Physics Flashcards

A proton (charge +e) moves perpendicular to a magnetic field B with speed v. What is the radius of its circular orbit? (mass = m)

Answer: mv/(eB)

The magnetic force provides centripetal force: evB = mv²/r → r = mv/(eB).

Which phenomenon explains why the sky appears blue?

Answer: Rayleigh scattering of shorter wavelengths

Rayleigh scattering is proportional to 1/λ⁴, so shorter (blue) wavelengths scatter far more than longer (red) wavelengths.

A 1000 W electric motor lifts a 50 kg load. What is the maximum speed at which the load can be raised? (g = 10 m/s²)

Answer: 2 m/s

Power = Fv = mgv; v = P/(mg) = 1000/(50 × 10) = 1000/500 = 2 m/s.

In the photoelectric effect, increasing the intensity of incident light (without changing frequency) will:

Answer: Increase the number of emitted electrons per second

Higher intensity means more photons per second, which ejects more electrons per second, but does not change each electron's kinetic energy.

Two parallel plates are separated by 2 mm and connected to a 100 V battery. What is the electric field between the plates?

Answer: 50,000 V/m

E = V/d = 100 / (2 × 10⁻³) = 50,000 V/m.

A gas expands isobarically (at constant pressure) from 2 L to 6 L at a pressure of 1 × 10⁵ Pa. How much work does the gas do?

Answer: 400 J

W = PΔV = 1 × 10⁵ × (6 − 2) × 10⁻³ = 1 × 10⁵ × 4 × 10⁻³ = 400 J.

A converging lens with focal length 20 cm forms a virtual image 40 cm from the lens on the same side as the object. What is the object distance?

Answer: 13.3 cm

Using 1/f = 1/do + 1/di with di = −40 cm (virtual): 1/20 = 1/do − 1/40 → 1/do = 1/20 + 1/40 = 3/40, do ≈ 13.3 cm.