A First Course in Physics · Ch. XVI, §§ 424–427 · Chapter 16: Induced Currents
49. The electric motor
Goal: Your student can explain the force on a current in a magnetic field, use the motor rule, explain how a motor turns, and explain back E.M.F.
You'll need
- The book, open to these pages
- A simple motor kit, or: an AA battery, a strong flat magnet, two large paper clips, tape, and about 1 m of enameled (varnished) copper wire with fine sandpaper
- A small toy motor (to take apart and look at, if you have an old one)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
A wire carrying a current is pushed sideways in a magnetic field; the motor rule (left hand); a dynamo run backward as a motor; street-car motors; back E.M.F. and the starting box. Ten problems follow.
The lesson, step by step
Hook question
SayA dynamo turns motion into electricity. Can you run one backward?
Read together
Read §§ 424–427. The key fact:
a wire carrying a current in a magnetic field tends to move in a direction at right angles both to the direction of the field and to the direction of the current.
Try it: build a simple motor
Wind the enameled wire into a coil about the size of a coin (10–15 turns), leaving two straight ends sticking out on opposite sides as an axle. Sand the varnish completely off one end, and off only the top half of the other end. Bend the paper clips into cradles, tape them to the ends of the battery, rest the coil in them, and set the magnet on the battery under the coil. Give it a flick: it spins. The half-sanded end acts as a simple commutator (§ 425).
Safety: the battery and wire get warm; disconnect when not spinning. Safety: use only 1.5-volt AA or AAA batteries for every circuit in this course. Never connect anything to a wall outlet, and don't hold a bare wire straight across a battery for more than a few seconds (it gets hot and drains the battery).
Explain the idea
The motor rule (§ 424): left hand, forefinger along the field, middle finger along the current, thumb gives the push. A motor is built just like a dynamo. Once it spins, it also acts as a dynamo and makes a back E.M.F. that opposes the current (§ 427). That is why a motor draws a big current when starting and less at full speed (problem 3).
What we know now
Electric motors are everywhere now: fans, washing machines, electric cars, the vibrator in a phone. Most streetcars are gone, but electric trains and subways still use the same ideas.
Say it back
Your student explains why an electric fan draws more current when it is first switched on.
Problems
Work problems 1–10 on book pages 329–330.
Afterwards
Count the electric motors in your home (don't forget the refrigerator, clocks, printers and the hard drive of a computer).
Answers to the book's problems
1. Toward the east (the force is at right angles to the downward current and the northward horizontal field). 3. At first there is no back E.M.F.; as it speeds up, the back E.M.F. cuts down the current. 5. No; the iron core greatly strengthens the field the wires move in. 6. Yes; with the circuit closed, the current's field opposes the turning (Lenz's law). 7. If the induced current helped the motion, a motor once started would speed itself up and do work for nothing. 8. Twice the current must be supplied at the same voltage, and the larger current in the armature pushes back twice as hard. 9. The current reverses in the field magnets and the armature together, so the push is always the same way. 10. 5 amperes ((500 − 450) ÷ 10).
Worked out for this site from the scan. If a number in your copy differs, trust the book.