Magnets and Electricity

Magnetic and electrical phenomena have a number of similarities but also some key differences. First, a similarity:
In the same way that the region of electrical influence around a positive electric charge is called an electric field, the region of magnetic influence around a pair of north and south magnetic poles is called a magnetic field. Either of these fields can be diagrammed as a system of lines of force that spread out from a positive charge (for electric lines of force) or a north pole (for magnetic lines of force) and come together at a negative charge, or a south pole, respectively. Such lines of force become visible when iron filings are placed on a sheet of paper immediately above a magnet.

Now, a difference: While positive and negative electric charges can occur separately, magnetic poles are always found in north-south pairs.

The relation between electric systems of charges and fields and magnetic systems of poles and fields is dynamic and three-dimensional, and took many years to discover even after people were looking for it (see history of magnets). Stationary electric charges and stationary magnet poles have no effect on each other. But moving charges (such as current in a wire), or fluctuating electric fields, create a magnetic field and will exert forces on magnets, and moving or fluctuating magnetic fields create electric fields and will exert forces on electric charges. The first of these effects is used to make electromagnets, and the second is used to make electrical generators.

When an electric current in a wire creates a magnetic field, the magnetic field lines do not line up with the electric field lines that are driving the charges along the wire -- instead, the magnetic field lines run in circles around the wire, at right angles to the electric field. If the wire is bent into a circle, the magnetic field lines run around it in circles that form a doughnut enclosing the wire -- funneling through the middle of the wire circle and flaring out to wrap back around the outside and funnel through again. If the wire is wound in multiple turns around its circle, the magnetic field strength of all the turns adds up to form a field of the same doughnut shape, but stronger by as many times as the number of additional turns of wire.

All this can be done — creating a magnetic field that will deflect compass needles or, with enough turns, will pick up paper clips — with a magnetic field established only by current in a wire, with no iron. But when iron is put within a coil that carries a current, the iron will act as a magnet while also intensifying the magnetic effect many times. It is therefore customary to wrap the wire around iron instead of air. The device that results is called an electromagnet.

Electromagnets have many uses and are extremely important in our lives. Every electric machine that does anything mechanical depends on them. Electric motors are made up either partly or completely of electromagnets, and most electric generators — all large ones — have electromagnets as a fundamental component. Electromagnets are also used in many automatic switches and valves, and in buzzers, doorbells, and similar devices. Compared to permanent magnets, they can easily be made much stronger, and also have the big advantage that they can be turned off or even reversed.