FARADAY'S LAW

An experiment whose results may be surprising, is shown in Figure (2). Here we have a magnetic field produced by an electromagnet so that we can turn B on and off. We have a wire loop that is large enough to surround but not lie in the magnetic field, so that B = 0 all along the wire. Again we have a gap and a voltmeter to measure any forces that might be exerted on the conduction electrons in the wire.
We have seen that if we pull the wire out of the magnet, Figure (1a), we will get a voltage reading while the loop is leaving the magnetic field. We have also seen, Figure (1b), that we get a voltage reading if the magnetic field is pulled out of the loop. In both cases we started with a magnetic field through the loop, ended up with no magnetic field through the loop, and got a reading on the voltmeter while the amount of magnetic field through the loop was decreasing.
Now what we are going to do in Figure (2) is simply shut off the electromagnet. Initially we have a magnetic field through the loop, finally no field through the loop. It may or may not be a surprise, but when we shut off the magnetic field, we also get a voltage reading.
We get a voltage reading if we pull the loop out of the field, the field out of the loop, or shut off the field. We are seeing that we get a voltage reading whenever we change the amount of magnetic field, the flux of magnetic field, through the loop.

Figure 1a: moving coil, magnets at rest

Figure 1b: From the point of view that the coil is at rest, the downward force on the electrons in the coil must be produced by an upward directed electric field.

Figure 2: Here we have a large coil that lies completely outside the magnetic field. Thus there is no magnetic force on any of the electrons in the coil wire. Yet when we turn the magnet on or off, we get a reading in the volt meter.

مواضيع ذات صلة

Thermal conductivity

Molecular diffusion

Ionic conductivity

The drift speed

Electrical transients

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