Physics: The First Science

By Peter Lindenfeld; Suzanne White Brahmia | Go to book overview

CHAPTER 10
Magnetism: Electricity’s Traveling
Companion
Again—force, field, and motion
Poles and currents
The force between two parallel currents
The magnitude of the magnetic field of a current
The direction of the magnetic field of a current
Motion of a charged object in a magnetic field
Solenoids
The earth’s magnetic field
Ampere’s law
The electron: an old friend turns out to be the elemental magnet
Spin and magnetic moment
Magnetic materials
Generating electricity: motional emf and Faraday’s law
The motional emf
The emf in terms of the change in the flux: Faraday’s law.

Magnetic forces have been known since iron-containing rocks were found in
antiquity. The magnetic compass was known in China in the second century,
and later made possible Columbus’s visit to America, but it was not until 1600
that Gilbert suggested that the earth was itself a giant magnet. Today current-
carrying coils can produce magnetic fields that are much stronger than those of
iron magnets. Most of the electric energy that we use comes from the motion of
wires in magnetic fields. Magnetic forces drive the motors in our fans and vac-
uum cleaners. And perhaps the most startling impact on our civilization comes
from the interplay of electric and magnetic fields that we call electromagnetic
waves.

We play with magnets from the time when we are children. We pick up pins
and nails with them and use them to stick notes to the refrigerator. The forces
that they exert are more familiar to us than electric forces, and it comes as a
surprise to learn that they are more complex.

The magnetic effects of electric currents were discovered in the early part
of the nineteenth century by Oersted, and Ampere soon suggested that the

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