03.01.2015 Views

handbook of modern sensors

handbook of modern sensors

handbook of modern sensors

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

3.3 Magnetism 51<br />

N<br />

S<br />

S<br />

N<br />

(A)<br />

(B)<br />

Fig. 3.10. Test magnet in a magnetic field (A); compass needle rotates in accordance with the<br />

direction <strong>of</strong> the electric current (B).<br />

By definition, the direction <strong>of</strong> this field at any point is given by the direction <strong>of</strong> the<br />

force exerted on a small-unit north pole. Directions <strong>of</strong> field lines are, by definition,<br />

from north to south. Figure 3.10A shows the direction <strong>of</strong> the field by arrows. A tiny<br />

test magnet is attracted in the direction <strong>of</strong> the force vector F. Naturally, approximately<br />

the same force but <strong>of</strong> opposite direction is exerted on the south pole <strong>of</strong> the test magnet.<br />

The above description <strong>of</strong> the magnetic field was made for a permanent magnet.<br />

However, the magnetic field does not change its nature if it is produced by a different<br />

device (e.g., electric current passing through a conductor). It was Hans Christian<br />

Oersted, a Danish pr<strong>of</strong>essor <strong>of</strong> physics, who in 1820 discovered that a magnetic field<br />

could exist where there were no magnets at all. In a series <strong>of</strong> experiments in which<br />

he used an unusually large Voltaic pile (battery) so as to produce a large current, he<br />

happened to note that a compass in the near vicinity was behaving oddly. Further<br />

investigation showed that the compass needle always oriented itself at right angles<br />

to the current-carrying wire and that it reversed its direction if either current was<br />

reversed, or the compass was changed from a position below the wire to one above<br />

(Fig. 3.10B). Stationary electric charges have no effect on a magnetic compass (in<br />

this experiment, a compass needle is used as a tiny test magnet). It was clear that<br />

the moving electric charges were the cause <strong>of</strong> the magnetic field. It can be shown<br />

that magnetic field lines around a wire are circular and their direction depends on<br />

the direction <strong>of</strong> electric current (i.e., moving electrons) (Fig. 3.11). Above and below<br />

the wire, magnetic field lines are pointed in the opposite direction. That is why the<br />

compass needle turns around when it is placed below the wire.<br />

A fundamental property <strong>of</strong> magnetism is that moving electric charges (electric<br />

current) essentially produce a magnetic field. Knowing this, we can explain the nature<br />

<strong>of</strong> a permanent magnet. A simplified model <strong>of</strong> a magnetic field origination process is<br />

shown in Fig. 3.12A. An electron continuously spins in an eddy motion around the

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!