A First Course in Physics · Ch. XIV, §§ 351–358 · Chapter 14: Electricity in Motion
40. Electric currents and their magnetic effect
Goal: Your student can explain that a moving charge makes a magnetic field, repeat Oersted's experiment with a battery and compass, use the right-hand rule, and explain how a galvanometer measures current.
You'll need
- The book, open to these pages
- A 1.5-volt AA battery (or two in a holder), a flashlight bulb in a holder, and two or three short insulated wires with bare ends
- A compass
- Tape
- A notebook and pencil
What's in the book
A Leyden-jar spark magnetizes a needle; a charge at rest has no magnetic effect; Galvani, Volta and the galvanic cell; Oersted's experiment (1819); the plates of a cell are charged; a cell gives a small voltage but a large steady current; the circular magnetic field around a wire and the right-hand rule; galvanometers; the ampere; the ammeter. Five questions follow, with portraits of Volta and Ampère.
The lesson, step by step
Hook question
SayIs there any connection between electricity and magnetism? People wondered for centuries, until 1819.
Read together
Read §§ 351–358. The key fact:
magnetic effects are always observable near the path of a moving electrical charge, while no such effects can ever be observed near a charge at rest.
Try it: Oersted's experiment
Make a circuit: battery, bulb and wires, so the bulb lights. Lay the compass flat and let it settle north–south. Tape one wire of the circuit so it runs straight north–south just above the compass. When the bulb lights, the needle swings. Swap the battery around: it swings the other way. Run the wire under the compass instead: it swings the other way again.
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).
The right-hand rule
Grasp the wire with your right hand, thumb pointing the way the current flows (from + to −, § 341). Your fingers curl the way the magnetic lines go around the wire (§ 355). Have your student predict which way the needle will swing before trying each setup.
Explain the galvanometer
Wind the wire several times around the compass: the swing grows, because every turn pushes the same way (§ 356). That is a galvanometer. Real ammeters (Fig. 272) use a coil that turns between magnet poles against a spring.
What we know now: the ampere is now defined by fixing the charge of the electron, but it is still about the same size; a flashlight bulb draws a few tenths of an ampere.
Watch only: the Leyden jar spark
Watch only: don't try this at home. § 351 magnetizes a needle with a Leyden-jar spark, and § 352 uses sulphuric acid in the cell. Today's AA battery does the same job safely.
Say it back
Your student explains why Oersted's experiment caused such excitement.
Questions
Work questions 1–5 on book page 268, using the right-hand rule.
Afterwards
Look at a car's dashboard ammeter or a multimeter if you have one, and find the coil-and-magnet idea inside it in a diagram.
Answers to the book's problems
1. Only when it is moving. 2. Toward the west (by the right-hand rule, with the current flowing south the magnetic lines above the wire run from east to west). 3. From north to south. 4. Down (a clockwise current, seen from above, makes the field inside the loop point downward). 5. As the needle turns toward the plane of the coil, less and less of the coil's force acts at right angles to the needle, while the earth's pull to turn it back grows.
Worked out for this site from the scan. If a number in your copy differs, trust the book.