General Science - Physics

General Science is one of the most important and scoring sections in SSC CGL General Awareness. Every year 4 to 7 questions come directly from General Science - covering Physics, Chemistry and Biology. Physics forms the largest portion of General Science questions asked in SSC CGL. Topics like Units and Measurement, Newton's Laws, Light, Sound, Electricity and Magnetism are asked repeatedly. Students who master this chapter can score full marks from this section consistently.

1. Units and Measurement

1.1 SI System (International System of Units)

The SI system is the globally accepted standard system for measurement. It has 7 base units:

1.2 Important Units - Match the Following (Frequently Asked)

1.3 Other Important Units

Note: Parsec is a unit of length/distance - not density or mass.

1.4 Measuring Instruments

1.5 Physical Quantities

Scalar Quantities - Have magnitude only:

• Mass, Distance, Speed, Work, Temperature, Time, Energy

Vector Quantities - Have both magnitude and direction:

• Velocity, Force, Acceleration, Displacement, Momentum

Important: Plane angle (radian) is the physical quantity that represents torque/radius. Its SI unit is radian.

2. Mechanics

2.1 Power

• Power is the rate of doing work
• If work W is done in time t, then Power = W/t
• SI unit: Watt (W) or Joule/second - named after scientist James Watt
• 1 Horsepower = approximately 746 Watts

2.2 Energy

Kinetic Energy:

• Energy possessed by a body due to its motion
• Formula: KE = ½mv²
• Depends on both mass and velocity of the moving body
• Examples: Falling coconut, fast-moving cars, rolling stones, flying aeroplanes

Potential Energy:

• Energy stored in a body due to its position or configuration
• Formula: PE = mgh
• Example: Water stored in a dam, a stretched spring, a book placed on a shelf

Energy Conversions (Frequently Asked):

• Photosynthesis: Light energy → Chemical energy
• Battery: Chemical energy → Electrical energy
• Wind turbine: Kinetic energy → Mechanical energy
• Electric motor: Electrical energy → Mechanical energy
• Hydroelectric power: Potential energy → Kinetic energy → Electrical energy

2.3 Newton's Laws of Motion

First Law (Law of Inertia):

• A body at rest remains at rest and a body in motion continues in uniform motion unless acted upon by an external force
• Also called Law of Inertia
• Example: Passengers jerk forward when a bus stops suddenly

Second Law (Law of Force):

• The rate of change of momentum of a body is equal to the applied force
• Formula: F = ma (Force = mass × acceleration)
• Gives the quantitative definition of force
• Newton is the SI unit of Force defined by this law

Third Law (Law of Action and Reaction):

• For every action there is an equal and opposite reaction
• Example: A cannon recoiling after firing; a rocket being propelled upward

2.4 Force

Four Fundamental Forces (in decreasing order of range):

Strong Nuclear Force range exponent: 10⁻¹⁵ m Weak Nuclear Force range exponent: 10⁻¹⁶ m

2.5 Speed and Velocity

Centripetal Acceleration Formula:

• For a body moving in a circular path: Ac = v²/R
• where Ac = centripetal acceleration, v = speed/velocity, R = radius

2.6 Projectile Motion

• When a particle is thrown vertically upward and reaches its highest point, its acceleration is directed downward (due to gravity)
• During projectile motion from Earth's surface, the horizontal component of velocity remains constant (when air resistance is neglected)

2.7 Acceleration

• Rate of change of velocity of a body
• Formula: a = (v - u)/t
• SI unit: m/s² or m·s⁻² (exponent of second is -2)
• Freely falling body is an example of uniform acceleration
• Acceleration = rate of change of Velocity

2.8 Friction

• Force that opposes relative motion between two surfaces in contact
• A rough surface increases friction force - this is why objects slide less on rough surfaces
• Brake force on a vehicle is friction force
• We can walk on Earth because of friction
• Without friction between tyre and road - tyres would slip

3. Gravitation

3.1 Gravity and Weight

• Weight is the gravitational force acting on a body
• g (acceleration due to gravity) is maximum at the poles of Earth
• g is minimum at the equator
• g decreases as you go above or below Earth's surface
• If Earth's rotation speed increases - body weight decreases
• Maximum weight of an object is at the poles of Earth

3.2 Weightlessness in Space

• When a spacecraft orbits Earth, the centrifugal force due to its orbital motion balances the gravitational pull of Earth
• The two opposing forces balance each other resulting in zero gravity on the spacecraft
• This is why astronauts feel weightless in space - due to zero gravitational pull (apparent)

4. Properties of Matter

4.1 Elasticity

• Spring balance works on Hooke's Law
• Hooke's Law: The change in length of an object is proportional to the force applied - F = -Kx
• Where K is the spring constant (Young's Modulus or Elasticity Coefficient)
• Stress is proportional to Strain (within elastic limit)

4.2 Surface Tension, Capillarity and Viscosity

• When a drop of water is placed on a glass plate it spreads into a thin layer whereas mercury remains spherical because:
  • Mercury's cohesive force (attraction between same molecules) is greater than its adhesive force (attraction between different molecules) with glass
  • Water's adhesive force with glass is more than its cohesive force - so it spreads
• Air bubbles in liquid rise due to viscosity and buoyancy

4.3 Buoyancy and Archimedes' Principle

• A fresh egg placed in salt water floats because the density of salt solution is greater than the density of the egg
• If gravitational force did not exist - upward pressure on liquids would not apply

4.4 Pressure

• In mountainous regions, the boiling point of water is less than at sea level because atmospheric pressure decreases with altitude - lower pressure means lower boiling point
• Pascal's Law: Pressure applied per unit area equals the force acting on the area it acts upon
• Pascal is the SI unit of pressure

4.5 Density

• A fresh egg floats in salt water because salt solution has higher density than the egg

4.6 Kinetic Theory

• The molecular weight of a gas is double its vapour density
• Molecular weight = 2 × Vapour Density

5. Heat

5.1 Temperature and Its Measurement

• At the boiling point of a liquid - its temperature remains constant
• Absolute Zero (0 K) is the temperature at which molecular motion of gases stops
• 0 K = -273°C
• Below absolute zero temperature is not possible
• Kelvin scale named after British physicist William Thomson (Lord Kelvin)

Unit Conversions:

• 1 Joule = 0.24 calories
• 1 Calorie = 4.186 Joules
• 1 Kilocalorie = 4.186 × 10³ Joules = 1000 calories

5.2 Thermal Energy, Conduction and Radiation

• During phase change of a substance - the absorbed heat is stored as kinetic energy of molecules
• After sunset, the air near Earth's surface receives heat through terrestrial radiation (radiation from Earth's surface)

5.3 Thermal Expansion

• The Eiffel Tower becomes longer in summer due to thermal expansion
• When a material is heated - the distance between its atoms increases
• Railway tracks have gaps at joints to allow for thermal expansion

5.4 Conductor, Insulator and Thermodynamics

• Ice covered with sawdust does not melt quickly because sawdust is a poor conductor (insulator) of heat
• The air trapped between sawdust particles prevents heat from reaching the ice

Thermodynamics Laws:

• Zeroth Law term was given by Ralph H. Fowler in 1931: If two systems are in equilibrium with a third system, they are also in equilibrium with each other
• William Thomson (Lord Kelvin) formulated the first two laws of thermodynamics and concluded that absolute zero temperature is -273.15°C
• When ice forms - the entropy of water decreases

6. Waves

6.1 Electromagnetic Spectrum

Order of radiations in decreasing frequency:

• X-rays > Infrared waves > Microwaves > Radio waves

Frequency Ranges:

• X-rays: 10¹⁸ Hz to 10¹⁶ Hz
• Infrared: 10¹² Hz to 10¹⁰ Hz
• Microwaves: 10¹⁰ Hz to 10⁸ Hz
• Long Radio waves: 10⁶ Hz to 10⁴ Hz

TV Remote Control works on the principle of Infrared waves

• Cathode rays are not electromagnetic waves
• Infrared waves were discovered by Herschel

6.2 Wave Properties

• The intensity (power) of a wave is proportional to the square of its amplitude
• Wave velocity formula: V = nλ where V = velocity, n = frequency, λ = wavelength

6.3 Polarisation of Light

• The phenomenon that establishes the transverse nature of light is polarisation
• Polaroids are used to produce polarised light
• Polaroid glasses are used in cinema halls to watch 3D films

6.4 Waves That Cannot Be Polarised

• Ultrasonic waves cannot be polarised because they are longitudinal waves (not transverse)
• Longitudinal waves (like sound waves in gases) cannot be polarised
• Transverse waves (electromagnetic waves) can be polarised

7. Sound

7.1 Nature of Sound Waves

• The Doppler Effect - change in frequency of sound/light - was discovered by Austrian scientist Christian Doppler in 1842
• Sound cannot travel in vacuum (requires a medium)
• Sound can travel in solids, liquids and gases
• Speed of sound: lowest in gases, higher in liquids, highest in solids

7.2 Speed of Sound

• Speed of sound in air at normal temperature: approximately 343 m/s
• At 0°C: 332 m/s

7.3 Characteristics of Sound

• The quality/timbre of musical sound from a stringed instrument depends on the waveform of the sound

8. Light

8.1 Nature of Light

• Light emitted by the Sun is white colour
• White light is a mixture of seven coloured lights
• When white light passes through a prism it splits into: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR)
• Violet has maximum bending (refraction) and Red has minimum

8.2 Scattering of Light

• At sunrise and sunset, the Sun appears red because:
  • All other colours scatter and only red light reaches our eyes (least scattering)
  • Red light has the least scattering among all colours
• Alpenglow: The phenomenon (due to scattering of light) where mountain peaks appear pink/rose coloured at sunrise and sunset

8.3 Reflection of Light

• Two mirrors inclined at 120° to each other produce 2 images
• Formula: Number of images = (360°/θ) - 1 = (360/120) - 1 = 3 - 1 = 2

8.4 Refraction of Light

• When light passes from one medium to another its direction changes - this is called refraction
• Refractive Index of Crown Glass (1.52) is closest to Canada Balsam (1.519-1.521)

8.5 Total Internal Reflection

• The shining of air bubbles in water is due to total internal reflection of light
• When angle of incidence exceeds the critical angle in a denser medium - total internal reflection occurs

8.6 Lens

• Power of a lens is measured in Dioptre (D) - D stands for Diopter
• If lens power is negative - lens is concave; if positive - lens is convex
• Formula: Power (P) = 1/f where f is focal length in metres

8.7 Human Eye and Eye Defects

• When coming from bright light to dim light - the iris contracts the pupil making it smaller - less light enters - so you cannot see immediately
• Far-sightedness (Hypermetropia): Corrected by Convex lens
• Near-sightedness (Myopia): Corrected by Concave lens

9. Electricity

9.1 Coulomb's Law

• When distance between two electric charges doubles - force between them becomes one-fourth
• Coulomb's Law: Force between two charges is proportional to the product of charges and inversely proportional to the square of distance between them - this is the Inverse Square Law

9.2 Electric Circuit

• Fuse wire rating is based on the electric current it can safely carry
• Fuse wire is made of alloy of Tin (63%) and Lead (37%)
• Fuse is always connected in series in a circuit

9.3 Electric Current

• Direct Current (DC) frequency: zero Hertz
• DC always flows in one direction - constant direction

9.4 Ohm's Law

• Discovered by German physicist George Simon Ohm from Germany in 1826
• V = IR (Voltage = Current × Resistance)
• Silver is the best conductor of electricity (followed by gold, copper, aluminium)
• Graphite is a good conductor of electricity among non-metals

9.5 Resistance

• Using Ohm's Law: R = V/I
• Example: V = 18V, I = 3A → R = 18/3 = 6 Ω
• Wire resistance depends on length and cross-sectional area: R = ρl/A
• To reduce electric resistance - connect resistors in parallel

9.6 Transformer

• Transformer cannot be built inside an integrated circuit
• Transformer converts: Low voltage AC to High voltage AC (step-up) or vice versa (step-down)

9.7 Battery/Electric Cell

• A battery converts chemical energy into electrical energy
• A common battery contains sulphuric acid

10. Magnetism

10.1 Electromagnetism

• André-Marie Ampère suggested that a magnet should also exert equal and opposite force on a current-carrying conductor
• The SI unit of electric current (Ampere) is named after him
• Electromagnetic Science was discovered and named by André-Marie Ampère
• Michael Faraday: Electromagnetic induction - invented Dynamo and Generator
• Hans Christian Oersted: Principle of deriving magnetic field
• James Clerk Maxwell: Electromagnetic theory

10.2 Electromagnetic Induction

• Induced current is maximum when the direction of coil's motion is perpendicular to the magnetic field

10.3 Types of Magnetic Materials

10.4 Permanent Magnet

• Steel (Iron alloy) is most suitable for making permanent magnets
• Soft iron is used for making electromagnets

10.5 Fleming's Left Hand Rule

• Fleming's Left Hand Rule is related to the effect of magnetic field on current-carrying conductor (electric motor)
• Fleming's Right Hand Rule is for electric generator
• Left Hand Rule explains force on current due to magnetic field (Magnetic field on current)

11. Electronics

11.1 Semiconductor

• The process of adding suitable impurities to a semiconductor is called Doping (Snehana)

11.2 Transistor

• Transistor is used as an amplifier device
• It is an amplifying component in audio/hearing devices
• It is a semiconductor device used to amplify or switch electronic signals

11.3 Diode

• Diode is used for rectification (converting AC to DC)
• A rectifier converts AC (alternating current) to DC (direct current)
• Diode was discovered by British scientist John Ambrose Fleming in 1904

11.4 Rectifier

• Rectifiers convert AC to DC
• Inverter converts DC to AC

11.5 Cathode Ray Tube (CRT)

• A sealed glass vacuum tube with an electron beam focused on a phosphorus-coated glass screen is a Cathode Ray Tube

12. Modern Physics

12.1 Photoelectric Effect

• For producing photoelectric effect - the metal used as cathode should have low reactivity (work function)

12.2 X-Rays

• X-rays are electromagnetic waves with wavelength 0.01 to 10 nanometres
• X-rays have high penetrating power
• They can reflect or refract
• X-rays travel at speed of light
• They do have high penetrating power (incorrect option in exams: "they have low penetrating power")
• X-rays were discovered by Wilhelm Röntgen

12.3 Infrared Rays

• Every hot object emits infrared radiation
• Infrared rays were discovered by William Herschel

12.4 Big Bang Theory

• The Big Bang Theory was proposed by Belgian astronomer and priest Abbé Georges Lemaître
• American scientist Edwin Hubble's name is associated with this theory
• Hubble proved that galaxies are not stationary but moving away from each other (1929)

13. Nuclear Physics

13.1 Nuclear Fusion

• When two light nuclei combine at high temperature to form one heavy nucleus - this is nuclear fusion
• Nuclear Fission: A heavy nucleus splits into two or more equal parts

14. Inventions

Discoverers of Subatomic Particles:

15. Miscellaneous Physics

15.1 Important Phenomena

Doppler Effect (1842): Change in frequency of sound or light when source and observer are in relative motion - discovered by Christian Doppler

Zeeman Effect: When a light source is placed in a strong magnetic field - each spectral line splits into two or more components of different frequency

Bernoulli's Principle: The total energy per unit mass of a fluid flowing smoothly - sum of kinetic energy, potential energy and fluid pressure energy remains constant

• Example: Person on railway platform pulled toward fast train; tin roofs blowing off in storms; physical balance lever dipping automatically

Super Cooling: Cooling a liquid or gas below its freezing point without it solidifying

Van Allen Radiation Belts: Vast bands of highly energetic charged particles trapped magnetically around Earth

Stars Twinkle but Planets Don't: Planets are close to Earth so more light reaches us and small variations in intensity go unnoticed; stars are very far so their light reaches us as a parallel beam after passing through atmosphere - causing apparent twinkling due to refraction in atmosphere

ARPANET: Considered the precursor to the Internet - started by US Defense Advanced Research Projects Agency (DARPA)

15.2 Insulators vs Conductors

• Mercury (para/quicksilver) is a conductor not an insulator
• Insulators: Ebonite, Glass, Dry paper, Wax

15.3 Best Absorbent

• Activated coconut wood charcoal is the best absorbent

15.4 Alcohol vs Water Volatility

• Alcohol is more volatile than water because its boiling point is lower than water
• Reason: Alcohol has lower polarity than water

15.5 Radio Tuning

• The component used for radio tuning is essentially a capacitor (condenser)

15.6 Solar Panels

• A group of solar cells connected together in a specific pattern is called a Solar Cell Panel

15.7 Isotopes and prefix ISO

• The prefix ISO is used to describe constant scientific terms

16. SSC CGL Important One-Liners - Physics

1. The SI unit of force is Newton
2. 1 Horsepower = 746 Watts
3. The rate of doing work is called Power
4. Kinetic energy depends on both mass and velocity
5. Newton's Second Law gives the quantitative definition of force
6. F = ma is derived from Newton's Second Law
7. A cannon recoiling after firing demonstrates Newton's Third Law
8. Absolute Zero = 0 K = -273°C
9. Kelvin scale was named after Lord Kelvin (William Thomson)
10. Pascal's Law: Pressure applied is transmitted equally in all directions in a closed container
11. Boiling point decreases at higher altitude due to lower atmospheric pressure
12. Silver is the best conductor of electricity
13. Graphite is a good electrical conductor among non-metals
14. Fuse wire is made of Tin (63%) and Lead (37%) alloy
15. Ohm's Law: V = IR - discovered by George Simon Ohm from Germany
16. Transformer converts low AC voltage to high AC voltage (and vice versa)
17. Battery converts chemical energy to electrical energy
18. DC current has frequency of zero Hertz
19. Doping is the process of adding impurities to semiconductors
20. Transistor is used as an amplifier in audio devices
21. Diode converts AC to DC (rectification)
22. Diode was discovered by John Ambrose Fleming in 1904
23. X-rays were discovered by Wilhelm Röntgen
24. Infrared rays were discovered by William Herschel
25. Every hot object emits infrared radiation
26. Doppler Effect was discovered by Christian Doppler in 1842
27. Sound cannot travel in vacuum
28. Speed of sound in air at normal temperature: 343 m/s
29. Total Internal Reflection causes the shining of air bubbles in water
30. Polarisation establishes the transverse nature of light
31. Refraction is bending of light when passing from one medium to another
32. White light splits into VIBGYOR colours on passing through a prism
33. Violet bends most, Red bends least in a prism
34. Sun appears red at sunrise/sunset because red scatters least - other colours scatter away
35. Alpenglow is when mountain peaks appear pink at sunrise/sunset due to scattering
36. Seismograph measures earthquake intensity
37. Barometer measures atmospheric pressure
38. Hygrometer measures relative humidity
39. Anemometer measures wind speed
40. Fathometer measures ocean depth
41. Parsec is a unit of astronomical distance (length)
42. 1 Parsec ≈ 3.262 Light years
43. Beaufort Scale measures wind speed (0 to 12)
44. Decibel is the unit of sound intensity
45. Ferromagnetic materials: Iron, Nickel, Cobalt
46. Steel is used for making permanent magnets
47. Soft iron is used for making electromagnets
48. André-Marie Ampère suggested magnet exerts force on current-carrying conductor
49. Neutron was discovered by James Chadwick in 1932
50. Nuclear Fusion: Two light nuclei combine to form one heavy nucleus

17. Chapter Summary

Physics forms the backbone of General Science for SSC CGL. The most important topics are: