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← IIT-JEE 2010 · Paper 1
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IIT-JEE 2010 · Paper 1, Physics question 74

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Paragraph for Questions 73 to 74

Electrical resistance of certain materials, known as superconductors, changes abruptly from a nonzero value to zero as their temperature is lowered below a critical temperature Tc(0)T_c(0). An interesting property of superconductors is that their critical temperature becomes smaller than Tc(0)T_c(0) if they are placed in a magnetic field, i.e., the critical temperature Tc(B)T_c(B) is a function of the magnetic field strength B. The dependence of Tc(B)T_c(B) on B is shown in the figure.

T_c(B) against B: starts at T_c(0) on the vertical axis and decreases, falling ever more steeply, to zero at a finite B.

A superconductor has Tc(0)=100T_c(0)=100 K. When a magnetic field of 7.5 Tesla is applied, its TcT_c decreases to 75 K. For this material one can definitely say that when

Choose one option
Answer and worked solution

Official answer: B

Monotonic decrease. Tc(B)T_c(B) falls from 100 K at B=0B=0 to 75 K at 7.5 T and keeps decreasing with BB.

  • At 5 T (between 0 and 7.5 T): 75 K<Tc<10075\text{ K}<T_c<100 K for certain. The exact value (e.g. 80 K) cannot be read off without knowing the curve's shape. (B) true, (A) not definite.
  • At 10 T: Tc<75T_c<75 K, so (C) is false, and the exact 70 K in (D) cannot be asserted.

(B)

Topics:Temperature dependence of resistance
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