A First Course in Physics · Ch. XVI, §§ 412–417 · Chapter 16: Induced Currents
47. Faraday's discovery: currents made by magnets
Goal: Your student can describe Faraday's discovery of induced currents, state Lenz's law and connect it to conservation of energy, and use the dynamo rule.
You'll need
- The book, open to these pages
- A strong magnet (a neodymium magnet if you have one, handled carefully) and a copper or aluminum tube, such as a length of copper plumbing pipe
- About 10 m of thin insulated wire wound into a coil, and a compass
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
A coil thrust over a magnet pole gives a momentary current; Faraday's discovery (1831); the direction of the induced current and Lenz's law; an E.M.F. is induced only when lines of force are cut; its strength (100,000,000 lines cut per second give 1 volt); the dynamo rule; a coil turning between magnet poles. Seven questions follow, with a portrait and short life of Michael Faraday.
The lesson, step by step
Hook question
SayAlmost all the electricity in your home is made without any batteries or chemicals. How?
Read together
Read §§ 412–417 and the note on Faraday (who was apprenticed to a bookbinder as a boy). Lenz's law:
Whenever a current is induced by the relative motion of a magnetic field and a conductor, the direction of the induced current is always such as to set up a magnetic field which opposes the motion.
Try it: the slow-falling magnet
Drop the strong magnet down the copper or aluminum tube and time it; then drop a non-magnetic object about the same size (a short wooden dowel, or a few coins taped into a roll). The magnet falls very slowly. Currents induced in the tube make a field that opposes the motion: Lenz's law you can see. (Copper isn't magnetic at all; try the magnet on it.)
Safety: strong magnets snap together hard and can pinch fingers; keep them away from phones and pacemakers.
Try it: Faraday's experiment
Wind 20 turns of the wire around a compass to make a simple galvanometer. Connect the ends to a separate coil (the rest of the wire wound into a loop about the size of a cup), placed a meter or more away from the compass. Push the magnet quickly into that coil: the compass twitches. Hold it still: nothing. Pull it out: it twitches the other way (§ 412). Faster motion, bigger twitch (§ 415). The effect is small: a strong magnet and many turns help.
Explain the idea
A current is made only when the conductor cuts magnetic lines, or the number of lines through a coil changes (§ 414). The induced current always pushes back against the motion that causes it, so you must do work to get electrical energy out (§ 413). That is the conservation of energy again. The dynamo rule (§ 416): right hand, forefinger along the field, thumb along the motion, middle finger gives the current.
What we know now
Faraday's 1831 discovery is the basis of every power-station generator, wind turbine and electric guitar pickup today. The book notes that Joseph Henry found it about the same time in America. The unit of magnetic flux today is the weber; a weber is 100,000,000 of the book's lines, so 1 volt is induced when one weber is cut per second.
Say it back
Your student explains why the magnet falls slowly through the copper pipe, using Lenz's law.
Questions
Work questions 1–7 on book page 317.
Afterwards
Find induction at work: an induction cooktop, a wireless phone charger, a bicycle dynamo light, the magnetic strip readers in stores.
Answers to the book's problems
1. When the magnet and a conductor move relative to each other so that lines of force are cut. 2. Counterclockwise as you look down (the induced current makes the coil's lower face an S pole to oppose the motion, so its upper face is N). 3. The induced current opposes the motion; if it helped the motion, the motion would speed itself up and make energy from nothing. 4. When the coil's circuit is closed, currents are induced in it as it swings, and by Lenz's law they oppose its motion. 5. In the northern hemisphere the E.M.F. is upward (from the bottom of the mast toward the top); the wire moves with the mast and has an equal E.M.F. induced in it, so the two cancel around the loop. 6. 100 volts (250 loops × 4,000,000 lines per turn × 10 turns per second = 10,000,000,000 lines per second). 7. When the plane of the coil is at right angles to the magnetic meridian (facing north and south), where the number of lines through it is greatest.
Worked out for this site from the scan. If a number in your copy differs, trust the book.