CIE AS Physics (9702) 2019-2021

Revision Notes

2.1.1 Measurement Techniques

Measurement Techniques

  • Common instruments used in Physics are:
    • Metre rules – to measure distance and length
    • Balances – to measure mass
    • Protractors – to measure angles
    • Stopwatches – to measure time
    • Ammeters – to measure current
    • Voltmeters – to measure potential difference
  • More complicated instruments such as the micrometer screw gauge and Vernier calipers can be used to more accurately measure length

 

Measuring Instruments, downloadable AS & A Level Physics revision notes

  • 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

Measuring Instruments Table, downloadable AS & A Level Physics revision notes

Micrometer Screw Gauge

Micrometer Screw Gauge, downloadable AS & A Level Physics revision notes

How to operate a micrometer

 

Vernier Calipers

Vernier Calipers, downloadable AS & A Level Physics revision notes

How to operate vernier calipers

 

The Galvanometer

  • A galvanometer is a type of sensitive ammeter used to detect electric current
  • It is used in a potentiometer to measure e.m.f between two points in a circuit
  • The circuit symbol is recognised by an arrow in a circle:

 

Galvanometer circuit symbol, downloadable AS & A Level Physics revision notes

Galvanometer circuit symbol

  • A galvanometer is made from a coil of wire wrapped around an iron core that rotates inside a magnetic field:

 

Galvanometer, downloadable AS & A Level Physics revision notes

The galvanometer

  • The arrow represents a needle which deflects depending on the amount of current passing through
    • When the arrow is facing directly upwards, there is no current
    • This is called null deflection
  • Ohm’s law tells us that the current through a conductor (wire) is directly proportional to the potential difference through it i.e. no p.d means no current flows through the galvanometer
  • A galvanometer has p.d of zero when the potential on one side equals the potential on the other side
  • This is at the position at which it is connected on the wire (which varies with the sliding contact) gives a p.d equal to the EMF of the cell connected to the galvanometer
  • The cell should be connected such that its potential opposes the potential on the wire i.e. the positive terminal of the power supply faces the positive terminal of the cell:

Galvanometer circuit diagram, downloadable AS & A Level Physics revision notes

 

  • When the sliding contact moves along the potentiometer wire, you add or remove resistance from/to the external circuit. This changes the potential drop across X and Y
  • Location of the sliding point is adjusted until the galvanometer reads zero. This is until the potential difference equals E2
  • The direction of the two e.m.fs oppose each other and there is no current

Worked example

Worked example - Galvanometer (1), downloadable AS & A Level Physics revision notes
Worked example - Galvanometer (2), downloadable AS & A Level Physics revision notes

Exam Tip

If you’re unsure as to whether the p.d will increase as the contact slider is moved along the wire, remember p.d is proportional to the length of the wire (from Ohm’s law and the resistivity equation). The longer the length of a wire, the higher the p.d

How to Use a Cathode-Ray Oscilloscope

  • Cathode-Ray Oscilloscopes (C.R.O) are used to measure voltage, frequency and phase
  • A C.R.O consists of:
    • An electron gun – a device which heats up a cathode to produce a narrow beam of electrons
    • A deflection system – a system which consists of two pairs of parallel plates, these plates deflect the electron beam to create the waveform which is observed on the screen
    • A fluorescent display – this is a screen with a thin coating of a material which produces a fluorescent light when electrons impact its surface

CRO diagram, downloadable AS & A Level Physics revision notes

Diagram of Cathode-Ray Oscilloscope display showing wavelength and time-base setting

C.R.O Controls

  • Brightness
    • The brightness of a C.R.O display is a measure of the numbers of electrons impacting the screen
    • A higher brightness means more electrons are formed into a beam current
  • Focus
    • The focus of the C.R.O display can be altered by controlling the diameter of the electron beam
    • The more focussed the beam, the narrower and faster the stream of electrons, which results in a smaller, sharper dot on the screen
  • Time-base
    • This controls how fast the dot moves across the screen
    • When the time-base is switched off, the dot appears static in the centre
    • When the time-base is turned up very high the dot appears as a horizontal line
    • The control has units of time per cm or time per division, and has a range of 100 ms – 1 μs per cm, or division
  • Gain (sensitivity)
    • This controls the vertical deflection, or amplitude, of the dot
    • It has units of volts per cm or volts per division
    • The peak voltage (V0) is the maximum vertical displacement measured from the time-line
    • The peak-to-peak voltage (VPP) is the vertical displacement between the minimum and maximum values of voltage

Analogue & Digital Displays

  • In physics experiments, you will come across a selection of devices for measuring quantities that can display either analogue or digital displays e.g. ammeter and voltmeters

Analogue displays

  • An analogue display normally involves a pointer which indicates a value depending on its position or angle on the scale

    Analogue display, downloadable AS & A Level Physics revision notes

    Analogue meter

  • The measurements taken are restricted over a range e.g. 0 – 1 A
  • Analogue meters are subject to zero errors
    • Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements
  • They are also subject to parallax error
    • Always read the meter from a position directly perpendicular to the scale
  • A galvanometer is an example of a sensitive analogue meter

 

Digital displays

  • Digital displays show the measured values as digits and are more accurate than analogue displays
  • They’re easy to use because they give a specific value and are capable of displaying more precise values

Digital display, downloadable AS & A Level Physics revision notes

Digital meter

 

  • Digital meters are also subject to zero error
    • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results
  • Most digital meters have an auto-range function, this means it can show very low or very high values depending on the readings
    • This saves time selecting an instrument with the correct range and precision for your experiment

 

Calibration Curves

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

Calibration curve, downloadable AS & A Level Physics revision notes

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

 

  • The accuracy of all measuring devices degrade over time. This is typically caused by normal wear and tear
    • Calibration improves the accuracy of the measuring device

 

Worked example

Worked example - calibration curves, downloadable AS & A Level Physics revision notes

  • The voltmeter has a systematic error as the reading it gives is always greater than the true value
  • If the true value is zero, the voltmeter would give a value greater than zero
  • Therefore, the curve doesn’t pass through the origin (0,0) as this would indicate that the reading is the same as the true value, and not greater – this rules out graph C
  • So, when the true value is zero, the meter would give a reading greater than zero. This is either graph A or B
  • The systematic error varies with voltage
    • So, the amount by which the meter reading is greater than the true value changes
  • Therefore, graph A is correct, because the difference between the meter reading and the true value increases with voltage

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