Edexcel A Level Physics:复习笔记1.2 Practical Skills

Using Equipment

• Good practical skills involves choosing the right equipment
• Knowing how to use it for a physics experiment is crucial
• When using measuring instruments, it is important to be aware of what each division on a scale represents
  • This is known as the resolution
• A list of common apparatus is shown below:

A selection of apparatus commonly used in physics experiments

Types of Display

• Scientific instruments can be digital or analogue

Analogue

• Analogue scientific instruments transfer information through electric pulses of varying amplitude
  • This means they cannot be read easily by a computer

   

• Analogue instruments are cheaper but they have lower accuracy and resolution
• They are also more sensitive, which can make it difficult to read fluctuating values
  • 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 - 10 A and a resolution of 1 A
• Analogue meters are subject to zero errors
  • This means the marker must be double-checked before each reading. If it is not at zero, then the value but be subtracted from all the measurements

   

• They are also subject to parallax error
  • Always read the meter from a position directly perpendicular to the scale

   

• A potentiometer is an example of a sensitive analogue meter

Digital

• Digital scientific instruments translate information into binary (0 or 1) format which can then be read and analysed by a computer
• They are more expensive but have greater accuracy and resolution than analogue instruments
• Digital displays show the measured values as digits
• 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 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

   

• A digital multi-meter is an example of a digital meter

Reading Distances

• Reading distances using calipers or micrometers will be expected for the practical examination

Micrometer Screw Gauge

• A micrometer, or a micrometer screw gauge, is a tool used for measuring small widths, thicknesses or diameters
  • For example, the diameter of a copper wire

   

• It has a resolution of 0.01 mm
• The micrometer is made up of two scales:
  • The main scale - this is on the sleeve (sometimes called the barrel)
  • The thimble scale - this is a rotating scale on the thimble

   

Components of a micrometer

• By rotating the rachet, the spindle and anvil are clamped around the object being measured
  • This should be tight enough so the object does not fall out but not so tight that is deformed
  • Never tighten the spindle using the barrel, only using the ratchet. This will reduce the chances of overtightening and zero errors

   

• The value measured from the micrometer is read where the thimble scale aligns with the main scale
  • This should always be recorded to 2 decimal places (eg. 1.40 mm not just 1.4 mm)

   

The micrometer reading is read when the thimble scale aligns with the main scale

Vernier Calipers

• Vernier calipers are another distance measuring tool that uses a sliding vernier scale
  • They can also be used to measure diameters and thicknesses, just like the micrometer
  • However, they can also measure the length of small objects such as a screw or the depth of a hole

   

• Vernier calipers generally have a resolution of 0.1 mm, however, some are as small as 0.02 mm - 0.05 mm
• The calipers are made up of two scales:
  • The main scale
  • The vernier scale

   

• The two upper or lower jaws are clamped around the object
  • The sliding vernier scale will follow this and can be held in place using the locking screw

   

Components of a vernier caliper

• The value read from the caliper when the vernier scale aligns with the main scale
  • This should always be recorded to at least 1 decimal place (eg. 12.1 mm not just 12 mm)

   

The vernier caliper reading is read when the vernier scale aligns with the main scale

• In general, the micrometer has a smaller measuring range than a vernier caliper
• However, the micrometer has a better accuracy (due to better resolution)
• The vernier caliper is quicker to use, whilst the micrometer involves rotating the thimble
  • Therefore, to take many measurements, a caliper would be easier to use

   

Signal Generators & Oscilloscopes

• Signal generators and oscilloscopes are commonly used in practicals to visualise waves

Signal Generator

•  A signal generator is an electronic test instrument used to create repeating or non-repeating waveforms
  • They can be adjusted for different shapes and amplitudes

   

• These are often used for designing and repairing electronic devices, to check they are working as expected
• Signal generators are used to create signals to then show on oscilloscopes

A signal generator can be used to create signals for a CRO

Cathode-Ray Oscilloscope

• A Cathode-Ray Oscilloscope (CRO) is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits
  • It can therefore be used as an a.c and d.c voltmeter

   

A cathode-ray oscilloscope displays the signal generated by the signal generator

• An a.c voltage on an oscilloscope is represented as a transverse wave
  • Therefore you can determine its frequency, time period and peak voltage

   

• A d.c voltage on an oscilloscope is represented as a horizontal line at the relevant voltage
• The x-axis is the time and the y-axis is the voltage (or y-gain)

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

• The period of the wave can be determined from the time-base
  • This is how many seconds each division represents measured commonly in s div-1 or s cm-

   

C.R.O Controls for an A.C waveform

• Time-base
  • When the time-base is switched off, only a vertical line on the voltage-gain axis is seen with its relevant amplitude
  • When the time-base is switched on, a wave will appear across the whole screen and the time period can be measured
  • This control has units of time cm-1 or time div-1 and has a range of 100 ms – 1 μs per cm, or division

   

• Voltage-gain (sensitivity)
  • This controls the vertical deflection, or amplitude, of the wave
  • The peak voltage (V0) is the maximum vertical displacement measured from the time axis
  • The peak-to-peak voltage is the vertical displacement between the minimum and maximum values of voltage
  • When the voltage-gain is switched off, only a horizontal line on the time axis will be seen
  • This control has units of volts cm-1 or volts div-1

   

C.R.O Controls for a D.C waveform

• For a d.c waveform, only a horizontal line is displayed at the relevant voltage
  • The time-base settings are irrelevant since there is no time period
  • The voltage-gain setting is relevant since this is used to read the value of the d.c voltage

   

Examples of an alternating and direct voltage on a CRO with and without the time base

Timing

• A stopwatch or light gates are common physics instruments used for measuring time
  • For example, the time taken for a ball to fall a certain distance

  

  A stopwatch is used to measure the time interval between the clap and when the sound is heard

   

  • The disadvantage of keeping time manually using a stopwatch is there will be a large error in the reading
  • This is caused by:
    • Human reaction time (on average, about 0.25 s)
    • The mechanism of the stopwatch (older stopwatches may have a slight delay)
    • Accidentally pressing the start or stop button too many times
    • Consistently starting the stopwatch too late or too early

     

  • Therefore, repeat readings are very important for experiments that require timekeeping

   

Light Gate

• A light gate is a digital switch-type sensor also used in time experiments
  • They consist of an infrared transmitter and receiver between the 'gate'
  • When this signal is obstructed by an object, a timer can either be started or stopped depending on its configuration

   

• If the distance between two light gates is known, the time interval between an object passes through both gates can then be used to measure its speed using the equation:

speed = distance ÷ time

• This is assuming the object is not accelerating

The first light gate starts a timer, and the second stops the timer when the flag passes between them. This is used to determine the speed of the object

• A light gate is much more accurate than a stopwatch, as it removes the errors caused by human reaction time
• They can also be connected to a digital timer or datalogger, which then output the time in which the signals are obstructed for data analysis

Computer Modelling & Data Loggers

• Computing modelling and data loggers are essential in all scientific experiments for obtaining and analysing reliable results

Data Loggers

• Data loggers are a tool that allows for the quick and efficient gathering of data
  • The information contained within a data logger can be inputted into a computer and formatted into a table
  • After this is done the computer is able to calculate the average and plot graphs using the data and calculate gradients, quicker and more accurately than humans

   

• They are electronic devices that automatically monitor and record environmental parameters over time such as temperature, pressure, voltage or current
  • It contains multiple sensors to receive the information and a computer chip to store it

   

A data logger measuring and displaying temperature using a probe

• The benefits of using data loggers and ICT (information and communication technology) include:
  • Readings are taken with higher degrees of accuracy
  • Reduction of human error (eg. human reaction times, subjectiveness)
  • Readings can be taken over a long period of time eg. hourly readings of temperature over many days
  • Readings can be taken in a very short period of time, which would be too quick for humans to see a difference
  • Reduction in safety risks with extreme conditions such as measuring the temperature of boiling water

   

Computer Modelling

Computer modelling uses a computer and sensors to analyse and display data