Magnetism and Electromagnetism

Magnetism and Electromagnetism • Electric Current and Magnetism • Electric Motors • Electric Generators and Transformers

Electric Current and Magnetism In 1819, Hans Christian Oersted, a Danish physicist and chemist, and a professor, placed a compass needle near a wire through which he could make electric current flow. When the switch was closed, the compass needle moved just as if the wire were a magnet.

Electric current and magnetism Electric current is made of moving charges (electrons), which creates the magnetic field around a current-carrying wire Magnetism is created by these moving charges.

The magnetic fields of straight wire • The magnetic field lines are concentric circles with the wire at the center of the circles. • The direction of the field depends on the direction of the current in the wire.

The magnetic fields of straight wire • The strength of the magnetic field near the wire depends on two factors: • The strength is directly proportional to the current, so doubling the current doubles the strength of the field. • The field strength is inversely proportional to the distance from the wire. (Decreasing the distance to the wire by half doubles the strength of the field.)

The magnetic fields of straight wire • Near a straight wire, the north pole of a compass needle feels a force in the direction of the field lines. • The south pole feels a force in the opposite direction. • As a result, the needle twists to align its north-south axis along the circular field lines.

The magnetic fields of loops and coils The magnetic field around a single wire is too small to be of much use. There are two techniques to make strong magnetic fields from current flowing in wires: Parallel wires can be bundled together. (10 wires, each with 1 A of current, create a magnetic field 10X as strong as 1 wire carrying 1 A). A wire can be looped into a coil so the magnetic field is concentrated at the center.

The magnetic fields of loops and coils The most common form of electromagnetic device is a coil with many turns called a solenoid. A coil takes advantage of these two techniques (bundling wires and making bundled wires into coils) for increasing field strength.

Magnetic forces and electric currents Two wires carrying electric current exert force on each other, just like two magnets. The forces can be attractive or repulsive depending on the direction of current in both wires.

Electric motors Electric motorsconvert electrical energy into mechanical energy. The disk in the motor is called the rotorbecause it can rotate. The disk will keep spinning as long as the external magnet is reversed every time the next magnet in the disk passes by. One or more stationary magnets reverse their poles to push and pull on a rotating assembly of magnets.

Using magnets to spin a disk • Reversing the magnet in your fingers attracts and repels the magnets in the rotor, making it spin.

Commutation The process of reversing the current in the electromagnet is called commutation and the switch that makes it happen is called a commutator.

Electric Motors All types of electric motors have three key parts: A rotating element (rotor) with magnets. A stationary magnet that surrounds the rotor. A commutator that switches the electromagnets from north to south at the right place to keep the rotor spinning.

AC and DC motors • Motors that run on alternating current (AC) electricity are easier to make because the current switches direction all by itself—a commutator isn’t needed.

Electric motors The rotating part of the motor, including the electromagnets, is called the armature. It has 3 electromagnets that correspond to the 3 coils.

Electric motors The permanent magnets are on the outside, and they stay fixed in place. The wires from each of the three coils are attached to three metal plates (commutator) at the end of the armature. commutator

Electric Motors As the rotorspins, the three plates come into contact with the positive and negative brushes. Electric current flows through the brushes into the coils.

Electromagnetic Induction If you move a magnet near a coil of wire, a current will be produced. This process is called electromagnetic induction, because a moving magnet induceselectric current to flow. Moving electric charge creates magnetism and conversely, changing magnetic fields also can cause electric charge to move.

Induction Current is only produced if the magnet is moving because a changingmagnetic field is what creates current. If the magnetic field does not change, such as when the magnet is stationary, the current is zero.

Induction If the magnetic field is increasing, the induced current is in one direction. If the field is decreasing, the induced current is in the opposite direction.

Faraday’s law of induction A moving magnet induces current in a coil only ifthe magnetic field of the magnet passes through the coil.

Faraday’s law of induction • Michael Faraday (1791–1867), an English physicist and chemist, was first to explain how moving magnets and coils induced voltage. • Faraday’s found that the induced voltage is proportional to the rate of change of the magnetic field through the coil.

Faraday’s law of induction Faraday’s law says the current in a coil is proportional to the rate at which the magnetic field changes.

Faraday's Law

Generators A generator is a device that uses induction to convert mechanical energy into electrical energy.

Electrical generators • The electrical energy created by a generator is not created from nothing. • Energy must continually be supplied to keep the rotating coil or magnetic disk turning. • In hydroelectric generators, falling water turns a turbinewhich spins a generator to produce electricity.

Producing and transporting energy • Hoover Dam is called a hydroelectric plant because it converts the energy of falling water into electricity. • Using the potential energy of water is one way to produce electricity.

Energy flow • With each transformation (green arrows), some energy is lost to the system in the form of heat (red arrows).

Electricity from different resources • A nonrenewable resourceis not replaced as it is used. • Any fossil fuelis an good example of nonrenewable resource. • Besides their growing scarcity, burning fossil fuels produces sulfur oxide emissions that reduce air quality and may be accelerating climate change.

Electricity from different resources • A renewable resourcecan be replaced naturally in a relatively short period of time. • Falling water, energy from the Sun, wind energy, and geothermal energy are examples of renewable resources.

Geothermal, biomass and hydroelectric energy • Geothermal power plants use Earth’s internal heat in the form of water or steam, to produce electricity. • Biomass, such as organic material from plants or animals or municipal waste, can be burned to produce steam for a turbine. • Impoundment and pumped storage hydroelectric power plants use falling water differently to generate electricity.

Electrical Power • Recall that electrical power (in watts) is the rate at which electrical energy is changed into other forms of energy such as heat, sound, or light. • Anything that “uses” electricity is actually converting electrical energy into some other type of energy. • Utility companies charge customers for the number of kilowatt-hours (kWh) used each month.

Transformers Transformers are extremely useful because they efficiently change voltage and current, while providing the same total power. The transformer uses electromagnetic induction, similar to a generator.

Transformers • Consider the transformer between the outside power lines and your house: • The primary coil is connected to outside power lines. Current in the primary coil creates a magnetic field through the secondary coil. The primary coil’s field is shown by the magnetic field lines (green arrows) • The current in the primary coil changes constantly because it is alternating current.

Transformers As the current changes, so does the strength and direction of the magnetic field through the secondary coil. The changing magnetic field through the secondary coil induces current in the secondary coil. The secondary coil connects to your home’s wiring. • The relationship between voltages and turns for a transformer is the result of two coils having a different number of turns.

Transformers • In the same changing magnetic field, a coil with 100 turns produces 10 times the voltage of the induced current as a coil with 10 turns.

Changing voltage in a transformer A cell-phone AC adapter reduces the 120 V AC to the 6 V DC needed by the phone’s battery. If the primary coil has 240 turns, how many turns must the secondary coil have? • Looking for:…no. of turns of the secondary coil. • Given:…voltage of each coil (120VAC and 6 VDC) and the no. of turns of the primary coil (240) • Relationships:…V1 = N1 Solve for N2 = V2 x N1 V2 N2V1 • Solution: … 6 V x 240 = 12 turns 120 V

Despite little formal schooling, Michael Faraday rose to become one of England’s top research scientists of the nineteenth century. He is best known for his discovery of electromagnetic induction, which made possible the large-scale production of electricity in power plants. Michael Faraday

Magnetism and Electromagnetism