CAPF-AC (Assistant Commandant) Exam Study Material :
General Science - Units & Measurements
Units & Measurements
Physics is a quantitative science, based on measurement
of physical quantities. Certain physical quantities have been chosen as
fundamental or base quantities (such as length, mass, time, electric
current, thermodynamic temperature, amount of substance, and luminous
Each base quantity is defined in terms of a certain basic,
arbitrarily chosen but properly standardised reference standard called unit
(such as metre, kilogram, second, ampere, kelvin, mole and candela). The units
for the fundamental or base quantities are called fundamental or base units.
Other physical quantities, derived from the base quantities,
can be expressed as a combination of the base units and are called derived
units. A complete set of units, both fundamental and derived, is called a system
The International System of Units (SI) based on seven base
units is at present internationally accepted unit system and is widely used
throughout the world. The SI units are used in all physical measurements, for
both the base quantities and the derived quantities obtained from them. Certain
derived units are expressed by means of SI units with special names (such as
joule, newton, watt, etc).
The SI units have well defined and internationally accepted
unit symbols (such as m for metre, kg for kilogram, s for second, A for ampere,
N for newton etc.). Physical measurements are usually expressed for small and
large quantities in scientific notation, with powers of 10. Scientific notation
and the prefixes are used to simplify measurement notation and numerical
computation, giving indication to the precision of the numbers.
- An atom is the smallest particle of the element that can exist
independently and retain all its chemical properties.
- Dalton’s atomic theory, which suggested that the atom was indivisible
and indestructible. But the discovery of two fundamental particles
(electrons and protons) inside the atom, led to the failure of this aspect
of Dalton’s atomic theory.
- Thomson proposed that:
- An atom consists of a positively charged sphere and the electrons are
embedded in it.
- The negative and positive charges are equal in magnitude. So, the atom
as a whole is electrically neutral.
- Rutherford’s alpha-particle scattering experiment led to the discovery
of the atomic nucleus. Rutherford’s model of the atom proposed that a very
tiny nucleus is present inside the atom and electrons revolve around this
nucleus. The stability of the atom could not be explained by this model.
- Temperature is a relative measure, or indication of hotness or coldness.
- Heat is the form of energy transferred between two (or more) systems or
a system and its surroundings by virtue of temperature difference. The SI
unit of heat energy transferred is expressed in joule (J) while SI unit of
temperature is kelvin (K), and °C is a commonly used unit of temperature.
- Thermometer is a device used for measuring temperatures. The two
familiar temperature scales are the Fahrenheit temperature scale and the
Celsius temperature scale. The Celsius temperature (tC) and the Farenheit
temperare (tF) are related by: tF = (9/5) tC + 32
- In principle, there is no upper limit to temperature but there is a
definite lower limit- the absolute zero. This limiting temperature
is 273.16° below zero on the celsius scale of temperature.
- Clinical thermometer is used to measure our body temperature. The range
of this thermometer is from 35°C to 42°C. For other purposes, we use the
laboratory thermometers. The range of these thermometers is usually from
–10°C to 110°C. The normal temperature of the human body is 37°C.
- To understand light you have to know that what we call light is what is
visible to us.Visible light is the light that humans can see. Other animals
can see different types of light. Dogs can see only shades of gray and some
insects can see light from the ultraviolet part of the spectrum.
- As far as we know, all types of light move at one speed when in a
vacuum. The speed of light in a vacuum is 299,792,458 meters per second.
- Any medium through which light can travel is an optical medium. If this
medium is such that light travels with equal speed in all directions, then
the medium is called a homogeneous medium. The homogeneous media through
which light can pass easily, are called transperant media. The media through
which light cannot pass, are called opaque media. Again the media through
which light can pass partly, are called translucent media.
Magnetism & Electricity
The word magnet is derived from the name of an island in Greece
calledMagnesia where magnetic ore deposits were found, as early as 600 BC.
Magnetite, an iron ore, is a natural magnet. It is called lodstone.
When a bar magnet is freely suspended, it points in
the north-south direction. The tip which points to the geographic north is
called the north pole and the tip which points to the geographic south is called
the south pole of the magnet. There is a repulsive force when north poles ( or
south poles ) of two magnets are brought close together. Conversely, there is an
attractive force between the north pole of one magnet and the south pole of the
The properties of a magnet are
- it attracts small piece of iron towards it.
- it always cmes to rest in north-south direction when suspended freely.
- like poles repel, unlike poles attracts each other
- Magnetic poles always exist in pairs.
- the strength of a magnet is maximum at poles located near the poends
- Motion: In physics, motion is change of location or position of an
object with respect to time. Mechanical motion is of two types, transitional
( linear ) and rotational ( spin).
- SPEED: The speed of a moving body is the rate at which it covers
distance i.e. the distance it covers per unit of time.
- Speed: (distance travelled/ time required.) The S.I. Unit of speed is
- VELOCITY: The distance covered by an object in a specified direction in
unit time interval is called velocity. The S.I. Unit of velocity is m/s.
- Average velocity can be calculated by dividing displacement over time.
- The instantaneous velocity shows the velocity of an object at one point.
- The difference betwwn speed and velocity is: Speed is the distance
travelled by an object in a particular time. Velocity is the speed in a
Properties of matters: A matter can neither be created nor it can be
destroyed but it can be transformed from one state to another. Matter is made of
basic building blocks commonly called elements which are 112 in number. The
matter is made of only one kind of element then the smallest unit of that
element is called an atom. If the matter is made of two or more different
elements then the smallest unit of matter is called a molecule.
Molecule is defined as the smallest unit of matter which has independent
existence and can retain complete physical and chemical properties of matters.
According to kinetic theory of matter:
- molecules are in the state of continuous motion in all possible
directions and hence they posses kinetic energy which increases with the
gain of heat energy or rise in temperature,
- the molecules always attract each other,
- the force of attraction between the molecules decreases with the
increase in intermolecular spaces
- Sound is a form of energy and like all other energies, sound is not
visible to us. It produces a sensation of hearing when it reaches our ears.
Sound can not travel through vacuum.
- Sound is produced due to vibration of different objects.The matter or
substance through which sound is transmitted is called a medium. It can be
solid, liquid or gas. Sound moves through a medium from the point of
generation to the listener.
- In longitudinal wave the individual particles of the medium move in a
direction parallel to the direction of propagation of the disturbance. The
particles do not move from one place to another but they simply oscillate
back and forth about their position of rest. This is exactly how a sound
wave propagates, hence sound waves are longitudinal waves. Sound travels as
successive compressions and rarefactions in the medium. In sound
propagation, it is the energy of the sound that travels and not the
particles of the medium.
WAVES: There are three types of waves:
1. Mechanical waves require a material medium to travel (air, water, ropes).
These waves are divided into three different types.
- Transverse waves cause the medium to move perpendicular to the direction
of the wave.
- Longitudinal waves cause the medium to move parallel to the direction of
- Surface waves are both transverse waves and longitudinal waves mixed in
2. Electromagnetic waves do not require a medium to travel (light, radio).
3. Matter waves are produced by electrons and particles.A point of maximum
positive displacement in a wave, is called crest, and a point of maximum
negative displacement is called trough.
Work, Power & Energy
- When a force acting on a body produces a change in the position of the
body, work is said to be done by the force. Work done on an object is
defined as the magnitude of the force multiplied by the distance moved by
the object in the direction of the applied force. The unit of work is
joule: 1 joule = 1 newton × 1 metre. Work done on an object by a force would
be zero if the displacement of the object is zero.
- Power is defined as the rate of doing work. Power = (work done) / (time
taken). The SI unit of power is watt. 1 W = 1 Joule/second. The unit of
power is also horse power. It is the power of an agent which can work at the
rate of 550 foot pounds per second or 33,000 foot pounds pwe minute. 1 horse
power = 746 watts.
- An object having capability to do work is said to possess energy. Energy
has the same unit as that of work.