A-Level H2 Physics 3 — Questions and Answers
Question 1: What is Coulomb's Law?
- F = GMm/r²
- F = BIL
- F = kQ₁Q₂/r² (Correct answer)
- F = qvB
Correct answer: F = kQ₁Q₂/r²
Coulomb's Law: F = kQ₁Q₂/r² (or F = Q₁Q₂/4πε₀r²), giving the electrostatic force between two point charges, where k = 9×10⁹ N m² C⁻².
Question 2: A particle of charge q moves with velocity v perpendicular to a magnetic field B. What force acts on it?
- F = qE
- F = qvB (Correct answer)
- F = mv²/r
- F = qv/B
Correct answer: F = qvB
The magnetic force on a moving charge: F = qvB sin θ. When velocity is perpendicular to B (θ = 90°), sin θ = 1, so F = qvB.
Question 3: What is meant by 'half-life' in radioactive decay?
- The time for half the atoms to stop emitting radiation
- The time for the activity (and number of undecayed nuclei) to halve (Correct answer)
- The average lifetime of a radioactive atom
- The time for all atoms to decay
Correct answer: The time for the activity (and number of undecayed nuclei) to halve
Half-life (t½) is the time taken for the number of undecayed nuclei (or the activity) of a radioactive sample to decrease to half its original value.
Question 4: A simple pendulum has period T = 2π√(L/g). If L is quadrupled, how does T change?
- T doubles (Correct answer)
- T quadruples
- T halves
- T is unchanged
Correct answer: T doubles
T ∝ √L. If L is quadrupled (×4), T changes by √4 = 2. So T doubles.
Question 5: What is the phenomenon of 'interference' in waves?
- A wave reflecting off a surface
- The bending of waves around obstacles
- The superposition of two coherent waves producing regions of constructive and destructive interference (Correct answer)
- The change in frequency due to motion
Correct answer: The superposition of two coherent waves producing regions of constructive and destructive interference
Wave interference occurs when two coherent waves superpose: where crests meet (in phase), constructive interference creates large amplitude; where crest meets trough (antiphase), destructive interference creates zero amplitude.
Question 6: What is the relationship between electric field strength E and potential V?
- E = V²/r
- E = -dV/dr (Correct answer)
- E = V/r
- E = Vr
Correct answer: E = -dV/dr
Electric field strength is the negative rate of change of electric potential: E = -dV/dr (or E = -ΔV/Δr for uniform fields). The field points in the direction of decreasing potential.
A-Level H2 Physics 3
What is Coulomb's Law?