1. Pearson Prentice Hall Physical Science: Concepts in Action Chapter 21 Magnetism

2. 21.1 Magnets and Magnetic Fields Objectives: 1. Describe how magnetic poles interact 2. Explain how a magnetic field affects a magnet that enters the field 3. Explain why some materials are magnetic and others are not

3. Magnetic Pole Interaction Def: magnetic force is the force a magnet exerts on another magnet, on iron or a similar metal, or on moving charges Def: magnetic poles are regions where the magnet’s force is the strongest All magnets have two poles : north and south If a magnet is cut, each piece will still have 2 opposite poles The poles exert a force on each other Like magnetic poles repel one another Opposite magnetic poles attract one another

4. Magnetic Fields Def: a magnetic field surrounds a magnet and can exert magnetic forces A magnetic field is strongest near a magnet’s poles A magnetic field will attract or repel another magnet that enters the field Magnetic field lines are used to represent the magnetic field of a bar magnet field lines that are close together mean a strong field The magnetic field gets weaker with distance from the magnet Def: the magnetosphere is the magnetic field that surrounds earth

5. Earth can be imagined as a giant bar magnet Earth’s magnetic field has direction and strength Earth has one magnetic point in northeastern Canada and an opposite point in Antarctica these magnetic poles are not the same as the geographic poles At the northern pole, the compass needle would point straight down At the southern pole, it would point straight up Earth’s magnetic field has changed direction throughout geologic time (at least 20 reversals in the past 5 million years)

6. Magnetic Materials Def: a magnetic domain is a region that has a very large number of atoms with aligned magnetic fields Domains are microscopic magnetic regions composed of a group of atoms whose magnetic fields are aligned in a common direction Magnetic fields of atoms in a domain point in the same direction Ferromagnetic materials like iron can be magnetized because they contain magnetic domains When a material is magnetized, most of its magnetic domains are aligned

7. 21.2 Electromagnetism Objectives: 1. Explain how an electric charge can create a magnetic field 2. Describe how an electromagnet is controlled 3. Explain how galvanometers and electric motors work

8. Electric Charge Can Create a Magnetic Field Electromagnetic force is a combination of electricity and magnetism Electric force comes from charged particles Magnetic force usually comes from movement of electrons in an atom Both electric and magnetic forces come from electric charges Moving electric charges create a magnetic field

9. Use the right hand rule to find the direction of the magnetic field produced by a current Imagine holding the wire in your right hand with your thumb in the direction of the positive current, the direction your fingers curl is the direction of the magnetic field Def: A solenoid is a long wound coil of insulated current carrying wire

10. The magnetic field of each loop of wire adds strength to the magnetic field Increasing the current also adds strength to the magnetic field The magnetic field has north and south poles and can be as strong as that of a bar magnet Inserting a rod made of iron or other potentially magnetic material increases the magnetic field and creates an electromagnet

11. Controlling Electromagnets Def: an electromagnet is a solenoid with a ferromagnetic core Changing the current in an electromagnet controls the strength and direction of its magnetic field Electromagnetic devices such as galvanometers and electric motors change electrical energy into mechanical energy

12. Galvanometers & Electric Motors Def: a galvanometer is a device that uses a solenoid to measure small amounts of current Example: the gas gauge in your car Def: an electric motor is device that uses an electromagnet to turn an axle An actual motor has many loops of wire around a central iron core to make the motor stronger

13. 21.3 Electrical Energy Generation & Transmission Objectives: 1. Discuss how voltage is induced in a conductor 2. Name two types of generators 3. Describe how a transformer changes voltage and current 4. Name some sources of electrical energy in the United States

14. Voltage Induction Def: electromagnetic induction is the process of generating current by moving an electrical conductor relative to a magnetic field In 1831, Michael Faraday discovered electromagnetic induction Electromagnetic induction is the production of current in a conducting circuit by a change in the strength, position or orientation of an external magnetic field Faraday’s law: An electric current can be produced in a circuit by a changing magnetic field

15. Two Types of Generators The two types of generators are AC generators and DC generators Generators convert mechanical energy into electrical energy For AC generators, a circuit is induced by a loop of wire moving between two magnetic poles A charged particle moving in a magnetic field will experience a force due to the magnetic field The force is at its maximum when the charge moves perpendicular to the field, and at its minimum when moving opposite the magnetic field lines When a wire full of charges is parallel to the field there is no current induced in the wire

16. Each half rotation of the loop inside the generator reverses direction producing alternating current (AC) Alternating current is an electric current that changes direction at regular intervals Most of the world uses AC Def: a commutator is a device used to make the current change direction every time the loop rotates Devices called brushes connect the wires to the commutator DC generators are similar to AC generators except that commutators are used

17. Transformers Voltage and Current Def: A transformer is a device that can change one AC voltage to a different AC voltage Transformers have primary and secondary coils around an iron core Transformers change voltage and current by inducing a changing magnetic field in one coil The changing field then induces an AC in the second coil with a different number of turns

18. The voltage of the secondary coil depends on the number of loops (turns) in the coil The transformer can be arranged for the secondary voltage to be greater than the primary (step up transformer) or less than the primary (step down transformer) Step down transformers have fewer loops than the primary coil Transformers are used in the transmission of electrical energy to homes and businesses (normally transmitted at 120 V)

19. Sources of Electrical Energy Most of the electrical energy generated in the United States is produced using coal as an energy source Other sources include water (hydroelectric), nuclear energy, wind, natural gas and petroleum Def: a turbine is a device with fanlike blades that turn when pushed (by water, wind or steam, for example) Burning fossil fuels and nuclear power electrical energy production require turbines to produce electricity