• O Level Physical Quantities

• Physical Quantities
• Base Quantities
• Prefixes
• Scalar and vector quantities
• Measurement Of Length
• Measurement Of Time
• How To Read A Vernier Caliper
• How To Read A Micrometer Screw Gauge
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·         Physical Quantities

·         The study of Physics is based on experiments, which allowed us to test and validate theories. The results of those experiments are obtained by accurate measurements using physical quantities.

·         All physical quantities consist of a numerical magnitude and a unit.

·         There are seven base quantities (E.g. Mass, length). From the seven base quantities, you can obtain all the other physical quantities.

·         Physical quantities had many different types of units in the past. In the modern times, the units have been standardised and are named the SI units, from the French “Le Systeme International d’Unites”.

Drive Unit

Physical quantity	Base SI unit
Mass (m)	Kilogram (Kg)
Length (l)	Metre (m)
Time (t)	Second (s)
Current (I)	Ampere (A)
Temperature (T)	Kelvin (K)
Amount of sub. (n)	Molar (mol)
Luminous Intensity (L)	Candela (cd)

A derived quantity is defined based on a combination of base quantities and has a derived unit that is the exponent, product or quotient of these base units.
An example of derived quantity is energy which has a derived unit of Joules which is kgm2s−2 in base SI units

Prefixes

Prefixes are useful for expressing units of physical quantities that are either very big or very small.

Some of the Greek prefixes and their symbols to indicate decimal sub-multiples and multiples of the SI units are:

Number Prefix
Number	Prefix	Number	Prefix
10−9	nano (n)	10−1	deci (d)
10−6	micro (μ)	103	Kilo (K)
10−3	milli (m)	106	Mega (M)
10−2	centi (c)	109	Giga (G)

Scalar and Vector Quantity

Scalar and vector quantities

Scalar quantities are quantities in which the magnitude is stated, but the direction is either not applicable or not specified.

Examples:

• Length
• Volume
• Mass
• Speed

Vector quantities are quantities in which both the magnitude and the direction must be stated.

Examples:

• Force
• Velocity
• Displacement
• Acceleration

Measurement of  Length

Measurement Of Length

SI unit for length is metre (m). It is a scalar quantity.

Things you need to know:

• Accuracy refers to the maximum error encountered when a particular observation is made.
• Error in measurement is normally one-half the magnitude of the smallest scale reading.
• Because one has to align one end of the rule or device to the starting point of the measurement, the appropriate error is thus twice that of the smallest scale reading.
• Error is usually expressed in at most 1 or 2 significant figures.

Vernier Caliper

Equipment: It is made up of a main scale and a vernier scale and can usually measure objects up to 15 cm in length. The smallest marking is usually 0.1 cm on the main scale.

It has:

• a pair of external jaws to measure external diameters
• a pair of internal jaws to measure internal diameters
• a long rod to measure depths

How to use: The jaws are first closed to find any zero errors. The jaws are then opened to fit the object firmly and the reading is then taken.

Accuracy: ± 0.01 cm

Micrometer Screw Gauge

Equipment: It is made up of a main scale and a thimble scale and can measure objects up to 5 cm in length. The smallest marking is usually 1 mm on the main scale (sleeve) and 0.01 mm on the thimble scale (thimble). The thimble has a total of 50 markings representing 0.50 mm.

It has:

• an anvil and a spindle to hold the object
• a ratchet on the thimble for accurate tightening (prevent over-tightening)

How to use: The spindle is first closed on the anvil to find any zero errors ( use the ratchet for careful tightening). The spindle is then opened to fit the object firmly (use the ratchet for careful tightening) and the reading is then taken.

Accuracy: ± 0.01 mm