Using Practical Equipment & Materials (OCR AS Physics)

• Choosing the right equipment and knowing how to use it for a physics experiment is crucial
• A list of common apparatus is shown below:

A selection of apparatus commonly used in physics experiments

• When using measuring instruments such as those listed above, it is important to be aware of what each division on a scale represents
  • This is known as the resolution

• The resolution is the smallest change in the physical quantity being measured that results in a change in the reading given by the measuring instrument
• The smaller the change that can be measured by the instrument, the greater the degree of resolution
• For example, a standard mercury thermometer has a resolution of 1°C whereas a typical digital thermometer will have a resolution of 0.1°C
  • The digital thermometer has a higher resolution than the mercury thermometer

• When planning and implementing a practical investigation, it is crucial to decide which apparatus is the most suitable for the intended purpose

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: