Page 12 - 10-phy-15 Electromagnetism

15. Electromagnetism eLearn.Punjab

The result is an electric motor, which is a device that converts electric energy into rotational kinetic
energy.
In a practical electric motor, the coil, called the armature, is made of many loops mounted on a
shaft or axle. The magnetic field is produced either by permanent magnets or by an electromagnet,
called a field coil. The torque on the armature, and, as a result, the speed of the motor, is controlled
by varying the current through the motor.

The total force acting on the armature can be increased by

• Increasing the number of turns of the coil Connection:

• Increasing the current in the coil Magnetic field lines help us to visualize the magnitude
• Increasing the strength of the magnetic field and direction of the magnetic field vectors just as electric
• Increasing the area of the coil field lines do for the magnitude and direction of E.

15.5 ELECTROMAGNETIC INDUCTION

Hans Christian Oersted and Ampere discovered that an electric

current through a conductor produces a magnetic field around it.

Michael Faraday thought that the reverse must also be true: that

a magnetic field must produce an electric current. Faraday found

that he could induce electric current by moving a wire through a

magnetic field. In the same year, Joseph Henry also showed that

a changing magnetic field could produce electric current. Now we Fig 15.10: Maximum strength of
shall discuss Faraday’s experiments for the production of e.m.f. in magnetic field
magnetic field.

The strength of magnetic field is

defined as the number of magnetic lines of force passing through any

surface. The number of lines of force is maximum when the surface is

held perpendicular to the magnetic lines of force (Fig.15.10). It will be

minimum when surface is held parallel to the magnetic lines of force

(Fig.15.11).

Fig 15.11: Minimum strength
of magnetic field

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