Page 13 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

FUNDAMENTALS OF ELECTRIC SYSTEMS

1.12 CHAPTER ONE

FIGURE 1.14 If we move the magnet toward the loop, the
induced current points as shown, setting up a magnetic field that
opposes the motion of the magnet.

FIGURE 1.15 A rectangular loop is pulled out of a magnetic field with velocity v.

Faraday’s law states that the induced voltage or emf is given by
d dx
d
B
(Blx) Bl Blv
dt dt dt
where dx/dt is the velocity υ of the loop being pulled out of the magnetic field. The
current induced in the loop is given by
Blv
i
R R
where R is the loop resistance. From Lenz’s law, this current must be clockwise because it is
opposing the change (the decrease in
). It establishes a magnetic field in the same direction
B
as the external magnetic field within the loop. Forces F and F cancel each other because they
2 3
are equal and in opposite directions. Force F is obtained from the equation (F il B)
1
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