmental conditions are accounted for, such as changes in temperature, and won’t skew the results obtained.
Why should the wire be rotated between readings of the diameter of the wire?
To ensure that the wire’s cross-section is circular.
