Physical quantity
• Any quantity that can be measured
• A physical quantity can be classified in to two:-
i) Fundamental quantity (Base quantity)
ii) Derived quantity
• Quantities that cannot be expressed in terms of other quantities are known as fundamental
quantities.
Eg:- mass, length, time etc.
• Quantities which are derived from fundamental quantities are known as derived quantities.
Eg:- force, velocity, area, volume ,etc
Unit :
Basic, internationally accepted reference
standard used for measurement is called unit.
The units for the fundamental or base
quantities are called fundamental or base
units.
The units of all other physical quantities can be expressed as combinations of the base units.
Units obtained for the derived quantities are
called derived units.
Systems of Units
A complete set of the base units and derived
units, is known as the system of units.
● In CGS system the base units for length, mass and time were centimetre, gram and
second respectively.
● In FPS system the base units for length, mass and time were foot, pound and
secondrespectively.
● In MKS system the base units for length, mass and time were metre, kilogram and
second respectively.
THE INTERNATIONAL SYSTEM OF UNITS
SI system is the internationally accepted system of unit at present.
In SI, there are seven base units and two
supplementary units.
MEASUREMENT OF LENGTH
A metre scale is used for lengths from 10-3 m to 10^2m.
A vernier callipers is used for lengths to an
accuracy of 10^-4m.
A screw gauge and a spherometer can be used to measure lengths as less as to 10^-5m.
Measurement of large distances
Parallax method
A method to measure distances to far away
objects.
The apparent shift in the position of an object
, when viewed from two different positions is
known as parallax.
The distance between the two points of
observation is called the basis
MEASUREMENT OF MASS
The unit used to measure mass of atoms or
molecules is unified atomic mass unit (u).
Mass of commonly available objects can be
determined by a common balance.
Large masses in the universe like planets, stars, etc., can be measured by using gravitational
method.
For measurement of small masses of
atomic/subatomic particles etc., we make use
of mass spectrograph
MEASUREMENT OF TIME
To measure any time interval we need a clock.
We now use an atomic standard of time, which
is based on the periodic vibrations produced in
a cesium atom- cesium clock or in general
atomic clock.
The cesium atomic clocks are very accurate.
In our country, the NPL( National Physical
Laboratory) has the responsibility of
maintenance and improvement of physical
standards, including that of time, frequency,
etc.
ACCURACY AND PRECISION OF A MEASUREMENT
Accuracy of a measurement is a measure of
how close the measured value is to the true
value of the quantity.
Precision is the resolution or limit with which
the quantity is measured.
The accuracy may depend on the limit or
resolution of the measuring instrument.
ERRORS IN MEASUREMENT
Error is the uncertainty in a measurement.
It is the difference between measured value
and true value of the quantity.
TYPES OF ERRORS
Errors can be classified as:
a) Systematic Errors
b) Random errors
c) Least count errors
Systematic errors
• Any quantity that can be measured
• A physical quantity can be classified in to two:-
i) Fundamental quantity (Base quantity)
ii) Derived quantity
• Quantities that cannot be expressed in terms of other quantities are known as fundamental
quantities.
Eg:- mass, length, time etc.
• Quantities which are derived from fundamental quantities are known as derived quantities.
Eg:- force, velocity, area, volume ,etc
Unit :
Basic, internationally accepted reference
standard used for measurement is called unit.
The units for the fundamental or base
quantities are called fundamental or base
units.
The units of all other physical quantities can be expressed as combinations of the base units.
Units obtained for the derived quantities are
called derived units.
Systems of Units
A complete set of the base units and derived
units, is known as the system of units.
● In CGS system the base units for length, mass and time were centimetre, gram and
second respectively.
● In FPS system the base units for length, mass and time were foot, pound and
secondrespectively.
● In MKS system the base units for length, mass and time were metre, kilogram and
second respectively.
THE INTERNATIONAL SYSTEM OF UNITS
SI system is the internationally accepted system of unit at present.
In SI, there are seven base units and two
supplementary units.
MEASUREMENT OF LENGTH
A metre scale is used for lengths from 10-3 m to 10^2m.
A vernier callipers is used for lengths to an
accuracy of 10^-4m.
A screw gauge and a spherometer can be used to measure lengths as less as to 10^-5m.
Measurement of large distances
Parallax method
A method to measure distances to far away
objects.
The apparent shift in the position of an object
, when viewed from two different positions is
known as parallax.
The distance between the two points of
observation is called the basis
MEASUREMENT OF MASS
The unit used to measure mass of atoms or
molecules is unified atomic mass unit (u).
Mass of commonly available objects can be
determined by a common balance.
Large masses in the universe like planets, stars, etc., can be measured by using gravitational
method.
For measurement of small masses of
atomic/subatomic particles etc., we make use
of mass spectrograph
MEASUREMENT OF TIME
To measure any time interval we need a clock.
We now use an atomic standard of time, which
is based on the periodic vibrations produced in
a cesium atom- cesium clock or in general
atomic clock.
The cesium atomic clocks are very accurate.
In our country, the NPL( National Physical
Laboratory) has the responsibility of
maintenance and improvement of physical
standards, including that of time, frequency,
etc.
ACCURACY AND PRECISION OF A MEASUREMENT
Accuracy of a measurement is a measure of
how close the measured value is to the true
value of the quantity.
Precision is the resolution or limit with which
the quantity is measured.
The accuracy may depend on the limit or
resolution of the measuring instrument.
ERRORS IN MEASUREMENT
Error is the uncertainty in a measurement.
It is the difference between measured value
and true value of the quantity.
TYPES OF ERRORS
Errors can be classified as:
a) Systematic Errors
b) Random errors
c) Least count errors
Systematic errors
