concerning the voltages contained in a series loop.
Much of today's modern electrical and
Kirchhoff's law states that
electronic equipment could not function without
magnetism. Modern computers, tape recorders,
and video reproduction equipment use magnetized
the algebraic sum of the voltage drops in
tape and/or discs. High-fidelity speakers use
any closed path in a circuit and the
magnets to convert amplifier outputs into audible
electromotive forces in that path are equal
sound. Electrical motors use magnets to convert
electrical energy into mechanical motion;
To state Kirchhoff's law another way, the
motion into electrical energy. Simply stated, the
voltage drops and voltage sources in a circuit are
equal at any given moment in time. If the voltage
law of magnetism states that
sources are assumed to have one sign (positive or
like poles repel, unlike poles attract.
negative) at that instant and the voltage drops are
assumed to have the opposite sign, the result of
MAGNETIC FIELDS.--The space surround-
adding the voltage sources and voltage drops will
ing a magnet where magnetic forces act is known
be zero. For more detailed information on the use
as the magnetic field. You can prove that the
and application of Kirchhoff's law, refer to
magnetic field is very strong at the poles and
chapter 3 of NEETS, module 1.
weakens as the distance from the poles increases
by conducting simple experiments. It will also be
NOTE: In applying Kirchhoff's law to
apparent that the magnetic field extends from one
direct current (dc) circuits, the term
pole to the other, constituting a loop about the
electromotive force (emf) applies to voltage
sources, such as batteries or power
Although magnetic lines of force are im-
supplies. Kirchhoff's law is also used in
aginary, a simplified version of many magnetic
alternating current (ac) circuits (which are
phenomena can be explained by assuming the
covered in NEETS, module 2).
magnetic lines to have certain real properties. The
lines of force can be compared to rubber bands,
which stretch outward when a force is exerted
upon them and contract when the force is
The relationship between attracting or
removed. The characteristics of magnetic lines of
repelling charged bodies was first discovered by
force can be described as follows:
a French scientist named Charles A. Coulomb.
Coulomb's law states that
1. Magnetic lines of force are continuous and
will always form closed loops.
charged bodies attract or repel each other
2. Magnetic lines of force will never cross one
with a force that is directly proportional
to the product of their individual charges,
3. Parallel magnetic lines of force traveling
and is inversely proportional to the square
in the same direction repel one another. Parallel
of the distance between them.
magnetic lines of force traveling in opposite
directions tend to unite with each other and form
Simply stated, the amount of attracting or
into single lines traveling in a direction determined
repelling force which acts between two electrically
by the magnetic poles creating the lines of force.
charged bodies in free space depends on two
4. Magnetic lines of force tend to shorten
things-(l) their charges and (2) the distance
themselves. Therefore, the magnetic lines of force
existing between two unlike poles cause the poles
to be pulled together.
Law of Magnetism
5. Magnetic lines of force pass through all
materials, both magnetic and nonmagnetic.
To properly understand the principles of
6. Magnetic lines of force always enter or
leave a magnetic material at right angles to the
the effects of magnetism on electrical equipment.
that knowledge of either subject would be
MAGNETIC EFFECTS.--The total number
incomplete without at least a basic knowledge of
of magnetic lines of force leaving or entering the