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Question #43312

Difficulty: 8/10 | 🟢 Free

The Question

A stationary circular conducting loop of radius rr, mass mm, and specific heat capacity ss is placed in a uniform magnetic field directed perpendicular to the plane of the loop. The magnetic field varies with time tt as B(t)=βtB(t) = \beta t, where β\beta is a positive constant. The initial resistance of the loop at t=0t=0 is R0R_0. The resistance varies with temperature TT as R(T)=R0[1+α(T−T0)]R(T) = R_0[1 + \alpha(T - T_0)], where T0T_0 is the initial temperature and α\alpha is the temperature coefficient of resistance. Assuming no heat is lost to the surroundings, negligible thermal expansion of the loop's dimensions, and negligible self-inductance, what is the time t0t_0 required for the resistance of the loop to become 2R02R_0?

Options

  • A.

    3msR02π2r4β2α\frac{3 m s R_0}{2 \pi^2 r^4 \beta^2 \alpha}

  • B.

    msR0π2r4β2α\frac{m s R_0}{\pi^2 r^4 \beta^2 \alpha}

  • C.

    4msR03π2r4β2α\frac{4 m s R_0}{3 \pi^2 r^4 \beta^2 \alpha}

  • D.

    2msR0π2r4β2α\frac{2 m s R_0}{\pi^2 r^4 \beta^2 \alpha}

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Variable_Constraint

Develop pure symbolic logic skills by mastering algebraic constraints without relying on numerical data.

Concepts Behind This Question

AC Generator Back-EMF|Relaxation Time Dependency|Resistivity vs Resistance

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to recognize and model the non-linear coupling between Joule heating and temperature-dependent resistance.

A standard trap is to treat the resistance as a constant R0R_0 throughout the heating process, which leads to the incorrect answer B.

Another trap is to incorrectly average the power dissipation over the interval (leading to answer C).

Because the power P=ε2/RP = \varepsilon^2/R is inversely proportional to the instantaneous resistance, the rate of temperature increase slows down as the loop heats up.

Students must set up a differential equation relating RdRR dR to dtdt rather than using macroscopic algebraic energy balances, properly integrating the non-ohmic transient behavior.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterElectromagnetic Induction - Class 12
ConceptAC Generator Back-EMF
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree