Measurement Techniques in IB Physics (DP IB Physics)

Measurement Techniques

• Common instruments used in Physics are:

  • Metre rules - to measure distance and length

  • Thermometers - to measure temperature

  • Measuring cylinders - to measure to volume of liquid or the volume of displaced liquid

  • Balances - to measure mass

  • Newtonmeters - to measure force

  • Protractors - to measure angles

  • Stopwatches - to measure time

  • Ammeters - to measure current

  • Voltmeters - to measure potential difference

  • Sound meter - to measure the intensity of sound

  • Light meter - to measure the intensity of light

• More complicated instruments such as the micrometer screw gauge and Vernier calipers can be used to measure thicknesses, diameters and lengths to a greater degree of accuracy

• When using measuring instruments like these you need to ensure that you are fully 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

Measuring Instruments Table

Controlling Variables

• For an experiment to be valid, it is essential that any variable that may affect the outcome of an experiment is controlled 

• Some of the key practical skills that are required to do so are as follows:

  • Calibration of measuring apparatus

  • Keeping certain environmental conditions constant

  • Insulation against heat loss or gain

  • Reduction of friction

  • Reduction of electrical resistance

  • Taking background radiation into account

Calibration of Measuring Apparatus

• Calibration is a comparison between a known measurement and the measurement you achieve using the instrument

• This checks the accuracy of the instrument, especially for higher readings

  • For example, checking whether a meter (e.g., voltmeter, micrometer, ammeter) reads zero before measurements are made

  • This helps to avoid zero error

Calibrating sensors

• Calibration curves are used to convert measurements made on one measurement scale to another measurement scale

• These are useful in experiments when the instruments used have outputs which are not proportional to the value they are measuring

  • For example, e.m.f and temperature (thermocouple) or resistance against temperature (thermistor)

• The calibration curve for a thermocouple, in which the e.m.f varies with temperature, is shown below:

A curve of voltage against temperature can be used as a temperature sensor

Maintaining Constant Conditions

• In an experiment, a variable is any factor that could change or be changed

• There are different types of variables within an experiment

  • The independent variable: the only variable that should be changed throughout an experiment

  • The dependent variable: the variable that is measured to determine the outcome of an experiment (the results)

  • The controlled variables: any other variables that may affect the results of the experiment that need to be controlled or monitored

• It is essential that any variable that may affect the outcome of an experiment is controlled in order for the results to be valid and to have a fair test

  • A fair test is one in which only the independent variable has been allowed to affect the dependent variable

Controlling Heat Losses & Gains

• Energy transfers by heating due to conduction are one of the most common sources of dissipated energy

• To reduce energy transfers by conduction, materials with a low thermal conductivity should be used

  • Materials with low thermal conduction are called insulators

• Insulation reduces energy transfers from both conduction and convection

• The effectiveness of an insulator is dependent upon:

1. The thermal conductivity of the material

• The lower the conductivity, the lower the amount of heat loss

2. The density of the material

• The more dense the insulator, the more conduction can occur

• In a denser material, the particles are closer together so they can transfer energy to one another more easily

3. The thickness of the material

• The thicker the material, the lower the amount of heat loss

Less energy is transferred by conduction and convection if the cavity is insulated

Reducing Friction

• In a mechanical system, there is often friction between the moving parts of the machinery

• This results in unwanted energy transfers by heating the machinery and the surroundings

• Friction can be reduced by:

  • Adding bearings to prevent components from directly rubbing together

  • Lubricating parts

Lubricating parts of a bicycle to reduce friction

Reducing Electrical Resistance

• In electric circuits, there is resistance as current flows through the wires and components

• This results in unwanted energy transfers by heating to the wires, components and the surroundings

• Resistance can be reduced by:

  • Using components with lower resistance

  • Reducing the current 

Adjusting for Background Radiation

• Although most background radiation is natural, a small amount of it comes from artificial sources, such as medical procedures (including X-rays)

• Levels of background radiation can vary significantly from place to place

• When conducting experiments to measure the radiation coming from radioactive sources, the background radiation must be taken into account, to do this:

  • Place a Geiger-Muller tube away from any radioactive sources and measure the background count

  • Carry out the experiment with the radioactive source

  • Subtract the background count from each reading to obtain the count rate from the source only