Required Practical 4

Equipment

• 2 1.5m long steel wires
• Main scale and vernier scale
• 1kg masses and 2 1kg holders
• Micrometer
• Metre ruler

Method

• Set up the apparatus as shown in the diagram.
• Measure the initial length l of the test wire with the metre ruler.
• Add a 1kg mass holder to both wires so they are taut and record the initial scale reading.
• Add an additional 1kg mass to the test wire and record the new scale reading. Find its extension e by subtracting the initial scale reading from this and record it.
• Add another 1kg mass and repeat this, adding 1kg each time up to around 8kg.
• Repeat the experiment twice more and find and record the mean e for each m, where m is the mass of the 1kg masses on the test wire’s holder.
• Measure the diameter d of the test wire at various points along it using the micrometer and find and record the mean diameter.
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Graphs and calculations

• Calculate the cross-sectional area A of the wire by A = 4 πd/ 2
• Find the force F on the test wire for each m by calculating mg and tabulate this.
• Plot a graph of F against e and draw a line of best fit. The young modulus E will be l multiplied by the gradient divided by A.
• E = stress/strain = lG/A where G is the gradient.

Safety

• The wire will be stretched very tightly and could break and injure eyes, so safety goggles must be worn.
• If the wire breaks, the masses could fall and cause injuries, so a sand tray should be placed beneath them to catch them.

Improvements and notes

• The comparison wire compensates for sagging of the beam and thermal expansion effects and provides a reference point against which to measure the extension.
• The original length l of the test wire should be as long as possible to reduce uncertainty in its measurement.