An electric motor has a circular armature with 250 turns of radius 5.0 cm throug

Question

An electric motor has a circular armature with 250 turns of radius 5.0 cm through which an 8.0-Acurrent flows. The maximum torque exerted on the armature as it turns in a magnetic field is 1.9 Nm.

Part A What is the orientation of the armature coil in the magnetic field when this torque is exerted on it (choose A or B)?

Part B Determine the magnitude of the field.

A. The angle between B and the normal of the coils surface equals zero. B. The angle between B and the normal of the coils surface equals 90.

Explanation / Answer

1)

N = 250

r = 0.05 m

I = 8 A

T = 1.9 N.m

Maximum torque :

T = BINA

where A = area of the coil = pi*r^2

For maximum torque,

the magnetic field must be perpendicular to the normal of the coil

So, the angle between B and the normal of the coils surface equals 90. <---- option B
2)

Now,

T = BINA

So, 1.9 = B*8*250*(pi*0.05^2)

So, B = 0.121 T <------answer

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