Topic 68: Physics – Motion, Energy, Light,

Study Material

Paper Paper II

Unit 2

1. Newton's Laws of Motion - First Law (Law of Inertia): body at rest or in uniform motion continues unless acted upon by an external force - Second Law: F = ma (force = mass × acceleration) - Third Law: for every action there is an equal and opposite reaction
2. Law of Conservation of Energy - Energy can neither be created nor destroyed — it can only be converted from one form to another - Total mechanical energy (KE + PE) is conserved in the absence of non-conservative forces
3. Newton's Law of Gravitation - F = Gm₁m₂/r² where G = 6.674 × 10⁻¹¹ N·m²/kg² - Escape velocity from Earth = 11.2 km/s; from Moon = 2.4 km/s - Moon has no atmosphere because its escape velocity is too low to retain gas molecules
4. Light and Its Phenomena - Light travels at 3 × 10⁸ m/s in vacuum - Reflection: angle of incidence = angle of reflection - Refraction: bending at medium boundary; refractive index n = c/v - Total Internal Reflection (TIR): basis of optical fibres and diamond brilliance; critical angle for glass ≈ 42°
5. Laws of Thermodynamics - Zeroth Law: thermal equilibrium concept — defines temperature - First Law: energy conservation; Q = ΔU + W - Second Law: heat flows naturally from hot to cold; entropy increases; no heat engine is 100% efficient
6. Ohm's Law and Electrical Energy - Ohm's Law: V = IR (voltage = current × resistance) - Power: P = VI = I²R = V²/R - Joule's Law of Heating: H = I²Rt - SI unit of electrical energy: kilowatt-hour (kWh); 1 kWh = 3.6 × 10⁶ J (one "unit" on electricity bill)
7. Electromagnetic Induction - Faraday (1831): a changing magnetic flux through a circuit induces an EMF - Principle behind generators, transformers, and induction motors - Lenz's Law: the induced current opposes the change causing it (negative sign in Faraday's law)
8. Sound Waves - Sound is a mechanical longitudinal wave requiring a material medium - Speed in air: 332 m/s at 0°C; ≈ 346 m/s at 25°C - Ultrasound (> 20,000 Hz): used in sonar, medical imaging, industrial flaw detection - Infrasound (< 20 Hz): detected by elephants; used in seismic monitoring
9. Electromagnetic Spectrum - Order by increasing frequency: Radio → Microwaves → Infrared → Visible (VIBGYOR) → Ultraviolet → X-rays → Gamma rays - All EM waves travel at c = 3 × 10⁸ m/s in vacuum - They differ in frequency and wavelength; relation: c = fλ
10. Medical Imaging — X-ray, CT, MRI - X-rays (Röntgen, 1895): penetrate soft tissue but absorbed by bones and metal; used for skeletal imaging - CT Scan: multiple X-ray images from different angles reconstructed into 3D cross-sections - MRI: uses strong magnetic fields (1.5–3 Tesla) and radio waves; images soft tissue without any radiation
11. PET Scan and Ultrasound - PET Scan: uses F-18 fluorodeoxyglucose (FDG); cancer cells consume more glucose → appear as "hot spots" - Positron emitted by F-18 annihilates with electron → two gamma photons detected - Ultrasound (USG): 2–15 MHz sound waves reflected from internal organs; safe in obstetrics and cardiac imaging
12. Radiation Safety — ALARA - ALARA principle: As Low As Reasonably Achievable - Time: minimise exposure duration - Distance: intensity ∝ 1/d² - Shielding: lead aprons for X-rays, concrete for nuclear reactors - Annual permissible dose for radiation workers = 20 mSv/year (ICRP)
13. Kepler's Laws of Planetary Motion - First Law: planets move in elliptical orbits with the Sun at one focus - Second Law: a line from Sun to planet sweeps equal areas in equal times (conservation of angular momentum) - Third Law: T² ∝ r³ (square of period proportional to cube of semi-major axis)
14. Semiconductor Physics and Band Theory - Conductors: overlapping valence and conduction bands (zero band gap) - Insulators: large band gap (> 3 eV) - Semiconductors (Si, Ge): small band gap (~1 eV); conductivity increases with temperature - p-n junction diode: allows current in one direction only (rectification) - LED: converts electrical energy to light at a p-n junction

Key facts

Newton's Law of Gravitation — F = Gm₁m₂/r² where G = 6.674 × 10⁻¹¹ N·m²/kg² — Escape velocity from Earth = 11.2 km/s; from Moon = 2.4 km/s
Light and Its Phenomena — Light travels at 3 × 10⁸ m/s in vacuum — Reflection: angle of incidence = angle of reflection
Laws of Thermodynamics — Zeroth Law: thermal equilibrium concept — defines temperature — First Law: energy conservation; Q = ΔU + W
Ohm's Law and Electrical Energy — Ohm's Law: V = IR (voltage = current × resistance) — Power: P = VI = I²R = V²/R — Joule's Law of Heating: H = I²Rt
Electromagnetic Induction — Faraday (1831): a changing magnetic flux through a circuit induces an EMF

Key Points at a Glance

1
Newton's Laws of Motion
- First Law (Law of Inertia): body at rest or in uniform motion continues unless acted upon by an external force
- Second Law: F = ma (force = mass × acceleration)
- Third Law: for every action there is an equal and opposite reaction
2
Law of Conservation of Energy
- Energy can neither be created nor destroyed — it can only be converted from one form to another
- Total mechanical energy (KE + PE) is conserved in the absence of non-conservative forces
3
Newton's Law of Gravitation
- F = Gm₁m₂/r² where G = 6.674 × 10⁻¹¹ N·m²/kg²
- Escape velocity from Earth = 11.2 km/s; from Moon = 2.4 km/s
- Moon has no atmosphere because its escape velocity is too low to retain gas molecules
4
Light and Its Phenomena
- Light travels at 3 × 10⁸ m/s in vacuum
- Reflection: angle of incidence = angle of reflection
- Refraction: bending at medium boundary; refractive index n = c/v
- Total Internal Reflection (TIR): basis of optical fibres and diamond brilliance; critical angle for glass ≈ 42°
5
Laws of Thermodynamics
- Zeroth Law: thermal equilibrium concept — defines temperature
- First Law: energy conservation; Q = ΔU + W
- Second Law: heat flows naturally from hot to cold; entropy increases; no heat engine is 100% efficient
6
Ohm's Law and Electrical Energy
- Ohm's Law: V = IR (voltage = current × resistance)
- Power: P = VI = I²R = V²/R
- Joule's Law of Heating: H = I²Rt
- SI unit of electrical energy: kilowatt-hour (kWh); 1 kWh = 3.6 × 10⁶ J (one "unit" on electricity bill)
7
Electromagnetic Induction
- Faraday (1831): a changing magnetic flux through a circuit induces an EMF
- Principle behind generators, transformers, and induction motors
- Lenz's Law: the induced current opposes the change causing it (negative sign in Faraday's law)
8
Sound Waves
- Sound is a mechanical longitudinal wave requiring a material medium
- Speed in air: 332 m/s at 0°C; ≈ 346 m/s at 25°C
- Ultrasound (> 20,000 Hz): used in sonar, medical imaging, industrial flaw detection
- Infrasound (< 20 Hz): detected by elephants; used in seismic monitoring
9
Electromagnetic Spectrum
- Order by increasing frequency: Radio → Microwaves → Infrared → Visible (VIBGYOR) → Ultraviolet → X-rays → Gamma rays
- All EM waves travel at c = 3 × 10⁸ m/s in vacuum
- They differ in frequency and wavelength; relation: c = fλ
10
Medical Imaging — X-ray, CT, MRI
- X-rays (Röntgen, 1895): penetrate soft tissue but absorbed by bones and metal; used for skeletal imaging
- CT Scan: multiple X-ray images from different angles reconstructed into 3D cross-sections
- MRI: uses strong magnetic fields (1.5–3 Tesla) and radio waves; images soft tissue without any radiation
11
PET Scan and Ultrasound
- PET Scan: uses F-18 fluorodeoxyglucose (FDG); cancer cells consume more glucose → appear as "hot spots"
- Positron emitted by F-18 annihilates with electron → two gamma photons detected
- Ultrasound (USG): 2–15 MHz sound waves reflected from internal organs; safe in obstetrics and cardiac imaging
12
Radiation Safety — ALARA
- ALARA principle: As Low As Reasonably Achievable
- Time: minimise exposure duration
- Distance: intensity ∝ 1/d²
- Shielding: lead aprons for X-rays, concrete for nuclear reactors
- Annual permissible dose for radiation workers = 20 mSv/year (ICRP)
13
Kepler's Laws of Planetary Motion
- First Law: planets move in elliptical orbits with the Sun at one focus
- Second Law: a line from Sun to planet sweeps equal areas in equal times (conservation of angular momentum)
- Third Law: T² ∝ r³ (square of period proportional to cube of semi-major axis)
14
Semiconductor Physics and Band Theory
- Conductors: overlapping valence and conduction bands (zero band gap)
- Insulators: large band gap (> 3 eV)
- Semiconductors (Si, Ge): small band gap (~1 eV); conductivity increases with temperature
- p-n junction diode: allows current in one direction only (rectification)
- LED: converts electrical energy to light at a p-n junction

Why does Topic 68 matter for RAS Science and Technology?

Topic 68 matters for RAS Science and Technology because it combines high past yield, repeatable formulas, and applied physics areas such as medical diagnostics, nuclear energy, electrical devices, and communication systems.

Topic 68 is the single highest-scoring topic in the entire Paper II Science & Technology unit with 66 marks over 6 years and an average of 13.2 marks per exam. It appeared in all five exam years.

According to the RPSC Mains syllabus, Paper II is a 200-mark General Knowledge and General Studies paper that includes General Science and Technology.

Physics shapes the examiner's favourite application zones: medical diagnostics, nuclear energy, everyday electricity, electromagnetic communication, semiconductors, and satellite-linked technologies. The RPSC syllabus explicitly places nuclear technology, telecommunication, electromagnetic waves, communication systems, computers, information technology, e-governance, renewable and non-renewable energy, and related human-health topics inside the same broad Science and Technology frame.

2026 High-Priority Sub-domains:

Medical diagnostics (MRI, CT, PET, Ultrasound) — appeared in 2021, heavily expected again
Nuclear fission/fusion — with India's expanding nuclear programme and ITER membership
EM waves and their applications — consistently tested
Semiconductor/band theory — tested 2016; likely revisited in 2026

Strategy for 50-word answers: Use the formulaic approach — state the law/principle → key equation or number → one practical application. Physics questions reward specific numbers (velocities, frequencies, energies), but the number must serve the explanation rather than sit as decoration.

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PREDICTED Predicted RAS Questions

Based on PYQ trends and 2026 syllabus analysis

1 5M What is MRI? How does it differ from CT scan and X-ray in its working principle and clinical applications? 5 marks · 50 words

Model Answer

MRI (Magnetic Resonance Imaging) uses a 1.5–3 Tesla magnetic field and radiofrequency pulses to image hydrogen atoms in body tissues — no ionising radiation. Best for soft tissues (brain tumours, MS, ligaments). CT scan uses multiple X-ray beams reconstructed into 3D slices — good for bone, chest, internal organs; moderate radiation dose. X-ray uses single X-ray beam — quick, low cost, ideal for bone fractures and chest infections; highest radiation per study area.

~50 words • 5 marks