Page 362 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 28] ELECTROMAGNETIC INDUCTION 347

28.14. The energy stored in the magnetic field of a 12-mH coil in which the current is5Ais
(a) 1.8 mJ (c) 0.15 J
(b) 30 mJ (d) 0.3 J

Supplementary Problems

28.1. What would happen if the primary winding of a transformer were connected to a battery?

28.2. What is it whose action on the secondary winding of a transformer causes an alternating potential difference to occur
across its ends, even though there is no connection between the primary and secondary windings?
28.3. Verify that L/R has the dimensions of time.

28.4. How fast should a wire 20 cm long be moved through a 0.05-T magnetic field for an emf of 1 V to appear across its
ends?

28.5. The magnetic flux through a 50-turn coil increases at the rate of 0.05 Wb/s. (a) What is the induced emf between
the ends of the coil? (b) If the coil’s resistance is 2 and it is connected to an external circuit whose resistance is
10 , how much current will flow?

28.6. A 30-turn coil 8 cm in diameter is in a magnetic field of 0.1 T that is parallel to its axis. (a) What is the magnetic
flux through the coil? (b) In how much time should the field drop to zero to induce an average emf of 0.7 V in the
coil?

28.7. A transformer has 50 turns in its primary winding and 100 turns in its seconary. (a) If a 60-Hz, 3-A current passes
through the primary winding, what are the nature and magnitude of the current in the secondary? (b) If a 3-A direct
current passes through the primary winding, what are the nature and magnitude of the current in the secondary?

28.8. The transformer in an electric welding machine draws 3 A from a 240-V ac power line and delivers 400 A. What is
the potential difference across the secondary of the transformer?

28.9. A 240-V, 400-W electric mixer is connected to a 120-V power line through a transformer. (a) What is the ratio of
turns in the transformer? (b) How much current is drawn from the power line?

28.10. A solenoid 2 cm long and 6 mm in diameter has 500 turns of thin wire. What is its inductance?

28.11. A 1-mH inductor is to be made by winding wire on a tube 2 cm in diameter and 10 cm long. How many turns are
needed?

28.12. Find the equivalent inductances of three 2-mH inductors when they are connected in series and in parallel.
28.13. Find the emf induced in a 0.1-H coil when the current in it is changing at the rate of 80 A/s.

28.14. What rate of change of current is needed to induce an emf of8Vina 0.1-H coil?

28.15. An average emf of 32 V is induced in a circuit in which the current drops from 10 to2Ain0.1 s. What is the
inductance of the circuit?

28.16. A 5-mH coil carries a current of 2 A. How much energy is stored in it?

28.17. How much current must flow through a 40-mH coil in order that it contain 0.1 J of magnetic potential energy?