18.4. A Practical Application of Magnetic Fields http://www.ck12.org
18.4 A Practical Application of Magnetic Fields
Objectives
The student will:
- Understand the basic operation of an electric motor.
Vocabulary
- brushes:
- commutator
- electric motor:A device which converts electrical energy into mechanical work.
Introduction
Theelectric motoris arguably, the most important invention based on the understanding that a magnetic field could
exert a force on a current-carrying wire. The average person uses many electrical appliances every day that depend
upon an electric motor. There are motors in washing machines, dryers, air conditioners, electric lawn mowers and
electric chain saws, electric blenders, electric can-openers, electric fans, DVD players, and in many children’s toys,
just to name a few.
How does an electric motor work?
The Rectangular Current Loop Revisited
In Example 19.2.1, a rectangular loop was placed perpendicular to the field lines of a uniform magnetic field. In
such a position we showed that the net force on the loop was zero. We now consider the forces on a rectangular
current-carrying loop of wire with constant currentI, when placed in a uniform magnetic fieldB, such that the plane
of the loop is parallel to the magnetic field. With such an arrangement, the magnetic field lines are perpendicular to
the side(b)of the loop and parallel to the side(a)of the loop as seen inFigure18.17.
By the right-hand rule, the force on the left side~F 1 of the loop is toward the reader, and the force on the right side~F 2
is away from the reader. We treat the result as a scalar quantity.
F 1 =ILBsinθ=ILBsin 90◦=ILB→F 1 =ILB F 2 =ILBsinθ=ILBsin 90◦=ILB→F 2 =−ILB
Since the forces are in opposite directions,~F 1 is taken as positive and~F 2 is taken as negative.
These are the only forces on the loop, since the angle between the magnetic field and the remaining sides of the
current-carrying loop is zero degrees.
F=ILBsinθ=ILBsin 0◦= 0