Section 1: Magnetism

• Magnets

  • More than 2,000 years ago Greeks discovered deposits of a mineral that was a natural magnet.

  • Magnetism: refers to the properties and interactions of magnets.

  • The strength of the force between two magnets increases as magnets move closer together and decreases as the the magnets move farther apart.

  • Magnetic Field: exerts a force on other magnets and objects made of magnetic materials.

    • The magnetic field is strongest close to the magnet and weaker far away.

    • A magnet’s magnetic field is represented by magnetic field lines.

  • Magnetic Poles: where the magnetic force exerted by the magnet is strongest.

  • Magnetic field lines always connect the north pole and the south pole of a magnet.

  • Two magnets can either attract or repel each other.

    • Two north poles or two south poles of two magnets repel each other.

    • North poles and south poles always attract each other.

    • Like magnetic poles repel each other and unlike poles attract each other.

  • When a compass is brought near a bar magnet, the compass needle rotates. The compass needle is a small bar magnet with a north pole and a south pole.

  • The north pole of a compass points in the direction of the magnetic field.

  • A compass can help determine direction because the north pole of the compass needle points north.

  • The north pole of a magnet is defined as the end of the magnet that points toward the geographic north.

• Magnetic Materials

  • Only a few metals, such as iron, cobalt, or nickel, are attracted to magnets or can be made into permanent magnets.

  • Electrons have magnetic properties.

  • Even though these atoms have their own magnetic fields, objects made from these metals are not always magnets.

  • In iron, cobalt, nickel, and some other magnetic materials, the magnetic field created by each atom exerts a force on the other nearby atoms.

  • Magnetic Domains: The groups of atoms with aligned magnetic poles

    • Each domain contains an enormous number of atoms, yet the domains are too small to be seen with the unaided eye.

  • Even though each domain behaves like a magnet, the poles of the domains are arranged randomly and point in different directions.

  • A permanent magnet can be made by placing a magnetic material, such as iron, in a strong magnetic field.

    • Even permanent magnets can lose their magnetic behavior if they are heated.

      • Heating causes atoms in the magnet to move faster.

  • Because every magnet is made of many aligned smaller magnets, even the smallest pieces have both a north pole and a south pole.

Section 2: Electricity and Magnetism

• Electric Current and Magnetism

  • In 1820, Hans Christian Oersted, a Danish physics teacher, found that electricity and magnetism are related.

  • It is now known that moving charges, like those in an electric current, produce magnetic fields.

  • When electric current flows through a wire, a magnetic field forms around the wire. The direction of the magnetic field depends on the direction of the current in the wire.

  • As the current in the wire increases the strength of the magnetic field increases.

  • As you move farther from the wire the strength of the magnetic field decreases.

• Electromagnet: a temporary magnet made by wrapping a wire coil carrying a current around an iron core.

  • An electromagnet is made from a current-carrying wire.

  • Solenoid: A single wire wrapped into a cylindrical wire coil

  • If the solenoid is wrapped around an iron core, an electro- magnet is formed.

  • Electromagnets are temporary magnets because the magnetic field is present only when current is flowing in the solenoid.

  • An electromagnet behaves like any other magnet when current flows through the solenoid.

  • What makes electromagnets so useful is that their magnetic properties can be controlled by changing the electric current flowing through the solenoid.

  • When current flows in the electromagnet and it moves toward or away from another magnet, electric energy is converted into mechanical energy to do work.

  • The electromagnet in a speaker converts electrical energy into mechanical energy to produce sound.

  • The forces exerted on an electromagnet by another magnet can be used to make the electromagnet rotate.

    • One way to change the forces that make the electromagnet rotate is to change the current in the electromagnet. Increasing the current increases the strength of the forces between the two magnets.

  • Galvanometers: devices that use an electromagnet to measure electric current.

    • In a galvanometer, the electromagnet is connected to a small spring.

• Electric Motor: a device that changes electrical energy into mechanical energy.

  • Electric motors are used in all types of industry, agriculture, and transportation, including airplanes and automobiles.

  • The main parts of a simple electric motor include a wire coil, a permanent magnet, and a source of electric current, such as a battery.

  • A simple electric motor also includes components called brushes and a commutator.

    • The brushes are conducting pads connected to the battery.

    • The brushes make contact with the commutator, which is a conducting metal ring that is split.

Section 3: Producing Electric Current

• From Mechanical to Electrical Energy

  • If the current in a wire changes with time, the changing magnetic field around the wire can also induce a current in a nearby coil.

  • Electromagnetic Induction: The generation of a current by a changing magnetic field

  • Generator: uses electromagnetic induction to transform mechanical energy into electrical energy.

  • The coil in a generator is rotated by an outside source of mechanical energy.

  • As the generator’s wire coil rotates through the magnetic field of the permanent magnet, current flows through the coil.

  • The frequency with which the current changes direction can be controlled by regulating the rotation rate of the generator.

  • The alternator provides electrical energy to operate lights and other accessories.

  • Each of these generators at Hoover Dam can produce over 100,000 kW of electric power. In these generators, a rotating magnet induces an electric current in a stationary wire coil.

  • Turbine: a large wheel that rotates when pushed by water, wind, or steam.

  • Both generators and electric motors use magnets to produce energy conversions between electrical and mechanical energy.

• Direct and Alternating Currents

  • Modern society relies heavily on electricity.

  • A battery produces a direct current.

    • Direct current: flows only in one direction through a wire.

  • Alternating Current: reverses the direction of the current in a regular pattern.

  • The electric current produced by a generator changes direction twice during each cycle, or each rotation, of the coil.

  • Electronic devices that use batteries as a backup energy source usually require direct current to operate.

• Transmitting Electrical Energy

  • The alternating current produced by an electric power plant carries electrical energy that is transmitted along electric transmission lines.

  • The heat produced in the power lines warms the wires and the surrounding air and can’t be used to power electrical devices.

  • One way to reduce the heat produced in a power line is to transmit the electrical energy at high voltages, typically around 150,000 V.

• Transformer: a device that increases or decreases the voltage of an alternating current.

  • A transformer is made of a primary coil and a secondary coil.

  • The voltage in the primary coil is the input voltage and the voltage in the secondary coil is the output voltage.

  • The output voltage divided by the input voltage equals the number of turns in the secondary coil divided by the number of turns in the primary coil.

  • A transformer that increases the voltage so that the output voltage is greater than the input voltage is a step-up transformer.

    • In a step-up transformer the number of wire turns on the secondary coil is greater than the number of turns on the primary coil.

  • A transformer that decreases the voltage so that the output voltage is less than the input voltage is a step-down transformer.

    • In a step-down transformer the number of wire turns on the secondary coil is less than the number of turns on the primary coil.

  • Power plants commonly produce alternating current because the voltage can be increased or decreased with transformers.

  • Many steps are involved in the creation, transportation, and use of the electric current in your home.