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CHAPTER 2 - AUTOMOTIVE ELECTRICAL CIRCUITS AND WIRING |
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ELECTROMAGNETIC INDUCTION
Current can be induced to flow in a conductor if it is moved through a magnetic field. In figure 1-22,
the wire is moved downward through the magnetic field between the two magnetic poles. As it moves downward cutting lines of force, current is induced in it. The reason for this is that the line of force resists cutting and tends to wrap around the wire as shown. With lines of force wrapping around the wire, current is induced. The wire movement through the magnetic field produces a magnetic whirl around the wire, which pushes the electrons along the wire.
If the wire is held stationary and the magnetic field is moved, the effect is the same. All that is required is that there be relative movement between the conductor and the magnetic lines of force to produce enough voltage to move the electrons along the conductor. Moving the magnet can move the magnetic field or, if it is a magnetic field from an electromagnet, starting and stopping the current flow in the electromagnet can move it. Suppose an electromagnet, such as the one shown in figure 1-20, has a wire held close to it. When the electromagnet is connected to a battery, current will start to flow through it. This current, as it starts to flow, builds up a magnetic field.
Figure 1-22.- Electromagnetic induction.
A magnetic field forms because of the current flow. This magnetic field might be considered to expand and move out from the electromagnet. As the lines of force move out, the wire will cut them. This wire will therefore have current induced in it. If the electromagnet is disconnected from the battery, these lines of force will disappear and current will stop flowing in the wire. It can be seen now that current can be induced in a wire by three methods: 1. The wire can be moved through the stationary magnetic field (the principle applied in a dc generator). 2. The wire can be held stationary and the magnet can be moved so the field is carried past the wire (the principle applied in an ac generator). 3. The wire and electromagnet can both be held stationary and the current turned on and off to cause the magnetic field to build up and collapse so the magnetic field moves one way or the other across the wire (the principle applied in an ignition coil).
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