13) A 80. volt DC motor draws 4.0 A at its operating speed of 1500. rpm with an

Question

13) A 80. volt DC motor draws 4.0 A at its operating speed of 1500. rpm with an 13) armature resistance of 4.0 . (a) What is the starting current? (b) What is the back emf when running normally? (c) What series resistance could be added to limit the starting current to 10. A? 14) A circular loop of wire of cross-sectional area 0.12 m2 consists of 200. tuns, each carrying 14) 0.50 A. It is placed in a magnetic field of 0.050 T oriented at 30° to the plane of the loop. What torque acts on the loop? 15) An eagle, with a wingspread of 2.0 m, flies due North at 8.0 m/s in a region where the 15) vertical component of the earth's magnetic field is 0.2 × 10-4 T. What emf would be developed between the eagle's wing tips? (It has been speculated that this phenomenon could play a role in the navigation of birds, but the effect is too small, in all likelihood.)

Explanation / Answer

13.

a)

starting current

I=V/R =80/4 =20 A

b)

Back emf when running normally

E=V-IR =80-4*4

E=64 Volts

c)

equivalent resistance

Req=80/10 =8 ohms

since Req=R+Rs

Rs=8-4=4 ohms

14.

Torque acting on the loop

T=NIABCos(o)=200*0.5*0.05*0.12*cos30

T=0.52 N-m

15.

Induced emf

E=BLVsin90=2*8*(0.2*10-4)

E=3.2*10-4 Volts

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