Gaokao Exam Gaokao Physics: Electromagnetism 7 — Questions and Answers
Question 1: A charged particle with charge q and mass m is accelerated from rest through a potential difference V, then enters a magnetic field B perpendicular to the field. The radius of its circular motion is:
- √(2mV/q)/B (Correct answer)
- √(2qV/m)/B
- 2mV/(qB)
- qBV/(2m)
Correct answer: √(2mV/q)/B
From ½mv² = qV: v = √(2qV/m); then r = mv/(qB) = m√(2qV/m)/(qB) = √(2mV/q)/B.
Question 2: Kirchhoff's current law (KCL) is based on the principle of:
- Conservation of charge (Correct answer)
- Conservation of energy
- Ohm's law
- Faraday's law
Correct answer: Conservation of charge
KCL states that the algebraic sum of currents at any junction is zero, reflecting charge conservation.
Question 3: The SI unit of inductance is the Henry (H). 1 H is equivalent to:
- 1 V·s/A
- 1 T·m²/A
- 1 Ω·s
- All of the above are equivalent (Correct answer)
Correct answer: All of the above are equivalent
1 H = 1 V·s/A (from EMF = L dI/dt) = 1 T·m²/A (from flux linkage) = 1 Ω·s; all three expressions are dimensionally equivalent.
Question 4: An electric dipole consists of charges +q and −q separated by distance d. The electric field at the midpoint between the charges is directed:
- From +q toward −q (Correct answer)
- From −q toward +q
- Perpendicular to the dipole axis
- Zero
Correct answer: From +q toward −q
Both charges produce fields at the midpoint; the +q field points away from +q (toward −q) and the −q field also points toward −q; they add in the same direction.
Question 5: A generator converts mechanical energy to electrical energy. The process is governed by:
- Faraday's law of electromagnetic induction (Correct answer)
- Ohm's law
- Ampere's law
- Gauss's law
Correct answer: Faraday's law of electromagnetic induction
Generators work by rotating coils in magnetic fields, inducing EMF according to Faraday's law.
Question 6: The dielectric constant of a material inserted between capacitor plates, compared to vacuum, causes the capacitance to:
- Increase by a factor equal to the dielectric constant (Correct answer)
- Decrease by a factor equal to the dielectric constant
- Stay the same
- Double regardless of the material
Correct answer: Increase by a factor equal to the dielectric constant
C = κε₀A/d; inserting a dielectric increases capacitance by factor κ (the dielectric constant).
Question 7: The phase relationship between current and voltage in a purely inductive AC circuit is:
- Current lags voltage by 90° (Correct answer)
- Current leads voltage by 90°
- Current and voltage are in phase
- Current lags voltage by 45°
Correct answer: Current lags voltage by 90°
For a pure inductor, V = L(dI/dt); if I = I₀sinωt then V = LωI₀cosωt, so voltage leads current by 90° (or current lags voltage by 90°).
Gaokao Exam Gaokao Physics: Electromagnetism 7
A charged particle with charge q and mass m is accelerated from rest through a potential difference V, then enters a magnetic field B perpendicular to the field.
The radius of its circular motion is: