Measuring Variables in Physics (DP IB Physics): Revision Note

Measuring variables in physics

• You need to know how to accurately measure variables to allow the collection of valid and high-quality data

• Sometimes, you will be required to make a decision as to what piece of equipment to use based on which is the most appropriate for that particular task

Measuring mass

• Mass is typically measured with a digital balance accurate to two decimal places

• Always tare (zero) the balance before weighing

• In physics, mass is usually recorded in kilograms (kg)

Measuring time

• Time is measured using a stopwatch or stop-clock, usually accurate to 1–2 decimal places

• The most common units are seconds and minutes, though hours may be used for very slow reactions (e.g. rusting)

  • 1 minute = 60 seconds

• An important factor when measuring time intervals is human reaction time

  • This can have a significant impact on measurements when the measurements involved are very short (less than a second)

Measuring length

• Rulers can be used to measure small distances of a few centimetres (cm).

  • They are able to measure to the nearest millimetre (mm)

• The standard unit of length is metres (m)

• Larger distances can be measured using a tape measure

• Many distances in physics are on a much smaller scale, therefore a micrometer may be used

Using a ruler to measure length

Measuring volume

• Measuring cylinders are used to measure the volume of liquids

• By measuring the change in volume, a measuring cylinder can also be used to determine the volume of an irregular shape

  • 1 ml of water = 1 cm³ of volume

Measuring temperature

• Temperature is measured using a thermometer or digital temperature probe

• Laboratory thermometers:

  • use the thermal expansion of a liquid (e.g. alcohol or mercury) in a capillary tube

  • commonly give readings to the nearest 1 °C or 0.5 °C

  • are simple, robust, and inexpensive

  • may take longer to equilibrate and are less precise than digital options

  • units are typically recorded in degrees Celsius (°C)

• Digital temperature probes:

  • use electronic sensors (e.g. thermistors or thermocouples) to detect temperature

  • often have a higher precision, reading to ±0.1 °C or even more accurately

  • provide fast, real-time readings

  • often used in data logging and continuous monitoring

  • can reduce human error and are ideal for remote or automated experiments

Measuring force

• Forces can be measured directly using a force meter, or spring balance

Measuring current

• Current is measured using an ammeter

• Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

  • They measure the amount of charge per unit time flowing through it

Digital or analogue?

• Ammeters can be either

  • analogue: scale and needle display

  • digital: electronic numerical readout

• Typical ranges for analogue ammeters are 0.1 - 1.0 A and 1.0 - 5.0 A

• Ammeters should be checked for zero errors before using

  • 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

• Analogue ammeters are also subject to parallax error 

  • Always read the meter from a position directly perpendicular to the scale

An analogue ammeter

• Digital ammeters can measure very small currents, in mA or µA

• 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

• However, digital displays may 'flicker' back and forth between values, and a judgment must be made as to which to write down

A digital ammeter

Measuring potential difference

• Potential difference (voltage) is measured using a voltmeter

• Voltmeters are always connected in parallel with the component being tested

  • They measure the difference in electrical potential between two points in a circuit

Analogue or digital?

• Voltmeters can be either

  • analogue: scale and needle display

  • digital: electronic numerical readout

• The common ranges are from 0.1–1.0 V or 0–5.0 V, though this may vary

• Voltmeters should be checked for zero errors before use

  • 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

• Analogue voltmeters are subject to parallax errors 

  • Always read the meter from a position directly perpendicular to the scale

An analogue and digital voltmeter

• Digital voltmeters can measure very small potential differences, in mV or µV

• 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

• However, digital displays may 'flicker' back and forth between values, and a judgment must be made as to which to write down

Measuring angle

• Angles should be measured with a protractor

• Align baseline carefully; read at eye level

• Precision: ±0.5° for protractor

• For repeated measurements, use a consistent reference point

  • For example, if measuring the angle of refracted light, measure from a drawn normal line to the centre of the light ray

Measuring sound intensity

• Sound intensity is measured with a sound level meter

• Precision: ±0.1 dB typically

• Hold at a fixed distance

  • Intensity decreases with distance (inverse-square law)

• Avoid background noise

Measuring light intensity

• Light intensity is measured with a light meter, photodiode, or lux meter.

• Precision: ±1 W m^-2 (depending on device)

• Keep the detector perpendicular to the source

• Shield from stray light

  • Results affected by ambient light; use a dark room if possible