MCAT Physics 5 — Questions and Answers
Question 1: A proton (charge +e) moves perpendicular to a magnetic field B with speed v. What is the radius of its circular orbit? (mass = m)
- mv/(eB) (Correct answer)
- eBv/m
- eB/(mv)
- mv²/(eB)
Correct answer: mv/(eB)
The magnetic force provides centripetal force: evB = mv²/r → r = mv/(eB).
Question 2: Which phenomenon explains why the sky appears blue?
- Reflection of light by clouds
- Absorption of red wavelengths by oxygen
- Rayleigh scattering of shorter wavelengths (Correct answer)
- Diffraction of light around Earth's curvature
Correct answer: Rayleigh scattering of shorter wavelengths
Rayleigh scattering is proportional to 1/λ⁴, so shorter (blue) wavelengths scatter far more than longer (red) wavelengths.
Question 3: 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)
- 1 m/s
- 2 m/s (Correct answer)
- 5 m/s
- 10 m/s
Correct answer: 2 m/s
Power = Fv = mgv; v = P/(mg) = 1000/(50 × 10) = 1000/500 = 2 m/s.
Question 4: In the photoelectric effect, increasing the intensity of incident light (without changing frequency) will:
- Increase the kinetic energy of emitted electrons
- Increase the number of emitted electrons per second (Correct answer)
- Increase the stopping potential
- Have no effect because intensity doesn't affect the photoelectric effect
Correct 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.
Question 5: Two parallel plates are separated by 2 mm and connected to a 100 V battery. What is the electric field between the plates?
- 200,000 V/m
- 50,000 V/m (Correct answer)
- 0.2 V/m
- 500 V/m
Correct answer: 50,000 V/m
E = V/d = 100 / (2 × 10⁻³) = 50,000 V/m.
Question 6: 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?
- 600 J
- 200 J
- 400 J (Correct answer)
- 800 J
Correct answer: 400 J
W = PΔV = 1 × 10⁵ × (6 − 2) × 10⁻³ = 1 × 10⁵ × 4 × 10⁻³ = 400 J.
Question 7: 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?
- 40 cm
- 60 cm
- 13.3 cm (Correct answer)
- 20 cm
Correct 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.
MCAT Physics 5
A proton (charge +e) moves perpendicular to a magnetic field B with speed v.
What is the radius of its circular orbit? (mass = m)