### Physics Class XI

Unit I: Physical World and Measurement
Chapter 1: Physical World
Unit II: Kinematics
Unit III: Laws of Motion
Chapter 5: Laws of Motion
Unit IV: Work, Energy and Power
Unit V: Motion of System of Particles and Rigid Body
Unit VI: Gravitation
Chapter 8: Gravitation
Unit VII: Properties of Bulk Matter
Unit VIII: Thermodynamics
Unit IX: Behaviour of Perfect Gases and Kinetic Theory of Gases
Unit X: Oscillations and Waves
Chapter 15: Waves

# Physical Quantity

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All matter possesses certain characteristics, with which they are identified. A pen, we use to write can be described by its color, color of ink, length, diameter, mass, transparency etc. These characteristics are known as properties of the pen. Some of them are measurable. Length, diameter, mass are the properties those can be measured. Others like color, transparency can be described. The properties of a matter which can be measured are Physical Quantities.

Physical Quantity: A Characteristic/ property of a system that can be measured to determine its amount is called a Physical Quantity.

Example: Mass, length, area, volume, density, speed, acceleration, force, pressure etc.

Based on their origin (dependency) physical quantities can be classified as: (i) basic or fundamental and (ii) derived physical quantities.

Fundamental Physical Quantity:

A physical quantity that has got its own origin and cannot be derived from any other physical quantity is called fundamental or basic physical quantity.

There are seven fundamental physical quantities. They are mass, length, time, temperature, electric current, amount of substance and luminous intensity. Along with these there are two supplementary fundamental quantities: plane angle and solid angle. Let us learn each of them in detail.

Length: The separation between two points is termed as length. It is generally used to denote the distance between two points or depict the size of an object.

Mass: If we are assigned to push a big rock and a small stone, the stone is easily displaced with less effort. Big rock is tough to be pushed and needs additional effort to displace. Mass is measure of resistance to motion of an object. Mass is a measure of resistance to motion of a body. The property of a body to oppose motion is known as inertia. We will learn in detail about inertia in later chapters of the subject. The inertia of a body is measure of its mass.

Time: The measure of sequence of events is termed as time. It is measured using a clock which is generally associated with a watch or modern days electronic gadgets.

Temperature: In summer days we feel hot while in winter we feel too cold. It is necessary to measure the degree of hotness or coldness of a body or region. It is measured using the physical quantity: temperature. Temperature is the measure of degree of hotness or coldness of a body or system.

Electric Current: Matter is composed of atoms. Atoms constitute positively charged protons, negatively charged electrons and neutral neutrons. When these charges particles move, they constitute an electric current. The rate of flow of charge is called electric current. It is measured using device named ammeter.

Amount of Substance: Representing a matter in terms of number of its constituent particles like atoms and molecules is the measure of amount of substance.

Luminous Intensity: It is advisable not to look at sun with bare eyes. Even though we have looked at it during the day resulting to which our eyes gets blank for some time. This is due to high brightness of light emitted by the sun. This brighteners is represented using luminous intensity.

Taking a look at the two supplementary physical quantities:

Plane Angle: The angle formed by two intersecting lines in a plane is called plane angle. In the figure below two lines and intersecting at a point form four angles as shown.

Solid Angle: The field of view of an object or surface obtained from a point is called solid angle. In the figure below, an object when viewed from forms a solid angle notified by the hatched region.

Derived Physical Quantities:

These physical quantities are obtained from combination of fundamental physical quantities.

They do not have origin of their own. Most of the physical quantities in nature are derived quantities which have obtained their existence from the seven fundamental physical quantities.

Consider a rectangular region with known length and breadth. These two adjacent lengths are fundamental physical quantitoes. Area being a physical quantity too, it is derived by multiplying the length of two adjacent sides of the rectangle. This simple example shows area to be considered as a derived physical quantity.

In another example, speed is obtained by dividing distance covered with time taken. This shows, speed is also a derived physical quantity which is obtained from the two fundamental quantities: length and time.

Representation of a physical quantity requires mentioning its magnitude (amount) in each case while some require an additional feature of depicting the direction along which it is acting. Classifying in terms of method of representation, physical quantities can be classified as; (i) scalar and (ii) vector.

Scalar Quantity: A physical quantity that requires mentioning its magnitude only is termed as a scalar.

The depiction of magnitude involves a number and associated unit. In an example, a bag of rice is quantified by specifying its mass say 25 kg. The representation 25 kg in which 25 is a numeral and kg is unit which is enough to mention its mass. Few more examples include volume, temperature, energy, time, electric current etc.

Vector Quantity: Unlike a scalar, a vector quantity requires both magnitude and direction for representation of the physical quantity.

Consider a case of moving an object on a flat surface from a location (point A) to another (point B). A certain minimum force is required to change its position. If a force of say 10 Newton is applied on it, the object will certainly move. If we applied a force of 10N but in direction of C, it won’t serve our purpose as it will not reach its desired destination B. But if the force is directed along direction of B, it will certainly reach point B. This indicates necessity of mentioning direction of force along with its magnitude. Not only in case of force, there are other physical quantities too that need direction along with magnitude to give a clear picture of them.

Other quantities like displacement, velocity, acceleration, momentum, etc. which need direction along with magnitude to be represented. There are different approaches to represent a vector quantity which we will learn in the topic of motion in a plane.

Area is a physical quantity which can be considered as a scalar as well as a vector based on its purpose. As a scalar quantity its magnitude i.e., the region occupied is represented. But while denoting it as a vector quantity, along with its magnitude the direction of area is also needed to be mentioned. The direction perpendicular to plane of area is considered as its direction.

A certain category of physical quantity called tensor which is quite different from both scalars and vectors will be briefed later in the topic of mechanical properties of solids.

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