11. Assume that a motor in which the coil has a total resistance of 40 ohm is su

Question

11. Assume that a motor in which the coil has a total resistance of 40 ohm is supplied a voltage of 120 V. When the motor is running at its maximum speed, the back emf is 60 V. (a) What is the power? Answer: (b) What is the power when the motor is running at one-half the maximum speed? Answer: 12. Find the magnetic energy stored in the air gap between two very large magnetic pole pieces, one North, one South, each with an area of 100 cm^2. Assume the 0.05 T magnetic field is uniform within the 3-cm gap. Answer:

Explanation / Answer

11) current i = V/R = 60/40 = 1.5 A..

A) power P = i*V = 1.5*60 = 90 W...

B) emf induced e = B*L*v...here v is the speed...if speed becomes half emf also becomes half..i.e emf = 30 V
current i = 30/40 = 0.75 A..
power p = i*V = 0.75*30 = 22.5 V

12) Energy per unit volume U/(A*d) = B^2/(2*mu_o)

energy stored is U = B^2*A*d/(2*mu_o) = (0.05^2*100*10^-4*3*10^-2)/(2*3.142*4*10^-7) = 0.298 J

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