A wheel with 8 metallic spokes each 50cm long is rotated with a speed of 120rev./min . The earth's magnetic field at the place is 0.4G and the angle of dip is 60 . Calculate the emf induced between the axle and the rim of the wheel. How will the value of emf be affected if the number of spokes were increased ?

Vertical component of earth magnetic field is given by:Bv= BsinδB = 0.4 Gδ = 60Bv = Bsin60Bv = 0.4×32Bv= 0.692 G = 0.692 ×10-4 teslalength of the stroke will act as the diameter.radius r = 502r = 25cm = 0.25 mArea a = πr2a = 3.14×0.25×0.25a = 0.19 m2
ω = 2πνν = 120 revolution per min = 2 rev per secondω = 2×3.14×2 ω= 12.56 rad per secondinduced emf is one stoke e = Bvaωcosωte = 0.692×10-4×0.19×12.56 cosωte = 0.1×10-3 volt  = 0.1 milli voltfor the 8 stokese = 8×0.1 milli volte = 0.8 milli voltas no of stokes increases.induced emg increases.

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