Unit 3 Lesson 5 Electromagnetism Copyright Houghton Mifflin

Unit 3 Lesson 5 Electromagnetism Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism Magnetic Attraction What is electromagnetism? • In 1820, Danish physicist Hans Christian Oersted observed a magnet move in the presence of an electric current. • Oersted hypothesized that the electric current had produced a magnetic field. Later experiments confirmed his hypothesis, demonstrating the relationship between electricity and magnetism. • Electromagnetism results when electric currents and magnetic fields interact with each other. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism How can you make a magnet using current? • A coil of wire that carries an electric current, and therefore produces a magnetic field, is called a solenoid. The more loops, the stronger the magnetic field. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism How can you make a magnet using current? • Wrapping a solenoid around an iron core makes an electromagnet, which combines the magnetic field of the solenoid with the magnetic field of the magnetized iron core. • Adding loops to the solenoid or increasing the electric current strengthens the electromagnet. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism What are some uses for electric magnets? • Electromagnets are useful for lifting and moving large metal objects containing iron. When current runs through the solenoid coils, it creates a magnetic field that attracts the metal objects. Turning off the current turns off the magnetic field so that the metal can be easily dropped in a new place. • Powerful electromagnets can raise a maglev train above its track. Just as poles of a bar magnet repel each other, electromagnets in the train and track repel each other when the electric current is turned on. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism What are some uses for electric magnets? • A galvanometer is a meter that measures the strength and direction of an electric current in a wire. • A galvanometer contains an electromagnet between the poles of a permanent magnet. When current is applied to the electromagnet, the two magnetic fields interact and cause the electromagnet to turn. • The indicator, attached to the electromagnet, moves to one side of the zero on the scale, indicting the strength and direction of the current. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism What are some uses for electric magnets? • The indicator on a galvanometer shows current direction and strength. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism A Look Inside • Magnetic resonance imaging (MRI) machines use powerful electromagnets and radio waves to produce detailed images of the body’s interior. • Doctors use MRI scans to diagnose many conditions, including broken bones and strained tendons. • Some MRI scans help scientists understand how the brain works. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism Let’s Motor! How do motors work? • An electric motor changes electrical energy into mechanical energy. • Electric motors range in size from large motors, used to power Ferris wheels, to small motors used in computer cooling fans. Almost every time a device uses electricity to make something move, there is a motor involved. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism How do motors work? • A simple motor has a coil or loop of wire called an armature mounted between the poles of a magnet. The armature becomes an electromagnet when current passes through it. • The armature rotates because its poles are pushed and pulled by the opposite poles of the magnet. The armature turns until its north pole is opposite the magnet’s south pole. • Then, a device called a commutator reverses the direction of the current in the wire, causing the armature to complete its turn. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism How do motors work? • Electric motors are very similar to galvanometers. However, in a motor, the electromagnet is made to rotate all the way around instead of back and forth in the magnetic field. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism A New Generation What are some uses for induction? • Using a magnetic field to create an electric current in a wire is called electromagnetic induction. • When electric charges move through a wire, the wire carries a current. Magnetic force from a magnet moving inside a coil of wire can make the electric charges in the wire move. When the magnet stops moving inside the coil, the electric current stops. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism What are some uses for induction? • The electric current increases if you move the magnet through the coil faster or if you add more loops of wire. The current can also be induced by moving the coil over the magnet. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism What are some uses for induction? • Transformers use induction to increase or decrease the voltage of alternating current. For example, transformers on power lines increase voltage to send current miles away and then decrease voltage for a single home. • Most transformers are iron “rings” with two coils of wire. The current in the primary side makes an electromagnet. Because the current alternates, the magnetic field changes. The changing magnetic field induces a current in the secondary side. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism What are some uses for induction? • In a step-up transformer, there are more turns of wire on the secondary side. In a step-down transformer, there are more turns of wire on the primary side. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism What are some uses for induction? • Electric generators use induction to change mechanical energy into electrical energy. • In power plants, mechanical energy is used to rotate turbines. The turbines turn magnets inside coils of wire, generating electricity. • Many power plants use rising steam to turn the turbines. The steam is produced from burning fossil fuels or using nuclear reactions to heat water. Other sources of mechanical energy to turn turbines are blowing wind, falling water, and ocean tides and waves. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 5 Electromagnetism What are some uses for induction? • Generators induce electric current when a magnet moves in a coil of wire or when a wire moves between the poles of a magnet. • In a simple generator, a wire loop at the end of a rod moves through the magnetic field of a magnet. In the first half of the turn, one side of the loop moves downward. • In the second half of the turn, the part of the loop that was moving downward now moves upward, reversing the current. This creates alternating current. Copyright © Houghton Mifflin Harcourt Publishing Company