When a conductor moves with respect to a magnetic field, a voltage is induced in the conductor only if the conductor cuts the lines of magnetic force. To cut lines of force means to move at right angles to the lines, as shown in Figure 5-3. In this example, conductor AB is moving downward through the magnetic field, and this motion is at right angles to the lines of force.

The lines of force are directed from the north pole to the south pole of the magnet. A voltage is
induced in the conductor. A negative potential appears at A, and a positive potential appears at B. If we connect a wire from A to B, electrons flow in the wire as shown.

Electricians need to know the polarity of the voltage induced in a conductor or the direction of induced current in the conductor (which amounts to the same thing). This is found by means of Lenz’s law. Figure 5-4 illustrates Lenz’s law. 

If you point in the direction of the magnetic flux with the index finger of your left hand and point your thumb in the direction that the conductor moves, your middle finger then points in the direction of electron flow in the conductor. Note that we hold the index finger, thumb, and middle finger at right angles to one another when we apply Lenz’s law.

The amount of voltage that is induced in a moving conductor depends on how fast the conductor cuts lines of magnetic force. If a conductor moves at a speed such that it cuts 108 lines of force per second, there will be 1 volt induced in the conductor.Of course, if we use a number of conductors, the induced voltage will be multiplied by the number of conductors. For example, if the coil in Figure 5-1

has 100 turns, the induced voltage will be 100 times greater than if only one turn were used. Since the amount of voltage induced in a conductor depends on how fast the conductor cuts lines of force, it might seem that the reading obtained on a fluxmeter would depend on how fast the flip coil is removed from a magnetic field. 

However, we will find that the speed with which the flip coil is moved has no effect on the meter reading. The reason that speed makes no difference is that the galvanometer indicates the total quantity of electricity induced in the flip coil. This total quantity of electricity is the same whether a small current is induced over a
long period of time or a large current is induced over a short period of time. A small current is produced by a small induced voltage, and a large current is produced by a large induced voltage.

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