Physics Electricity and Magnetism 3 — Questions and Answers
Question 1: A long straight wire carries 5 A of current. What is the magnetic field at a distance of 0.1 m from the wire? (μ₀ = 4π×10⁻⁷ T·m/A)
- 1×10⁻⁵ T (Correct answer)
- 10⁻⁶ T
- 2π×10⁻⁵ T
- 10⁻⁷ T
Correct answer: 1×10⁻⁵ T
B = μ₀I/(2πr) = (4π×10⁻⁷ × 5)/(2π × 0.1) = 10⁻⁵ T.
Question 2: Which of the following best describes a diamagnetic material?
- Strongly attracted to magnetic fields
- Weakly attracted to magnetic fields
- Weakly repelled by magnetic fields (Correct answer)
- Retains magnetization after field removal
Correct answer: Weakly repelled by magnetic fields
Diamagnetic materials have a small negative susceptibility and are weakly repelled by external magnetic fields.
Question 3: An EMF of 20 V is induced in a coil when the current changes at a rate of 4 A/s. What is the self-inductance of the coil?
- 80 H
- 5 H (Correct answer)
- 0.2 H
- 16 H
Correct answer: 5 H
EMF = L(dI/dt), so L = EMF/(dI/dt) = 20/4 = 5 H.
Question 4: In a series RLC circuit at resonance, the impedance is:
- Zero
- Equal to R only (Correct answer)
- Equal to X_L - X_C
- Maximum
Correct answer: Equal to R only
At resonance X_L = X_C, so they cancel and the total impedance Z = R.
Question 5: A proton moves east in a uniform magnetic field directed north. In which direction is the magnetic force on the proton?
- Downward
- Upward (Correct answer)
- West
- South
Correct answer: Upward
Using F = qv×B with v east (+x) and B north (+y), F = q(x̂ × ŷ) = qẑ, which is upward.
Question 6: The electric potential at a distance r from a point charge Q is given by:
- kQ/r²
- kQ/r (Correct answer)
- kQ²/r
- kQ·r
Correct answer: kQ/r
Electric potential V = kQ/r, which decreases as 1/r (not 1/r² like the field).
Question 7: Three resistors of 6 Ω each are connected in parallel. What is the equivalent resistance?
- 18 Ω
- 6 Ω
- 3 Ω
- 2 Ω (Correct answer)
Correct answer: 2 Ω
1/R_eq = 1/6 + 1/6 + 1/6 = 3/6 = 1/2, so R_eq = 2 Ω.
Physics Electricity and Magnetism 3
A long straight wire carries 5 A of current.
What is the magnetic field at a distance of 0.1 m from the wire? (μ₀ = 4π×10⁻⁷ T·m/A)