# Use it to predict the polarity

(a) State Lenz’s law. Use it to predict the polarity of the capacitor in the situation given below:

(b)A jet plane is travelling towards west at a speed of 1800 km/h.
(i) Estimate voltage difference developed between the ends of the wing having a span of 25 m if the earth’s magnetic field at the location has a magnitude of 5\times { 10 }^{ -4 } T and dip angle is 30°.
(ii)How will the voltage developed be affected if the jet changes its direction from west to north ? [O.D. I, II, III 2015]

(a) Lenz’s law: The polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it. Polarity A —> (+ve); B —> (-ve)
(b) (i) V=Blv
Here B = vertical component of Earth’ s magnetic field
$B=(5\times { 10 }^{ -4 }\sin { { 30 }^{ 0 } } )T$
$=2.5\times { 10 }^{ -4 }T$
$[\therefore V=\left[ 2.5\times {{10}^{-4}}\times 25\times \frac{1800\times {{10}^{3}}}{60\times 60} \right]]$volts
=3.125 volts
(ii)Now B = horizontal component of Earth’s magnetic field
$=B\cos { { 30 }^{ 0 } } =\frac { B\sqrt { 3 } }{ 2 }$
$\therefore V’=\sqrt { 3 } V=1.732\times 3.125volt$
$\cong 5.4volt$