The Magnetic Field
We have discussed how a charged plastic rod produces a vector field-the electric field E -at all points in the space around it. Similarly, a magnet produces a vector field-the magnetic field B-at all points in the space around it. You get a hint 01 that magnetic field whenever you attach a note to a refrigerator door with a small magnet, or accidentally erase a computer disk by bringing it near a magnet. TI)e magnet acts on the door or disk by means of its magnetic field. In a familiar type of magnet, a wire coil is wound around an iron core and current is sent through the coil; the strength of the magnetic field is determined by e size of the current. In industry. such electromagnets are used for sorting scrap metal (Fig. 29-1) among many other things. You are probably more familiar with permanent magnets-‘-magnets, like the refrigerator-door type, that do not need current to have a magnetic field.In Chapter 23 we saw that an electric charge sets up an electric field that can then affect other electric charges. Here, we might reasonably expect that a magnetic charge sets up a magnetic field that can then affect other magnetic charges. AlthoughSUCh magnetic charges, magnetic monopoles. are predicted by certain tones. their existence has not been confirmed. How then are magnetic fields setup? There are two ways. (I) Moving electrically charged particles, such as a current in a wire. create magnetic fields. (2) Elementary particles such as electrons have an intrinsic magnetic field around them: that is. this field is a basic characteristic of the particles, just as are their mass and electric charge (or lack of charge). As we shall discuss in Chapter 32. the magnetic fields of the electrons in certain materials add together to give a net magnetic field around the material. This is true for the material in permanent magnets (which is good, because they can then hold notes to a refrigerator door). In other materials. the magnetic fields of all the electrons cancel out. giving no net magnetic field surrounding the material. This is true for the material in your body (which is also good, because otherwise you might be slammed up against a refrigerator door everytime you passed one). Experimentally we find that when a charged particle (either alone or as part of a current) moves through a magnetic field. a force due to the field can act on the particle. In this chapter we focus on the relation between the magnetic field and this force.