Magnetic Field and Magnetic Field Lines MCQs

Q1. Consider the following statements about magnetic field lines: (I) They are closed continuous loops. (II) They never intersect each other. (III) They are closer where the field is weaker. Which of the above statement(s) is/are correct?

• A.Only III
• B.Only I
• C.None of these
• D.Only I and II

Answer: D. Only I and II

NCERT fact (physics, chapter 'Magnetic Effects of Electric Current'): Correct: I, II. Why option D fits - Correct. Distractor analysis: option A) 'Only III' doesn't hold - Re-check each statement individually; option B) 'Only I' is wrong because Re-check each statement individually. Final answer - D) Only I and II.

Q2. A long straight transmission wire carries a steady current of 5 A; what is the magnitude of the magnetic field it produces at a point 0.1 m away from it?

• A.4*10^-9 T
• B.6.28*10^-5 T
• C.5*10^-6 T
• D.1*10^-5 T

Answer: D. 1*10^-5 T

For a long straight wire B = mu0*I/(2*pi*r). Here mu0 = 4*pi*10^-7, I = 5 A, r = 0.1 m, so B = (4*pi*10^-7 * 5)/(2*pi*0.1) = (2*10^-7*5)/0.1 = 1*10^-5 T.

Q3. An electrician runs a single straight cable carrying 10 A; at a perpendicular distance of 0.05 m from the cable, what magnetic field strength does a sensor read?

• A.1*10^-9 T
• B.4*10^-5 T
• C.2.51*10^-4 T
• D.2*10^-5 T

Answer: B. 4*10^-5 T

For a long straight wire B = mu0*I/(2*pi*r). Here mu0 = 4*pi*10^-7, I = 10 A, r = 0.05 m, so B = (4*pi*10^-7 * 10)/(2*pi*0.05) = (2*10^-7*10)/0.05 = 4*10^-5 T.