OAT Physics Principles 5 — Questions and Answers
Question 1: A charge of +2 μC is placed 3 cm from a charge of −2 μC. What is the magnitude of the electrostatic force between them? (k = 9 × 10⁹ N·m²/C²)
- 0.4 N
- 40 N (Correct answer)
- 4 N
- 400 N
Correct answer: 40 N
F = kq₁q₂/r² = (9×10⁹)(2×10⁻⁶)(2×10⁻⁶)/(0.03)² = 36×10⁻³/9×10⁻⁴ = 40 N.
Question 2: Which of the following best explains why a spinning ice skater speeds up when they pull their arms in?
- Torque increases as radius decreases
- Conservation of angular momentum requires angular velocity to increase as moment of inertia decreases (Correct answer)
- Friction between the ice and skate increases rotational speed
- Kinetic energy is created by the inward arm movement
Correct answer: Conservation of angular momentum requires angular velocity to increase as moment of inertia decreases
With no external torque, L = Iω is constant; pulling arms in decreases I, so ω must increase.
Question 3: A sound source produces a 400 Hz tone. An observer moves toward the source at 34 m/s. If the speed of sound is 340 m/s, what frequency does the observer hear?
- 360 Hz
- 400 Hz
- 440 Hz (Correct answer)
- 480 Hz
Correct answer: 440 Hz
f' = f(v + v_observer)/v = 400 × (340 + 34)/340 = 400 × 374/340 ≈ 440 Hz.
Question 4: In a transformer, the primary coil has 100 turns and the secondary has 500 turns. If the primary voltage is 120 V, what is the secondary voltage?
- 24 V
- 120 V
- 600 V (Correct answer)
- 6000 V
Correct answer: 600 V
V_s/V_p = N_s/N_p → V_s = 120 × (500/100) = 600 V.
Question 5: A fluid flows through a pipe that narrows from a cross-sectional area of 0.04 m² to 0.01 m². If the initial flow speed is 2 m/s, what is the speed in the narrower section?
- 0.5 m/s
- 2 m/s
- 4 m/s
- 8 m/s (Correct answer)
Correct answer: 8 m/s
By the continuity equation: A₁v₁ = A₂v₂ → v₂ = (0.04 × 2)/0.01 = 8 m/s.
Question 6: A radioactive isotope has a half-life of 10 days. What fraction of the original sample remains after 30 days?
- 1/2
- 1/4
- 1/8 (Correct answer)
- 1/16
Correct answer: 1/8
30 days = 3 half-lives; fraction remaining = (1/2)³ = 1/8.
Question 7: Which thermodynamic process occurs at constant pressure?
- Isothermal
- Isochoric
- Isobaric (Correct answer)
- Adiabatic
Correct answer: Isobaric
An isobaric process occurs at constant pressure, so work done by the gas equals PΔV.
OAT Physics Principles 5
A charge of +2 μC is placed 3 cm from a charge of −2 μC.
What is the magnitude of the electrostatic force between them? (k = 9 × 10⁹ N·m²/C²)