2.1.1 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

• 当使用你不像这样的测量仪器ed to ensure that you are fully aware of what each division on a scale represents
  • This is known as theresolution

• 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

Micrometer Screw Gauge

How to operate a micrometer

Vernier Calipers

How to operate vernier calipers

• 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

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

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 callednulldeflection

• 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 currentflows 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 theposition 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 potentialopposes钢丝上的潜在即积极的时间al of the power supply faces the positive terminal of the cell:

• 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

ANSWER: D

• Cathode-Ray Oscilloscopes (C.R.O) are used to measure voltage, frequency and phase
• A C.R.O consists of:
  • Anelectron gun- a device which heats up a cathode to produce a narrow beam of electrons
  • Adeflection 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
  • Afluorescent display- this is a screen with a thin coating of a material which produces a fluorescent light when electrons impact its surface

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
  • Thepeak voltage(V₀) is the maximum vertical displacement measured from the time-line
  • Thepeak-to-peak voltage(V_PP) is the vertical displacement between the minimum and maximum values of voltage

• 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 meter

• The measurements taken on this analogue ammeter are restricted over a range e.g. 0 - 1 A and a resolution of 1 A
• Analogue meters are subject tozero 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 toparallax 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 meter

• The measurements taken on this digital ammeter have a much wider range and a resolution of 0.01 A
• Digital meters are also subject tozero error
  • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results

• 大多数数字米有auto-range函数,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 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:

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

ANSWER:A

• 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 graphC
• So, when the true value is zero, the meter would give a reading greater than zero. This is either graphAorB
• The systematic error varies with voltage
  • So, the amount by which the meter reading is greater than the true value changes

• Therefore, graphAis correct, because the difference between the meter reading and the true value increases with voltage