1. Chapter 22 Magnetism and its uses

2. 22-1 Characteristics of Magnets Greeks experimented more than 2000 years ago with a mineral that pulled iron objects toward it. They described this mineral as being magnetic. Together, magnetic and electric forces can generate electricity and operate electric motors. Magnetism is a property of matter in which opposite magnetic poles attract and like poles repel each other. Magnetic forces are strongest near the poles (north and south). Only a few materials are naturally magnetic.

3. 22-1 The area around a magnet where the attractive or repulsive forces are felt is called the magnetic field. The field can be seen if iron filings are placed around a magnet. A compass is a device that uses the magnetic field of magnets and the magnetic field of Earth to help individuals navigate. Magnets always point north. What makes some materials magnets, other materials attracted to magnets and many materials that are neither?

4. 22-1 Each spinning electron causes a magnetic field to form around it. Most materials have electrons that exist in pairs that spin in opposite directions thus canceling out the magnetic field. The atoms in materials such as iron, cobalt and nickel have unpaired electrons, so the electrons' magnetic fields do not cancel. As a result, each atom of these elements act like a very small magnet. The magnetic fields of each atom cause groups of atoms to align into poles. These groups of atoms are called magnetic domains. If a permanent magnet is rubbed against a nail it will become a temporary magnet until the nail's atoms return to their random arrangement.

5. 22-2 Uses of Magnetic Fields In 1820, a Danish physics teacher, Hans Christian Oersted, observed that a current moving through a wire moved the needle on a nearby compass. When the current was reversed, the compass needle moved in the opposite direction. Oersted hypothesized, the electric current must produce a magnetic field around the wire. A magnetic field forms around any wire that has current flowing through it. If the wire is coiled the magnetic field lines overlap making the magnetic field stronger. When a current passes through a coiled wire the temporary magnet that is formed is called an electromagnet.

6. 22-2 Electromagnets can be strengthened in two ways - by adding more coils to the wire - by adding an iron core inside the coil By adding the iron core, the iron becomes a magnet and its magnetic field aligns with the electromagnet. Electromagnets operate doorbells, loudspeakers and to lift large metal objects in construction machines. They can be turned on and off by controlling the flow of current through the coil. They change electrical energy to mechanical energy to do work.

7. 22-2 Because electromagnets are sensitive to electrical currents, they can detect electric current. An instrument used to do so is called a galvanometer. A galvanometer consists of a coil of wire connected to a circuit and suspended so it can rotate in the magnetic field of a permanent magnet. Galvanometers can be calibrated to measure current or electrical potential depending on whether it will be used as an ammeter or a voltmeter. Ammeters measure current passing though a series circuit, whereas, voltmeters measure potential different in parallel circuits in volts.

9. 22-2 Electric motors Electric motors like galvanometers contain an electromagnetic that is free to rotate. To make the coil in an electric motor spin steadily the direction of the current through the coil must be reversed after each half revolution. The device used to change the direction of the current is called the commutator. The commutator is a reversing switch that rotates with the electromagnet.

10. Electric Motors

11. 22-3 Producing Electric Current Oersted found that magnetism could be produced from electrical current. It was later discovered that moving a wire through a magnetic field induced a current in the wire. This process is called electromagnetic induction and can be done in two ways 1.Surrounding a moving wire with a magnetic field 2.Moving a magnet in and out of a coil of wire. A generator works using the first method. The wire loop is connected to a source of mechanical energy and placed between the poles of a magnet.

12. Generator

13. 22-3 The design of a generator is very similar to that of an electric motor. The difference is that in an electric motor the wire loop is made to rotate by external forces (battery) and in the generator the wire crosses through the magnetic forces causing it to rotate. Motors consume electricity to make mechanical power, whereas, generators consume mechanical power to produce electricity. If a motor is using a battery it is using direct current (DC). DC flows only in one direction through a wire. Electrons always move from the negative to the positive terminal. When plugging an appliance in to the wall it is using alternating current (AC). AC reverses its direction in a regular pattern.

14. 22-3 Current flowing from power plants to homes through power lines are AC and have extremely high voltage. Before the AC can enter your home its voltage must be decreased. The device that can raise or lower the voltage of an AC is a transformer. A transformer reduces voltage to 120 volts before it enters your home. A transformer that reduces voltage is called a step-down transformer. In step-up transformers the output voltage is greater than the input voltage. This helps transmit AC current from power plants long distances.

15. 22-4 Superconductivity Remember that all conducting materials have some resistance to electron flow and thus electricity moving through the conductor is lost as heat. Also as the temperature rises, the resistance rises and more current is lost. A material that has no electrical resistance is called a superconductor. In 1911 scientists discovered that some materials when cooled to temperatures near absolute zero (0K) -273 o C lost all electrical resistance. One way to cool a material to superconducting temperatures is to submerge it in liquid helium. Helium is normally a gas but liquefies at low temperatures. The use of superconductors could eliminate electrical energy waste and improve the function of electric motors, generators and computer parts.