UPSC NDA Physics Flashcards

24 sample cards from the Physics deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What is latent heat?

   Latent heat is the heat absorbed or released by a substance during a change of state (melting, freezing, boiling, condensing) at constant temperature. Latent heat of fusion is for solid↔liquid; latent heat of vaporisation is for liquid↔gas.

2. Convert between Celsius and Kelvin scales.

   K = °C + 273.15 (often approximated as +273). Absolute zero is 0 K = −273.15°C. Also °F = (9/5)°C + 32.

3. What is the difference between reflection and refraction of light?

   Reflection is the bouncing back of light from a surface (angle of incidence = angle of reflection). Refraction is the bending of light as it passes from one medium to another due to a change in its speed.

4. State the laws of reflection.

   1) The angle of incidence equals the angle of reflection. 2) The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

5. State Snell's Law of refraction.

   The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media: n₁ sin θ₁ = n₂ sin θ₂, where n is the refractive index.

6. What is total internal reflection and one condition for it?

   Total internal reflection occurs when light travelling from a denser to a rarer medium is completely reflected back. It requires the angle of incidence to exceed the critical angle and the light to travel from denser to rarer medium.

7. Differentiate a concave and a convex lens.

   A convex (converging) lens is thicker in the middle and converges parallel rays to a real focus; used to correct hypermetropia. A concave (diverging) lens is thinner in the middle and diverges rays; used to correct myopia.

8. What is the colour sequence of the visible spectrum?

   VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red. Violet has the shortest wavelength and red the longest.

9. List the electromagnetic spectrum in order of increasing wavelength.

   Gamma rays, X-rays, Ultraviolet, Visible light, Infrared, Microwaves, Radio waves. Increasing wavelength means decreasing frequency and energy.

10. State Ohm's Law and give its equation.

    At constant temperature, the current through a conductor is directly proportional to the potential difference across it: V = IR, where V is voltage, I is current and R is resistance.

11. Give the SI units of current, voltage, resistance and charge.

    Current: ampere (A); Voltage/potential difference: volt (V); Resistance: ohm (Ω); Electric charge: coulomb (C). 1 A = 1 C/s.

12. How are resistors combined in series and in parallel?

    In series: R = R₁ + R₂ + R₃ (resistances add). In parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃, and the total resistance is less than the smallest individual resistance.

13. State the formula for electrical power.

    P = VI = I²R = V²/R, where P is power (watts), V is voltage, I is current and R is resistance. Electrical energy = power × time (commonly measured in kilowatt-hours).

14. What is the difference between AC and DC current?

    Alternating Current (AC) periodically reverses direction (e.g. mains supply, 50 Hz in India). Direct Current (DC) flows in one constant direction (e.g. from batteries and cells).

15. State Faraday's Law of Electromagnetic Induction.

    A changing magnetic flux through a circuit induces an electromotive force (EMF). The magnitude of the induced EMF is proportional to the rate of change of magnetic flux. It is the principle of the generator.

16. State Lenz's Law.

    The direction of an induced current is always such that it opposes the change in magnetic flux that produced it. It is a consequence of the conservation of energy.

17. What is the right-hand thumb rule (Maxwell's rule)?

    If the right-hand thumb points in the direction of conventional current in a straight wire, the curled fingers give the direction of the magnetic field around the wire.

18. Differentiate a transverse wave and a longitudinal wave.

    In a transverse wave the particles vibrate perpendicular to the direction of wave travel (e.g. light, water waves). In a longitudinal wave particles vibrate parallel to the direction of travel through compressions and rarefactions (e.g. sound waves).

19. Write the wave equation relating speed, frequency and wavelength.

    v = fλ, where v is wave speed (m/s), f is frequency (Hz) and λ is wavelength (m).

20. What is the approximate speed of sound in air and how does it compare with light?

    Sound travels at about 340 m/s in air at room temperature. Light travels at about 3×10⁸ m/s in vacuum, roughly a million times faster, which is why we see lightning before hearing thunder.

21. What is the audible range of frequency for humans, and what are infrasonic and ultrasonic sounds?

    Humans hear roughly 20 Hz to 20,000 Hz. Infrasonic sound is below 20 Hz; ultrasonic sound is above 20,000 Hz (used in SONAR and medical imaging).

22. What is the Doppler Effect?

    The apparent change in the frequency (pitch) of a wave due to the relative motion between the source and the observer. Frequency appears higher when they approach and lower when they recede.

23. Define refractive index of a medium.

    Refractive index (n) is the ratio of the speed of light in vacuum to the speed of light in the medium: n = c/v. A higher refractive index means light slows down and bends more.

24. What are the SI base units for length, mass, time, temperature and electric current?

    Length: metre (m); Mass: kilogram (kg); Time: second (s); Temperature: kelvin (K); Electric current: ampere (A). The other two base units are mole (amount of substance) and candela (luminous intensity).

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