Exams › JEE Advanced › Physics

A capacitor with circular plates of radius R is charged by a constant current I. Which of the following statements are correct? (A) The Poynting vector points radially inward everywhere on the cylindrical surface around the capacitor. (B) The magnetic field inside the capacitor is clockwise as viewed from above (in the direction of current flow). (C) The magnetic energy density inside the capacitor is increasing with time. (D) The flux of the Poynting vector through the cylindrical surface equals the rate of change of stored electric energy.

1. Poynting vector points radially inward everywhere on the circle.
2. Magnetic field is clockwise as seen from above.
3. Magnetic energy density is increasing with time.
4. Poynting vector flux is equal to the rate of change of stored energy.

Correct answer: Poynting vector flux is equal to the rate of change of stored energy.

Solution

Poynting vector S = (1/mu0)(E x B) points radially inward (energy flows into the capacitor). Magnetic field from constant displacement current I is constant, so magnetic energy density is constant, not increasing. Poynting flux equals rate of change of stored (electric) energy. Options A and D are correct.

Related JEE Advanced Physics questions

• A pulse of light of duration 100 ns is absorbed completely by a small object initially at rest. Power of the pulse is 30 mW and the speed of light is 3 × 10⁸ m/s. The final momentum of object is
• A floodlight filtered to pass only red light produces a sinusoidal plane electromagnetic wave whose electric field component is given by Eₓ = 36 sin(1.20 * 10⁷ * z - 3.6 * 10¹⁵ * t) V/m. The average intensity of this beam is closest to sqrt(n) W/m², where n is an integer. Find n.
• The electric field of an electromagnetic wave is E = 20 sin(w*(t - x/c)) j V/m, where w is the angular frequency and c is the speed of light. Calculate the total electromagnetic energy stored in a volume of 5 * 10^(-4) m³. (Given: epsilon₀ = 8.85 * 10^(-12) C²/(N*m²))
• A plane electromagnetic wave of frequency 25 GHz propagates in vacuum along the +z direction. At a particular point in space and time, the magnetic field is B = 5 * 10^(-8) T in the +x direction. What is the corresponding electric field vector E? (Speed of light c = 3 * 10⁸ m/s)
• The electric field component of an electromagnetic wave in vacuum is given by E = (j-hat + b*k-hat) * 10⁻³ * sin[10⁷(x + 2y + 3z - beta*t)] in SI units. Using c = 3 * 10⁸ m/s, which of the following statements are correct?
• The magnetic field of a plane electromagnetic wave is given by B = 3 * 10^(-8) * sin[200*pi*(y + c*t)] * i_hat T, where c = 3 * 10⁸ m/s. What is the corresponding electric field?

⚔️ Practice JEE Advanced Physics free + battle 1v1 →