A First Course in Physics · Ch. XVI, §§ 428–443 · Chapter 16: Induced Currents
50. Induction coils, transformers, power lines and the telephone
Goal: Your student can explain how changing current in one coil induces current in another, how a transformer steps voltage up or down, why power is sent at high voltage, and how the telephone works.
You'll need
- The book, open to these pages
- A phone charger or doorbell transformer (to read its label only; don't open it)
- Two paper cups and string (a string telephone), or an old pair of earbuds
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Making and breaking a current in one coil induces currents in another; direction by Lenz's law; secondary voltage proportional to turns; the large E.M.F. at break; the induction coil and its condenser; laminated cores and eddy currents; the transformer, step-up and step-down; efficiency; long-distance transmission (Niagara to Buffalo); Bell's telephone, the carbon transmitter, and the central-battery exchange. Seven questions follow.
The lesson, step by step
Hook question
SayPower lines carry tens of thousands of volts, yet the outlet in your wall gives about 120. Where does the voltage change?
Read together
Read §§ 428–443, skimming §§ 438 and 443. The transformer rule:
In general, the ratio between the voltages at the terminals of the primary and secondary is the ratio of the number of turns of wire upon the two.
Explain the transformer
A changing current in the primary coil makes changing magnetic lines in the iron core, which induce a current in the secondary (§ 428). Volts go as the number of turns: 20 times as many turns on the primary steps 2200 volts down to 110 (§ 436). Power in ≈ power out, so if the voltage goes down, the current goes up. That only works with alternating current, which is why AC won.
Why high voltage on long lines
The heat wasted in a line is C²R (§ 406). Sending the same power at 10 times the voltage needs 1/10 the current, so only 1/100 of the loss (§ 439). Work problem 5 together.
Watch only: induction coils and power lines
Watch only: don't try this at home. § 431 describes holding the ends of an induction coil to feel a shock; never do this. Induction coils, spark gaps and transformers can deliver dangerous high voltages, and power lines can kill. Stay far away from downed lines, never climb poles or trees near lines, and never open a transformer or charger.
Try it: a telephone without electricity
Make a cup-and-string telephone and talk across the room with the string pulled tight. Sound makes the cup bottom vibrate, and the string carries the vibration. Bell's telephone (§ 441) did the same thing with a vibrating iron disk, a magnet and induced currents in a wire instead of a string.
What we know now: today's phones turn sound into digital signals carried by radio waves and light in glass fibers, but every phone still has a tiny speaker that works like the receiver in Fig. 360.
Say it back
Your student explains why an induction coil sparks at "break" and not at "make" (§ 431).
Questions
Work questions 1–7 on book page 342.
Afterwards
Read the label on a phone charger: input (about 120 volts AC) and output (5 volts or so DC). It contains a small transformer.
Answers to the book's problems
1. At break, because the current, and so the magnetic field, can be stopped far faster than it can be started, and the induced E.M.F. depends on how fast the lines change. 3. Currents would be induced around the solid cylinders (eddy currents), wasting energy as heat; flat disks break up those paths. 4. The primary must have about 9.1 times as many turns as the secondary (1000 ÷ 110). 5. The 100-volt line carries 100 times the current, so to waste the same heat (C²R) its resistance must be 10,000 times smaller: a cross section 10,000 times as great.
Worked out for this site from the scan. If a number in your copy differs, trust the book.