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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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An easy way to remember is by means of the left-hand rule. In your imagination, place your left hand around a conductor, as in Figure 11-6, so that your thumb points in the direction of the current (negative to positive). Then your fingers will be pointing in the direction of the lines of force.
Refer to Figure 11-7 to determine the direction in which a compass needle will be deflected.
Parallel conductors carrying currents in the same direction attract each other. See Figure 11-8. Conductors A and B are carrying current away from us, so the lines of force are counterclockwise.
Electromagnetism 131
DIRECTION OF CURRENT Figure 11-6 Current and
I direction of lines of flux.
(A) Current going into the page. (B) Current coming out of the page.
Figure 11-7 Direction of deflection of a compass needle.
Figure 11-8 Wires carrying current in the same direction attract each other.
132 Chapter 11
Instead of circling A and B separately, as at C and D, they combine and encircle both conductors, as at E and F.
Wires carrying current in opposite directions repel each other. See Figure 11-9. Conductor A is carrying current away from us and conductor B is carrying current to us. Lines of force around A are counterclockwise and clockwise around B. Since the lines of force are oriented in opposite directions, they won’t combine as in Figure
11-8, but will tend to push the wires apart.
Figure 11-9 Wires carrying current in opposite directions repel each other.
Maxwell’s rule states that every electrical circuit is acted upon by a force that urges it in such a direction as to cause it to include within its embrace the greatest possible number of lines of force.
In explanation of Maxwell’s rule we shall use Figure 11-10. In Figure 11-10A, there is a circuit doubled back on itself. The lines of
(A) Initial configuration of current-carrying conductor.
(B) Configuration resulting from Figure 11-10 Illustration of
electromagnetic repulsion. Maxwell's rule.
Electromagnetism 133
force around half the wire will oppose the lines of force around the other half, as was covered in the explanation of Figure 11-9. This means the wire will have a tendency to be pushed apart and, if free to move, would theoretically take the shape of the circuit shown in Figure 11-10B, which would be a circle.
A paraphrase of Maxwell’s rule is that every electrical circuit tends to so alter its shape as to make the magnetic flux through it a maximum. In this paraphrase, you have the answer that explains motor action and also the action of many measuring instruments.
In explanation, every electric motor has a loop of wire that carries current. This loop is placed in such a position in a magnetic field that the lines of force pass parallel to, but not through, it. From Maxwell’s rule, the loop tends to turn in such a direction so as to include within it the lines of force of the magnetic field. It is suggested that this action be reviewed and remembered, as it will have a far-reaching effect in later chapters.
One of these far-reaching effects will be covered under a chapter concerned with fault currents, but it will be good to touch on this matter now, while we are involved in one important effect caused by fault currents.
On large-capacitance circuits, which will have high currents available should the conductors of a circuit short together, Maxwell’s rule must be prepared for before the short occurs. When the short occurs, the magnetic forces tending to cause the circuit to embrace the greatest possible number of lines of force will tend to throw the conductors apart. These magnetic stresses become very great. In switchgear, for instance, where bus bars are the conductors, it becomes an engineering problem to design the bus bars so that they won’t be torn from their mountings. Therefore, they have to be rigidly supported and bolted into place.
The same must be done with cable trays. Here the conductors should be tied down securely so that they won’t be thrown out of the tray or destroy the tray.
In Figure 11-11, the lines of force from the magnet go from N to S and the current in the conductor is coming toward us, so the flux
Figure 11-11 Composite of a magnetic field and a current-carrying conductor.
FORCE
134 Chapter 11
around the conductor is clockwise. One might compare these lines of force to rubber bands. Thus the lines of force of the magnetic poles tend to straighten out and push the conductor down.
The right-hand rule is illustrated in Figure 11-12. The right hand is cupped over the pole piece as shown; the thumb represents the direction of motion, the index finger the direction of the flux, and the middle finger the direction of the current.
Galvanoscope
Figure 11-1 showed a magnetic needle under a wire carrying current, as did Figure 11-7. A galvanoscope is such a device. Figure 11-13 will be used in the explanation. Figure 11-13A shows a simple gal-vanoscope, similar to the one previously illustrated. Current will deflect the magnetic needle, which is suspended by a thread. If the current is very feeble, the deflection will be hard to notice. To overcome this, more turns are added, as in Figure 11-13B. Thus, if there are 100 turns, the effect will be 100 times as much as the effect of
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