Chapter 7: Magnetism and Its Uses

1. Chapter 7: Magnetism and Its Uses Section 1: Magnetism Section 2: Electricity and Magnetism Section 3: Producing Electric Current

2. N N S S N S N S Section 1: Magnetism Magnetism – the properties and interaction of magnets • Magnetic field – the region around a magnet that exerts themagnetic force • The magnetic fieldweakens with distance • The magnetic field can be representedby curved lines called magnetic field lines • Magnetic poles – regions on a magnet where the magnetic force is strongest • All magnets have a north and south pole • Magnetic field lines start at the north pole and end on the south pole • Two magnets will either attract or repel each other • Like poles will repel, unlike poles will attract

3. Section 1: Magnetism • Earth’s magnetic field and compasses • A compass is a device consisting of a tiny bar magnet that is free to rotate • When a compass is placed near a magnet, the needle will align with the field lines of the magnet • The Earth acts like a huge bar magnet, so a compass needle will align with the Earth’s magnetic field line and the need le will point to Earth’s magnetic north pole • Magnetic materials – not all metals are attracted to a magnet • Magnetic metals include iron, cobalt, and nickel • All electrons have magnetic properties. In the atoms of most elements these properties cancel out • The magnetic properties in the atoms of iron, cobalt, and nickel do not cancel, so each atom of these metals behave like little magnets

4. Section 1: Magnetism • Magnetic domains – groups of atoms with aligned magnetic poles • Placing a magnetic metal near a permanent magnet will cause the atoms in the metal to align, and the metal will become a temporary magnet • Because of the random motion of the atoms in the metal this alignment is soon lost and the metal will no longer act as a magnet • In permanent magnets the magnetic field inside the magnet is several thousand times stronger than the magnetic field outside the magnet so the magnetic domains do not get bumped out of alignment

5. Magnetic field lines Electron flow Electron flow Magnetic field lines Section 2: Electricity and Magnetism Electric Current and Magnetism • Moving charges, like an electric current, produce magnetic fields • The field forms a circular pattern around a current-carrying wire • The direction of the field depends on the direction of the of the current • The strength of the magnetic field depends on the amount of current Electromagnetism – the interaction between electric charges and magnets • Electromagnetic force – the attractive or repulsive force between electric charges and magnets

6. Section 2: Electricity and Magnetism Electromagnets • An electromagnet is a temporarymagnet made by placing a piece or iron inside a current-carrying coil of wire • Electromagnets are temporary magnets because the magnetic field is present only when the current is flowing Electric Motors • An electric motor is a device that converts electrical energy into mechanical energy • Contains an electromagnet that is free to rotate between the poles of a permanent, fixed magnet • Converts alternating current to rotary motion

7. Section 3: Producing Electric Current Electromagnetic induction – the production of an electric current by moving a loop of wire through a magnetic field or moving a magnet through a wire loop • Generator – a device that produces electric current by rotating a coil of wire in a magnetic field • Power plants use huge generators to produce electric current • Turbine– a large wheel that rotates when pushed by water, wind, or steam Direct and Alternating Currents • Direct current – the electric current flows in only one direction • Ex.: the current produced by a battery • Alternating current – reverses the flow of current in a regular way • In North America, generators produce alternating current at a frequency of 60 Hz (cycles per second) • The electric current produced changes direction twice during each cycle, so a 60 Hz alternating current changes direction 120 each second

8. Section 3: Producing Electric Current Transformers • Transformer – a device that increases or decreases the voltage of an alternating current • Made of two coils of wire, called the primary and secondary coils, wrapped around the same iron core • As the alternating current passes through the primary coil the core becomes an electromagnet • Because the current is changing direction many times each second, the magnetic field of the iron core changes direction. • The changing magnetic field induces an alternating current in the secondary coil • Step-up transformer – if the secondary coil has more turns of wire than the primary, then the transformer increases voltage • Step-down transformer – if the secondary coil has fewer turns of wire than the primary, then the transformer decreases voltage • Before the electric current enters your house it must travel through a step-down transformer

9. Section 3: Producing Electric Current Example 1: What is the output voltage of a transformer if the input voltage is 240 V and the primary coil has 20 wraps and the secondary coil has 30 wraps? Solution Example 2: What is the output voltage of a transformer in the input voltage is 1,200 Vand the primary coil has 40 wraps and the secondary coil has 4 wraps? Solution