## What are Physical Quantities?

### Definition

• The quantities which can be expressed in terms of Laws of Physics like time, length, volume and density etc. are called Physical Quantities.
• These can also be measured.
• These Physical Quantities are represented by a number followed by a unit.

The number along with the unit is called the ‘magnitude‘ of the physical quantity.

### TYPES OF PHYSICAL QUANTITIES

• Basic/Fundamental Physical Quantity
• Derived Physical Quantity

#### Fundamental Physical Quantity

• Are expressed in terms of standard units.
• There are seven (7) basic physical quantities.
• Examples- mass, length, time, electric current, temperature, luminous intensity and amount of substance.
• All other quantities are expressed in terms of these quantities.

#### Derived Physical Quantities

• When a physical quantity expresses itself in terms of two or more fundamental physical quantities.
• It is derived by multiplying or dividing one basic physical quantity with another basic physical quantity.
• Examples- density, volume, force, power, velocity, acceleration etc.

## INTERNATIONAL SYSTEM OF UNITS (SI Units)

### METRE

• It is the SI Unit of Length.
• A metre is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

1 kilometre (km) = 1000 metres (m)

1 centimeter (cm) = 0.01 metre

1000 millimeters (mm) = 1 metre

1 micrometre = 10-6 metres

### KILOGRAM

• It is the SI Unit of mass.
• Its symbol is kg.
• its other units which are commonly used are gram, milligram.

1 quintal = 100 kg

1 tonne = 1000 kg

### SECOND

• It is the SI Unit of time.

### AMPERE

• It is the SI Unit of current.

### KELVIN

• Kelvin is the SI Unit of temperature.
• The Kelvin is the 1/273.16 of the thermodynamic temperature of the triple point of water.

### CANDELA

• It is the SI Unit of Luminous Intensity i.e. brightness of Light.
• If a monochromatic light produces a frequency of 540 × 1012 in a definite direction and if its intensity is 1/683 watt per steradian then Luminous Intensity is 1 candela.

### MOLE

Mole is the SI Unit of the amount of substance.

It is the amount of substance which contains as many elementary particles (atoms, molecules or ions) as there are in 12 grams of carbon-12.

NOTE- Remember this table

 VALUES PREFIX SYMBOL 1018 exa E 1015 peta P 1012 tera T 109 giga G 106 mega M 103 kilo K 102 hecto H 101 deca da 10-1 deci d 10-2 centi c 10-3 milli m 10-6 micro µ 10-9 nano n 10-12 pico p 1015 femto f 10-18 atto a

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