MAGNETISM AND ITS USES CHAPTER 8 SLIDE 1 The Greeks found a mineral, they called (lode stone), now called magnetite. It had natural magnetic properties. What explanations do you think they gave for the behavior of magnetite? SECTION 1: MAGNETISM A. Magnetism – the properties an interactions of magnets 1. Interaction between two magnets called magnetic force increases as magnets move closer together. 2. A magnetic field, which exerts the magnetic force, surrounds a magnet, and is strongest closest to the magnet. SLIDE 2 B. Magnetic poles – the regions of a magnet where the magnetic force exerted by the magnet is strongest 1. All magnets have a north pole and a south pole. 2. Like poles repel. Unlike poles attract. SLIDE 3 The magnetic field lines around horseshoe and disk magnets are closest together at the magnet’s poles. Magnets can attractSLIDE or4 repel each other. Unlike poles attract. When unlike poles are brought together, their magnetic field lines seem to connect with each other. Like poles repel. When like poles are brought together, their magnetic field lines seem to push away from each other. C. Magnetic materialsSLIDE – iron, cobalt, and 5 nickel 1. The magnetic field created by each atom exerted a force on nearby atoms. 2. Magnetic domains – groups of atoms with aligned magnetic poles. a. In a magnet, the like poles of all the domains point in the same direction. b. Permanent magnets are made by placing a magnetic material in a strong magnetic field, forcing a large number of magnetic domains to line up. SLIDE 6 3. Earth has magnetic poles. a. A compass needle is a small bar magnet that can freely rotate. b. A compass needle always points north. A normal iron nail is made up of billions of domains that are arranged randomly. The domains will align themselves along the magnetic field lines of a nearby magnet. Each piece of a broken magnet still has a north and a south pole. SECTION 2: A. ELECTRICITY & MAGNETISM Moving charges and magnetic fields. 1. Moving charges, like those in an electric current, produce magnetic fields. a. The magnetic field around a current-carrying wire forms a circular pattern about the wire. b. The direction of the field depends on the direction of the current. c. The strength of the magnetic field depends on the amount of current flowing in the wire SLIDE 2 B. Electromagnet – a temporary magnet made by placing a piece of iron inside a currentcarrying coil of wire. 1. Magnetic field is present only when current is flowing in the wire. 2. Increase strength of the magnetic field by adding more turns to the wire coil or increasing the current passing through the wire. 3. Magnetic properties of electromagnets can be controlled by changing the electric current flowing through the wire. 4. Converts electrical energy into mechanical energy to do work. SLIDE 2 B. When many loop of currentcarrying wire are formed into a coil, the magnetic field is increased inside the coil. The coil has a north pole and a south pole. A. Magnetic field lines circle around a loop of current-carrying wire. C. An iron core inserted into the coil becomes a magnet. SLIDE 3 The electromagnet in a speaker turns electrical energy into mechanical energy to produce sound. A. B. SLIDE 4 When current flows through the coil, an electromagnet is formed that is attracted to and repelled by the poles of the permanent magnet. The magnetic forces on the coil cause it to rotate, aligning it with the field of the permanent magnet. C. Galvanometer – a device that uses an electromagnetic to measure electric current. D. Electric motor – a device that changes SLIDE 5 electrical energy into mechanical energy. 1. Contains an electromagnet that is free to rotate between the poles of a permanent, fixed magnet. The coil in the electromagnet is connected to a source of electric current. 2. When a current flows through the electromagnet, a magnetic field is produced in the coil. 3. Changing the direction of the current causes the coil in an electric motor to keep rotating. 4. Rotation speed of electric motors can be controlled. a. Vary the amount of current flowing through the coil. b. When more current flows through the coil, the electromagnet’s magnetic field becomes stronger, the magnetic force between turns faster. A. A battery causes an electric current to flow through the coil of the electromagnet. B. Unlike poles of the two magnets attract each other, and the like poles repel. This causes the coil to rotate until the opposite poles are next to each other. C. If the current in the coil is switched, the direction of the coil’s magnetic field also switches. The north and south poles of the magnet trade places. D. The coil is repelled by and attracted once again to the poles of the permanent magnet. The coil rotates until it is again aligned with the permanent magnet’s field. SECTION 3: PRODUCING ELECTRIC CURRENT A. From mechanical to electrical energy 1. Electromagnetic induction – the production of an electric current by moving a lop of wire through a magnetic field or moving a magnet through a wire loop 2. Generator – a device that produces electric current by rotating a coil of wire in a magnetic field a. The wire coil is wrapped around an iron core and placed between the poles of a permanent magnet. b. Coil is rotated by an outside source of mechanical energy. c. As the coil turns within the magnetic field of the permanent magnet, an electric current flows through the coil. d. Direction of the current in the coil in a generator changes twice with each revolution. SLIDE 2 The electromagnet in a generator is rotated by some outside source of mechanical energy. In this setup, a student can rotate a crank to turn the electromagnet. SLIDE 3 The direction that current flows in a wire coil depends on how the wire coil is aligned with the permanent magnet. SLIDE 4 3. Generating electricity a. Electricity used in the home comes from a power plant with huge generators. b. Coils of electromagnets in the generators usually connected to a turbine – a large wheel that rotates when pushed by water, wind, or steam.