A First Course in Physics · Ch. XVI, §§ 418–423 · Chapter 16: Induced Currents
48. Dynamos: alternating and direct current
Goal: Your student can explain how an alternator makes alternating current, how a commutator turns it into direct current, and the difference between series, shunt and compound dynamos.
You'll need
- The book, open to these pages
- A hand-crank flashlight or radio, a bicycle dynamo light, or a small toy motor and an LED (spin the motor shaft fast to make the LED blink)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
A coil spinning between electro-magnet poles; ring and drum armatures; slip rings and alternating current; multipolar alternators; the commutator; ring- and drum-armature direct-current dynamos; series, shunt and compound winding; lamps in parallel and arc lamps in series. Twelve problems follow.
The lesson, step by step
Hook question
SayThe electricity from a wall outlet is called "alternating current." What is alternating about it?
Read together
Read §§ 418–423, studying Figs. 329–338. The commutator's job:
By the use of a so-called commutator it is possible to transform a current which is alternating in the coils of the armature to one which always flows in the same direction through the external portion of the circuit.
Explain the alternator
As a coil turns between N and S poles, each side cuts lines first one way and then, half a turn later, the other way, so the current reverses twice each turn (§ 418). Slip rings (R and R′) carry this alternating current out. A commutator (§ 420) swaps the connections every half turn, so the outside circuit always gets current in one direction.
Try it: be the power station
Crank a hand-crank flashlight or radio and feel how much harder it gets when the light is on (Lenz's law again). Or connect an LED to a small toy motor and spin the shaft fast between your fingers: the motor works as a dynamo and the LED flickers. (If it doesn't light, reverse the LED.)
What we know now
The book's 1906 power stations made both kinds of current. Alternating current won for power grids, because transformers (two lessons from now) work only with AC. In the United States it alternates 60 times a second (120 alternations); in much of the world, 50. Batteries, phones and computers still use direct current inside.
Say it back
Your student explains, with Fig. 335, how the split ring keeps the outside current flowing one way.
Problems
Work a selection of problems 1–12 on book pages 325–326: try 1, 3, 6, 8, 9, 10, 11 and 12.
Afterwards
Look for the label on an appliance that says "AC" or "DC" and "60 Hz" (or 50 Hz).
Answers to the book's problems
1. 66⅔ alternations per second (20 poles × 200/60 revolutions per second). 3. The commutator reverses the coil's connections to the brushes just as the current in the coil reverses. 6. Twelve brushes. 8. The field magnets grow stronger, so the E.M.F. rises. 9. The field magnets grow weaker, so the E.M.F. falls. 10. In a compound dynamo the series coil strengthens the field as the shunt coil weakens it, and the two effects balance. 11. 5000 amperes; 550 kilowatts, about 737 horse power. 12. About 218,000 lamps (12,000,000 watts ÷ 55 watts, taking 0.5 ampere per lamp).
Worked out for this site from the scan. If a number in your copy differs, trust the book.