Electromagnets vs. Permanent Magnets Permanent magnets are made by placing pieces of iron or certain alloys in a strong magnetic field. At its curie point, a permanent magnet loses it magnetism. An electromagnet is produced by placing a piece of iron in a current-carrying wire coil. When current is turned off, the electromagnet loses it magnetism.

Hans Christian Ørsted On 21 April 1820, during a lecture, Ørsted noticed a compass needle deflected from magnetic north when an electric current from a battery was switched on and off, confirming a direct relationship between electricity and magnetism. His initial interpretation was that magnetic effects radiate from all sides of a wire carrying an electric current, as do light and heat. Three months later he began more intensive investigations and soon thereafter published his findings, showing that an electric current produces a circular magnetic field as it flows through a wire. This discovery was not due to mere chance, since Ørsted had been looking for a relation between electricity and magnetism for several years.

The magnetic field that surrounds a current-carrying conductor can be shown by arranging a large number of small compasses around a wire and passing a current through it. In Figure 4-1a, when no current is flowing through the wire, the compasses line up with the Earth’s magnetic field and all point n the same direction (North). In Figure 4- 1b, when a current is flowing through the wire, the compass align with a stronger magnetic field surrounding the wire. Notice that the magnetic field forms concentric circles around the wire. Thus when current flows through a conductor a magnetic field surrounds the conductor.

The discovery that electric current causes magnetism is used everyday The discovery that electric current causes magnetism is used everyday. Any item you listen to an audio tape, watch a video tape, use a hair dryer, talk on the phone, or ring a doorbell you are using electromagnetism. Each of the device uses electromagnet. An electromagnet is a conductor with electric current flowing through it. For example, when you push the button of a chiming doorbell, you close a circuit and send current through a coil. The coil becomes an electromagnet and repels a permanent magnet inside the coil. The permanent magnet then bounces against a chime, creating the sound.

In an old classic telephone handset, there is a small disc called a diaphragm. The diaphragm vibrates when you speak, disturbing a magnetic field and resulting in an electric current. At the receiving end, the changes in current switch an electromagnet on and off, vibrating another diaphragm and reproducing the sound.

English physicist and inventor who made the first electromagnets William Sturgeon English physicist and inventor who made the first electromagnets

In 1825 William Sturgeon, a self-taught experimenter in electricity, made the discovery that adding an iron coil of wire would increase the magnetic force (see Figure 4-2). He took a bar of soft iron and coated it with varnish. Around it, he wrapped a coil of bare wire 18 turns. The weight of Sturgeon’s magnet was 7 ounces. When it was connected to one cell, it was able to support a weight of 9 pounds.

Joseph Henry The first practical electromagnet was built in 1831 by the American physicist Joseph Henry. Instead of insulating the iron core, he wrapped the wire in silk and wound a large number of turns on the bar in the same direction. His experiment proved that the more turns of wire in coil and the larger the current, the stronger the electromagnet is.

Electromagnets are very useful because their magnetism can be turned on and off, much the same way a light switch operates. One example of the use of an electromagnet is a crane that is used to move scrap metal or old cars in a salvage yard. The crane has a strong electromagnet suspended from a cable. When the current is on, the electromagnet can lift the scrap metal. Turning off the current makes the electromagnet lose its magnetic field and the metal falls off.

The discovery that electricity could be used to produce magnetism led many scientist to feel that it must also be possible to use magnetism to produce electricity. In 1831, Joseph Henry and the English scientist Michael Faraday discovered that moving a magnet through a wire coil caused an electric current to flow through the wire (Figure 4-3). The process by which magnetism produces electric current is called electromagnetic induction. Electromagnetic induction occurs when either the magnetic field moves past a stationary conductor (wire) or when the conductor moves through a stationary magnetic field. An electric current is produced whether either or both move.

The amount of voltage induced depends on how quickly the magnetic field lines are crossed by the wire. Very slow motion produces hardly any voltage. Quick motion induces a greater voltage. Also, the greater the number of coils in the loop of wire, the greater the induced voltage (Figure 4-4). Pushing a magnet into twice as many loops induces twice as much voltage. Pushing it into five times as many loops induces five times as much voltage. The strength of the magnet affects the amount of voltage induced; the stronger the magnet, the greater the induced voltage.