Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r Physical Quantities A physical quantity is one that can be measured and consists of a magnitude and unit. SI units are common today Measuring length 70 km/h  4.5 m  Vehicles Not Exceeding 1500 kg In Unladen Weight

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Are classified into two types: Base quantities Derived quantities 1.1 Physical Quantities Base quantity is like the brick – the basic building block of a house Derived quantity is like the house that was build up from a collection of bricks (basic quantity)

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r SI Units SI Units – International System of Units Base QuantitiesName of UnitSymbol of Unit lengthmetrem masskilogramkg timeseconds electric currentampereA temperaturekelvinK amount of substancemolemol luminous intensitycandelacd

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 1.A physical quantity is a quantity that can be measured and consists of a numerical magnitude and a unit. 2.The physical quantities can be classified into base quantities and derived quantities. 3.There are seven base quantities: length, mass, time, current, temperature, amount of substance and luminous intensity. 4.The SI units for length, mass and time are metre, kilogram and second respectively.

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r Scalars and Vectors Scalar quantities are quantities that have magnitude only. Two examples are shown below: Measuring Mass Measuring Temperature

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r Measurement of Length and Time Accurate Measurement No measurement is perfectly accurate Some error is inevitable even with high precision instruments Two main types of errors – Random errors – Systematic errors

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r Measurement of Length and Time Accurate Measurement Random errors occur in all measurements. Arise when observers estimate the last figure of an instrument reading Also contributed by background noise or mechanical vibrations in the laboratory. Called random errors because they are unpredictable Minimise such errors by averaging a large number of readings Freak results discarded before averaging

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Accurate Measurement Systematic errors are not random but constant Cause an experimenter to consistently underestimate or overestimate a reading They Due to the equipment being used – e.g. a ruler with zero error may be due to environmental factors – e.g. weather conditions on a particular day Cannot be reduced by averaging, but they can be eliminated if the sources of the errors are known 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Length Measuring tape is used to measure relatively long lengths For shorter length, a metre rule or a shorter rule will be more accurate 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Correct way to read the scale on a ruler Position eye perpendicularly at the mark on the scale to avoids parallax errors Another reason for error: object not align or arranged parallel to the scale 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Many instruments do not read exactly zero when nothing is being measured Happen because they are out of adjustment or some minor fault in the instrument Add or subtract the zero error from the reading shown on the scale to obtain accurate readings Vernier calipers or micrometer screw gauge give more accurate measurements 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Vernier Calipers Allows measurements up to 0.01 cm Consists of a 9 mm long scale divided into 10 divisions 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Vernier Calipers The object being measured is between 2.4 cm and 2.5 cm long. The second decimal number is the marking on the vernier scale which coincides with a marking on the main scale. 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r Measurement of Length and Time Here the eighth marking on the vernier scale coincides with the marking at C on the main scale Therefore the distance AB is 0.08 cm, i.e. the length of the object is 2.48 cm

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r Measurement of Length and Time The reading shown is 3.15 cm. The instrument also has inside jaws for measuring internal diameters of tubes and containers. The rod at the end is used to measure depth of containers.

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Micrometer Screw Gauge To measure diameter of fine wires, thickness of paper and small lengths, a micrometer screw gauge is used The micrometer has two scales: Main scale on the sleeve Circular scale on the thimble There are 50 divisions on the thimble One complete turn of the thimble moves the spindle by 0.50 mm 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Micrometer Screw Gauge Two scales: main scale and circular scale One complete turn moves the spindle by 0.50 mm. Each division on the circular scale = 0.01 mm 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Precautions when using a micrometer 1. Never tighten thimble too much – Modern micrometers have a ratchet to avoid this 2. Clean the ends of the anvil and spindle before making a measurement – Any dirt on either of surfaces could affect the reading 3. Check for zero error by closing the micrometer when there is nothing between the anvil and spindle – The reading should be zero, but it is common to find a small zero error –Correct zero error by adjusting the final measurement 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r Measurement of Length and Time Time Measured in years, months, days, hours, minutes and seconds SI unit for time is the second (s). Clocks use a process which depends on a regularly repeating motion termed oscillations.

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Caesium atomic clock NIST-F1 begins operation with an uncertainty of 1.7 × 10 −15, or accuracy to about one second in 20 million years 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Time The oscillation of a simple pendulum is an example of a regularly repeating motion. The time for 1 complete oscillation is referred to as the period of the oscillation. 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Pendulum Clock Measures long intervals of time Hours, minutes and seconds Mass at the end of the chain attached to the clock is allowed to fall Gravitational potential energy from descending mass is used to keep the pendulum swinging In clocks that are wound up, this energy is stored in coiled springs as elastic potential energy. 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Watch also used to measure long intervals of time most depend on the vibration of quartz crystals to keep accurate time energy from a battery keeps quartz crystals vibrating some watches also make use of coiled springs to supply the needed energy 1.5 Measurement of Length and Time

Physical Quantities, Units and Measurement T H E M E O N E : M E A S U R E M E N T C h a p t e r 1 Stopwatch Measure short intervals of time Two types: digital stopwatch, analogue stopwatch Digital stopwatch more accurate as it can measure time in intervals of 0.01 seconds. Analogue stopwatch measures time in intervals of 0.1 seconds. 1.5 Measurement of Length and Time