Chapter 1 Physical Quantities Units and Measurement 1

Chapter 1 Physical Quantities, Units and Measurement 1. 1 Physical Quantities • A physical quantity is one that can be measured and consists of a magnitude and unit. Measuring length 70 km/h SI units are common today THEME ONE: 4. 5 m Vehicles Not Exceeding 1500 kg In Unladen Weight MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 1 Physical Quantities Are classified into two types: • • Base quantities Derived quantities Base quantity is like the brick – the basic building block of a house THEME ONE: MEASUREMENT Derived quantity is like the house that was build up from a collection of bricks (basic quantity)

Chapter 1 Physical Quantities, Units and Measurement 1. 2 SI Units • SI Units – International System of Units Base Quantities Name of Unit Symbol of Unit length metre m mass kilogram kg time second s electric current ampere A temperature kelvin K amount of substance mol luminous intensity candela cd THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 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. THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 4 Scalars and Vectors • Scalar quantities are quantities that have magnitude only. Two examples are shown below: Measuring Mass THEME ONE: MEASUREMENT Measuring Temperature

Chapter 1 Physical Quantities, Units and Measurement 1. 5 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time Length • Measuring tape is used to measure relatively long lengths • For shorter length, a metre rule or a shorter rule will be more accurate THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time • 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time • 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time Vernier Calipers • Allows measurements up to 0. 01 cm • Consists of a 9 mm long scale divided into 10 divisions THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time 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. THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 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. THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and 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. THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time Caesium atomic clock • 1999 - NIST-F 1 begins operation with an uncertainty of 1. 7 × 10− 15, or accuracy to about one second in 20 million years THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and 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. THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time 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. THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time 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 THEME ONE: MEASUREMENT

Chapter 1 Physical Quantities, Units and Measurement 1. 5 Measurement of Length and Time 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. THEME ONE: MEASUREMENT