Q. Two parallel rails of a railways track insulated from each other and with the ground are connected to a millivoltmeter. The distance between the rails is one metre. A train is travelling with a velocity of 72 km-$h^{- 1}$ along the track. The reading of the millivotmeter (in mV) is : (Vertical component of the earth’s magnetic induction is$\ 2 \times 10^{- 5}T)$
A) 1.44
B) 0.72
C) 0.4
D) 0.2
Explanation
(c) Induced emf $e = Bvl$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ v = velocity\ of\ train$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 72 \times \frac{5}{18} = 20{ms}^{- 1}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 2 \times 10^{- 5} \times 20 \times 1$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 2 \times 10^{- 5} \times 20$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 40 \times 10^{- 5}\ V$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 40 \times 10^{- 2}mV = 0.4mV$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.