Physics Flashcards

A photon has a wavelength of 400 nm. What is its energy? (h = 6.63 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s)

Answer: 4.97 × 10⁻¹⁹ J

E = hc/λ = (6.63 × 10⁻³⁴ × 3 × 10⁸) / (400 × 10⁻⁹) ≈ 4.97 × 10⁻¹⁹ J.

An ideal gas undergoes an isothermal compression. Which of the following remains constant?

Answer: Temperature

By definition, isothermal means constant temperature; kinetic energy of molecules (∝ T) also stays constant, but 'temperature' is the direct answer.

A wave on a string has a frequency of 50 Hz and a wavelength of 0.4 m. What is the wave speed?

Answer: 20 m/s

Wave speed v = fλ = 50 × 0.4 = 20 m/s.

A 70 kg person stands on a scale in an elevator accelerating upward at 2 m/s². What does the scale read? (g = 10 m/s²)

Answer: 840 N

Normal force N = m(g + a) = 70 × (10 + 2) = 70 × 12 = 840 N.

Light travels from glass (n = 1.5) into air (n = 1.0). If the angle of incidence is 30°, what is the angle of refraction?

Answer: 48.6°

By Snell's law, n₁sinθ₁ = n₂sinθ₂: 1.5 × sin30° = 1.0 × sinθ₂, so sinθ₂ = 0.75, θ₂ ≈ 48.6°.

Which thermodynamic process occurs when a gas expands against zero external pressure (free expansion)?

Answer: Neither heat nor work is exchanged with the surroundings

In free expansion, no work is done (P_ext = 0) and for an ideal gas in an insulated container no heat is exchanged, so ΔU = 0.

A 0.5 kg ball is thrown vertically upward with an initial velocity of 20 m/s. What maximum height does it reach? (g = 10 m/s²)

Answer: 20 m

Using v² = v₀² − 2gh, at maximum height v = 0: h = v₀²/(2g) = 400/20 = 20 m.