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6.4 ELECTRIC CURRENTS AND e
MAGNETISM

As Oersted discovered, electric charges in motion produce a

magnetic fi eld. The direction of the magnetic field around a

current-carrying wire can be determined by using a magnetic
compass. The north-seeking pole of the compass needle will

point in the direction of the magnetic field lines. If you move

the compass around the wire, the needle will always move to a A e B
position that is tangent to a circle around the wire. Evidently, the
magnetic field lines are closed concentric circles that are at right

angles to the length of the wire (Figure 6.25). S
FIGURE 6.26 (A) Forming a wire into a loop causes the magnetic
CURRENT LOOPS field to pass through the loop in the same direction. (B) This gives

The magnetic field around a current-carrying wire will interact one side of the loop a north pole and the other side a south pole.

with another magnetic field, one formed around a permanent

magnet or one from a second current-carrying wire. Th e two

fields interact, exerting forces just like the forces between the

fi elds of two permanent magnets. The force could be increased
by increasing the current, but there is a more effi cient way to
obtain a larger force. A current-carrying wire that is formed into
a loop has perpendicular, circular field lines that pass through

the inside of the loop in the same direction. This has the eff ect of
concentrating the field lines, which increases the magnetic fi eld S N

intensity. Since the field lines all pass through the loop in the

same direction, one side of the loop will have a north pole and
the other side a south pole (Figure 6.26).
Many loops of wire formed into a cylindrical coil are called
a solenoid. When a current passes through the loops of wire in
a solenoid, each loop contributes field lines along the length e – – e –

of the cylinder (Figure 6.27). The overall effect is a magnetic +

field around the solenoid that acts just like the magnetic fi eld

of a bar magnet. This magnet, called an electromagnet, can be Battery

turned on or off by turning the current on or off. In addition, the

strength of the electromagnet depends on the magnitude of the
current and the number of loops (ampere-turns). Th e strength FIGURE 6.27 When a current is run through a cylindrical coil
of the electromagnet can also be increased by placing a piece of of wire, a solenoid, it produces a magnetic field like the magnetic

soft iron in the coil. The domains of the iron become aligned field of a bar magnet.

by the influence of the magnetic fi eld. This induced magnetism

Wire e –
increases the overall magnetic field strength of the solenoid

as the magnetic field lines are gathered into a smaller volume

Magnetic within the core.
compass
APPLICATIONS OF ELECTROMAGNETS

The discovery of the relationship between an electric current,
magnetism, and the resulting forces created much excite-

ment in the 1820s and 1830s. This excitement was generated
because it was now possible to explain some seemingly sepa-
e – rate phenomena in terms of an interrelationship and because
people began to see practical applications almost immediately.
Within a year of Oersted’s discovery, André Ampère had fully
FIGURE 6.25 A magnetic compass shows the presence and

direction of the magnetic field around a straight length of explored the magnetic effects of currents, combining experi-

current-carrying wire. ments and theory to find the laws describing these eff ects.
158 CHAPTER 6 Electricity 6-20