Using Practical Equipment & Materials (OCR A Level Physics)

The diagram below shows one possible method for determining the Young modulus of a metal in the form of a wire.

Describe how you can use this apparatus to determine the Young modulus of the metal. The sections below should be helpful when writing your answers.

• The measurements to be taken.

• The equipment used to take the measurements.

• How you would determine Young modulus from your measurements.

Answer:

Step 1: State the necessary measurements to be taken

• The diameter of the wire

• The initial length of the wire

• The extension of the wire (final length – initial length)

• The mass of the hanging masses or the weight applied to the wire

Step 2: State and explain which equipment would be the most suitable

For measuring the diameter of the wire:

• A micrometer screw gauge or vernier callipers

• Micrometer would be best as this has the highest resolution when measuring small areas

For measuring the original length / extension of the wire:

• A metre ruler or a tape measure

• The wire has a moderate length which cannot be measured using a vernier scale

For measuring extension:

• Travelling microscope

• These are designed for measuring small changes in length

For measuring the mass:

• Scales or a top-pan balance

• A top-pan balance would be best as this has the higher resolution

• W = mg equation can be used to calculate weight

For measuring the weight directly:

• A newton-meter

• This is useful if ‘known’ weights are used and to check if the quoted masses are accurate

Step 3: Explain how Young's modulus can be determined from these measurements

• Young modulus is equal to the gradient of a stress-strain graph (in the linear region)

• Stress is equal to:

• Strain is equal to:

Where:

• F = weight (N)

• A = cross-sectional area of wire (m²)

• ΔL = extension (m)

• L = original length (m)

• Young's modulus for this metal is then calculated using the following equation: