General Knowledge for SSC Exams (Units Of Measurements And Important Scientific Instruments)

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General Knowledge for SSC Exams (Units Of Measurements And Important Scientific Instruments)

Quantity Units (S.I.)
Length Metre
Time Second
Mass Kilogram
Area Square metre
Volume Cubic metre
Velocity Metre/second
Acceleration Metre/second square
Density Kilogram/metre Cube
Work Joule
Energy Joule
Force Newton
Pressure Pascal or Newton/sq. metre Charge
Frequency Hertz
Power Watt
Weight Newton or Kilogram
Impulse Newton-second
Angular velocity Radian /second
Quantity Units (S.I.)
Viscosity Poise
Surface tension Newton/square metre
Heat Joule
Temperature Kelvin
Absolute temperature Kelvin
Resistance Ohm
Electric current Ampere
Electromotive force Volt
Electrical conductivity Ohm/metre
Electric energy Kilo watt hour
Electric power Kilo watt or watt
Magnetic intensity Orsted
Coulomb  
Magnetic induction Gauss
Luminous flux Candela
Intensity of sound Decibel
Power of lens Dioptre
Depth of sea Fathom




Scientific Explanations Of Common Phenomena

  • It is dangerous to sleep in an unventilated room with fire burning inside because the fire produces carbon monoxide and carbon dioxide gases. Carbon monoxide is poisonous and can cause death

  • The filament of an electric bulb is made of tungston because it has a high melting point and can be heated to a high temperature to emit light.

  • Water kept in an earthen pitcher becomes cold because the pitcher has minute pores on its surface which absorb water. Water in the pores evaporates when it comes in contact with air, and produces a cooling effect.

  • A sliced apple, when exposed to air, turns brown after some time as it contains iron which gets oxidised and gives the fruit a brownish colour.

  • The freezing compartment inside a refrigerator is at the top because the air in contact with it becomes cold and heavy and sinks downwards, cooling the rest of the compartment.

  • A copper vessel left in the air for a long time turns green. This is due to the formation of copper carbonate when copper reacts with carbon dioxide and moisture present in the air.

  • A wick in a stove keeps burning continuously as kerosene rises in the wick due to capillary action.

  • The Earth rotates on its axis from west to east. This rotation makes the Sun and the stars appear to be moving across the sky from east to west.

  • The sky appears blue because the light of the Sun is spread or scattered by the dust particles in the air. In space the sky would appear black as there are no dust or air particles to scatter the light.

  • Food is cooked quickly in a pressure cooker because the boiling point of water increases at high pressure. Food cooks faster at high temperature.

  • In mountainous regions, the atmospheric pressure is less than it is at sea level. Water, therefore, boils at a lower temperature (less than 100° C) and food takes more time to cook.

  • When ice floating in a glass of water melts, the level of water remains unchanged because as a solid, ice displaces an equal volume of water.

  • A man weighs more at the poles than at the equator because the polar radius of the Earth is less than the equatorial radius. Hence the gravitational pull is more at the poles than at the equator.

  • Standing in double-decker buses, particularly on the upper floor, is not allowed because on tilting, the centre of gravity of the bus gets changed and it is likely to overturn.

  • The boiling point of seawater will be more than the boiling point of pure water because the former contains salt and other impurities.

  • An ordinary clock loses time in summer because the length of its pendulum increases, and therefore, its time period also increases. The pendulum takes more time to complete each oscillation and thus loses time.

  • Whenever there is water loss from the body, secretion of saliva is reduced resulting in dryness of the mouth and stimulating the sensation of thirst. Intake of fluid then helps in restoring the loss of the water.

  • A swimmer just out of the river feels cold particularly if it is windy, because of the evaporation of water from his body surface. The evaporation is more on a windy day.

  • Alcohol is sometimes rubbed on the body of a person suffering from fever. As soon as it is applied on the body, it evaporates taking away some heat from the body. Since evaporation has a cooling effect, the body temperature can be reduced by rubbing alcohol.

  • Soft iron is used as an electromagnet because it remains a magnet only while the current passes through the coil around it and loses its magnetism when the current is switched off.

  • The person jumping out of a moving train is carried forward in the direction of the train because the person himself is in motion sharing the velocity of the train and will continue in its state of uniform motion unless it exercises some force to prevent it (Newton’s first law of motion).

  • A lightning conductor is fixed to tall buildings to protect them from the destructive effects of the lightning.

  • An electric bulb makes a bang when it is broken because there is a vacuum inside the electric bulb; when the bulb is broken air rushes in at great speed from all sides to fill the vacuum. The rushing of air produces a noise generally referred to as the “bang”.

  • A small space is left between each set of two rails of railway line to allow for their expansion in summer.

  • A bad egg floats in water because the up-thrust produced on account of displaced water by the immersed portion of the bad egg is greater than the weight of the egg.

  • Moisture gathers on the outer surface of a glass tumbler containing cold water because the water vapours present in the air get cooled and appear as droplets of water on coming in contact with the cold surface of the glass tumbler.

  • The launching of Earth satellites should be from a place near the equator to take the fullest advantage of the Earth’s movements. The regions of the Earth closer to the equator are moving faster through space as compared to regions elsewhere. Thus at the equator, rockets would start off with a speed of nearly 1600 km/hour. Further, rockets are usually fired in the direction of the Earth’s rotation.

  • In deserts, day temperatures are very high and night temperatures are extremely low because the specific heat of sand is very low. It, therefore, absorbs the heat readily and its temperature rises by a large degree during the day. At night sand radiates the heat equally readily, making the temperature low.

  • If you are sweating, you will feel cooler on a hot day than on a cooler moist day because on a hot dry day the perspiration gets evaporated quickly producing more cooling effect.

  • The air escaping from a punctured tyre feels cold because the air escaping from a punctured tyre enters a region of low pressure from high pressure and thus suffers a fall in temperature.

  • White roof keeps the house cooler in summer than black roof because white roof reflects more and absorbs less heat rays whereas black roof absorbs more and reflects less heat rays.

  • It is hotter on a cloudy night than on a clear night because clouds prevent the heat radiated by the Earth from escaping into the sky.

  • Ice wrapped in a blanket does not melt away quickly because woollen blanket is a bad conductor of heat.

  • A rose appears red when day light falls on it because it absorbs all the constituent colours of white light except red, which it reflects to us.

  • Grass appears dark in blue light because it has the property of absorbing all other colours except its own colour. The blue rays falling on the grass are absorbed by it and hence it appears dark in colour.

  • A group of soldiers on a bridge are advised not to walk in steps because their movement causes the bridge to vibrate. It they walk in step the frequency of vibration may match the natural frequency of the bridge structure, thereby causing resonance. This resonance of frequency can cause the bridge to collapse.

  • When the wind is blowing at high speed, the roofs of the hutments are sometimes blown away. The pressure at the top falls much below the pressure below the roof. This difference in pressure across the roof causes it to be blown away.

  • Human breath is visible in winter but not in summer because in winter air is cold and so water vapours present in the human breath condense, making it visible.

  • Steam causes more severe burns than boiling water because steam at the same temperature has more latent heat which causes more severe burns than boiling water at the same temperature.

  • It is not advisable to wash a clinical thermometer with boiling water because the boiling point of water is much above the maximum provided in the clinical thermometer. As a result, pressure from undue expansion of mercury may break it.

  • The face turns red when one feels hot and turns blue when one feels cold. This is because in the first case the blood rushes towards the skin to get cooled, and in the second case it rushes away from the skin to keep itself warm.

  • A petrol fire cannot be extinguished by pouring water over it because of the high temperature of burning petrol the water breaks up into hydrogen and oxygen which only helps in burning. Further, petrol being lighter than water, floats on water and continues to burn.

  • We experience difficulty in breathing on mountains because the pressure of the air outside is less as compared to the pressure of air inside the lungs.

  • If a highly corked glass bottle full of water is left out of doors on a frosty night it will burst because the water contained in the bottle will freeze on a frosty night and convert into ice. There is no room available for the increased volume and this may result in bursting of the bottle.

  • Water extinguishes fire because as it evaporates, the temperature of the burning body is lowered, thus retarding or stopping the burning action. Also the resulting vapour surrounds the burning substance cutting off the oxygen supply thus inhibiting the burning process. In fact, hot water will extinguish fire more quickly than cold water as hot water will vaporise faster.

  • A perspiring man feels relief when air floats by his side because the flow of air increases the rate of evaporation of perspiration from the body.

  • Water from a hand pump is warm in winter and cold in summer because in winter outside temperature is low and in summer outside temperature is higher as compared to the temperature of water obtained from underground which remains practically unchanged due to Earth being bad conductor of heat.

  • A thick glass tumbler often cracks when very hot liquid is poured into it because the inner surface of the thick glass tumbler coming in contact with the hot liquid expands more in comparison with the outer surface which has relatively lower temperature. This unequal expansion of inner and outer surfaces causes the tumbler to crack.

  • When a gun is fired at a visible distance, the sound is heard a little after the smoke is seen because the velocity of light is much higher than that of sound.

  • A train stops when the chain is pulled because of the following mechanism. When the chain is pulled, one small valve gets opened and air enters to the under side of the piston head of the brake cylinder through pipe connections. Initially there is vacuum on both top and bottom side of the piston head. When air enters the under side of the piston head, the piston raises up due to difference of pressure and pulls the brake thus stopping the train.

Important Laws And Principles

  • Archimedes Principle: When a body is immersed either wholly or partially in a fluid at rest, the apparent loss of weight suffered by it is equal to the weight of the fluid displaced by it.

  • Avogadro’s Law : Equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules.

  • Black Body Radiation : A black body absorbs heat or radiates heat more quickly than any other body.

  • Boiling Point : It increases with the increase of pressure. The presence of impurities also raises the boiling point of a liquid.

  • Boyle’s Law : At constant temperature, the volume of a given mass of gas varies inversely to its pressure.

  • Centre of Gravity : A body will remain at rest only if the vertical line through its centres of gravity passes through the base of support of the body.

  • Charle’s Law : The volume of a given mass of gas is directly proportional to its absolute temperature when the pressure remains constant.

  • Coulomb’s Law : The force between the two electric charges reduces to a quarter of its former value when the distance between them is doubled.

  • Dalton’s Law : At a specific temperature and for a container of fixed volume, the total pressure of a mixture of non-reacting gases is the sum of their respective partial pressures.

  • Doppler’s Principle : When the distance between the source of a wave and the observer increases due to their relative motion, the frequency of the wave appears to decrease. The converse condition is also true.

  • Faraday’s Laws of Electrolysis
    1. The amount of chemical change during electrolysis is proportional to the charge passed.
    2. The masses of substances liberated or deposited by the same quantity of electric charge are proportional to their chemical equivalents.

  • Heisenberg’s Uncertainty Principle : The velocity and position of an electron in the orbit of an atom cannot be simultaneously determined.

  • Inverse Square Law : The force of attraction between two unlike magnetic poles and the force of repulsion between two like poles is inversely proportional to the square of the distance between them. A similar law is true of electrical charges also.

  • Law of Conservation of Matter : In chemical changes, matter is neither created nor destroyed. The sum total of the masses of all the products of a chemical change is exactly equal to the sum total of the substances from which these products have been ,formed.

  • Laws of Thermodynamics
    1. The amount of heat given to a system is equal to the sum of the increase in the internal energy of the system and the external work done.
    2. It is impossible to construct a continuous self-acting machine that can pump heat energy from a body at lower temperature to a body at higher temperature.

  • Lenz’s Law : When an electric current is induced by a change in magnetic field, the induced current is always in such a direction that its magnetic field opposes the change of field which causes the induction.

  • Mass-Energy Equation : E = mc2, where E = quantity of energy released from the annihilation of matter of mass ‘m’, c = velocity of light. It implies that mass and energy are interchangeable.

  • Newton’s Law of Cooling : The rate at which a body cools or looses its heat to its surroundings is proportional to the excess of mean temperature of the body over that of the surroundings, provided this temperature excess is not too large.

  • Newton’s Universal Law of Gravitation : Everybody in the universe attracts every other body with a force, directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

  • Newton’s Laws of Motion
    1. Everybody continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by a force [called Law of inertial].
    2. The rate of change of momentum of a moving body is proportional to the applied force and takes place in the direction of the force.
    3. For every action there is an equal and opposite reaction.

  • Ohm’s Law : The amount of current flowing in an electric circuit is governed by the voltage of the battery or dynamo which powers it. In other words, the current through a conductor is directly proportional to the potential difference across the conductor and inversely proportional to its resistance.

  • Pascals’ Laws
    1. When pressure is applied to a fluid, the pressure change is transmitted to every part of the fluid without loss. Hydraulic machines work on this principle.
    2. Atmospheric pressure decreases with increase in height.

  • Rectilinear Propagation of Light : Light travels in a straight line. Total internal reflection takes place when a ray of light tries to pass from a denser medium to a rarer medium at an angle of incidence more than the critical angle.

Alloys & their uses

Alloys Uses
Brass In making utensils
Bronze In making coins, bell and utensils
German Silver In making utensils
Rolled gold In making cheap ornaments
Gun metal In making gun, barrels, gears and bearings
Delta metal In making blades of aeroplane
Munz metal In making coins
Dutch metal In making artificial ornaments
Monel metal For base containing container
Rose metal For making automatic fuse
Solder For soldering
Magnalium For frame of aeroplane
Duralumin For making utensils
Type metal In printing industry
Bell metal For casting bells, statues
Stainless steel For making utensils and surgical cutlery
Nickel steel For making electrical wire, automobile parts

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