Transcript Magnets?
Chapter 12 What is Magnetism? Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons. Magnets? •The earliest magnets were found naturally in the mineral magnetite which is abundant the rock-type lodestone. These magnets were used by the ancient peoples as compasses to guide sailing vessels. *A magnetic field is produced by the motion of electric charge. * * Two kinds of electron motion make magnetism: 1. * Electron spin – electrons spin about their own axes like tops 2. * Electron revolution – electrons revolve about the atomic nucleus like planets revolving around the sun In most common magnets, electron spin is the main contributor to magnetism. •Every spinning electron is a tiny magnet. A pair of electrons spinning in the same direction makes a stronger magnet. A pair of electrons spinning in opposite directions, however work against each other. The magnetic fields cancel. This is why most substances are not magnets. Magnets? (cont) * Iron, Nickel, and Cobalt are materials that are easily magnetized because their fields do not cancel each other entirely. Most common magnets are therefore made from alloys containing iron, nickel, and cobalt in various proportions. * Some materials are easy to magnetize, but they lose their magnetism quickly these are called temporary magnets. * Materials that are more difficult to magnetize, but they tend to stay magnetized, are called permanent magnets. Magnets have two ends or poles, called north and south poles. * Unlike poles of magnets attract each other and like poles of magnets repel. * Electric Charges can be isolated, but Magnetic Poles cannot. If you cut a magnet in half – you get 2 Magnets both with a North and South Pole * At the poles of a magnet, the magnetic field lines are closer together. Magnetic Fields * A magnetic field is produced by the motion of electric charge. The region where the magnetic forces act is called the “magnetic field” The filings line up with the magnetic field lines that spread out from one pole and return to the other. Where the lines are closer together, the field is stronger. The earth is like a giant magnet! The nickel iron core of the earth gives the earth a magnetic field much like a bar magnet. Magnetic South Pole * The needle of a compass always points toward the magnetic south pole. We call this direction “North” (remember, opposites attract) Magnetic North Pole What are magnetic domains? Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. * These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. * How to weaken a magnet: 1. Drop it 2. Heat it 3. Hit it with a hammer Some of the domains are jostled out of alignment so they become random again Electricity and Magnetism – how are they related? * A single moving charge produces a magnetic field. * When an electric current passes through a wire a magnetic field is formed. LEFT HAND RULE We can use the fingers and thumb of the left hand to determine either the direction of Current Flow or the direction of the Magnetic Field or direction of the Magnetic North Pole Conductors Thumb pointing in Direction of Current Flow Fingers are pointing in the direction of the magnetic field. Electromagnets Fingers pointing in Direction Current Flow Thumb points in Direction of North Pole. What is a Solenoid? * A long coil of wire with many loops in it is called a solenoid. Magnetic field lines about a current-carrying wire crowd up when the wire is bent into a loop. More loops mean more magnetic field intensity. * In summary then a Solenoid acts as a Magnet when a Current passes through it * Three things that can strengthen the magnetic field of a solenoid: 1. Increasing the # of Coils 2. Increasing the Current (Amps) flowing through it 3. Placing a piece of Iron in the Center of it (Electromagnet) What is an electromagnet? * When an electric current is passed through a coil of wire (solenoid) wrapped around a iron core, the alignment of magnetic domains in the iron produces a very strong magnetic field. This is called an electromagnet. * What are the differences between an electromagnet and a permanent magnet? You can make an electromagnet stronger Increase the current (amps) You can change the direction of the poles in an electromagnet Change the direction of current You can turn an electromagnet on and off Close or open the circuit (flip a switch) An electromagnet would then be classified as a temporary magnet Superconducting Electromagnets Ceramic superconductors have the interesting property of expelling magnetic fields. Because magnetic fields cannot penetrate the surface of a superconductor, magnets levitate above them. Used for the levitation of high-speed trains for transportation. Can travel vibration-free at high speeds because they make no physical contact with the guideway they float above. Magnetic Forces are Exerted on Moving Charges A charged particle has to be moving to interact with a magnetic field (charges at rest don’t respond to magnets) The force is greatest when the particles move at right angles to the magnetic field lines (at other angles, the force is less and becomes zero when they move parallel to the field lines) The force is always perpendicular to the magnetic field lines and perpendicular to the velocity of the charged particle. So a moving charge is deflected when it crosses through a magnetic field, but when it travels parallel to the field no deflection occurs. Forces – Gravitational, Electrical, Magnetic Gravitation force act in a direction parallel to the line between masses Electrical force acts in a parallel direction between charges Magnetic force acts at right angles to the magnetic field and the velocity of the charged particle What is a galvanometer? A galvanometer is an electromagnet that interacts with a permanent magnet. The stronger the electric current passing through the electromagnet, the more it interacts with the permanent magnet. Galvanometers are used as gauges in cars and many other applications such as electric meters (ammeter, voltmeter) What are electric motors? * An electric motor is a device which changes electrical energy into mechanical energy. How does an electric motor work? In an electric motor the current is made to change direction each time the coil makes a half rotation. Current in a motor is reversed during each half revolution by stationary contacts on the shaft. These contacts are called Brushes. Larger Motors Larger motors, DC or AC, are usually made by replacing the permanent magnet by an electromagnet that is energized by some power source. Many loops of wire are wound about an iron cylinder, called an armature, which then rotates when the wire loops carry current. What is Electromagnetic Induction? We have seen how electricity can produce a magnetic field, but a magnetic field can also produce electricity! How? * In 1831 two physicists, Michael Faraday and Joseph Henry both discovered that electric current can be produced in a wire simply by moving a magnet in or out of a coiled part of the wire. * The voltage is caused, or induced, by the relative motion between a wire and a magnetic field. 1. Magnetic Field moves near a stationary conductor 2. Conductor moves in a stationary magnetic field * The greater the number of loops of wire moving in a magnetic field, the greater the induced voltage (10x as many loops induces 10x as much voltage) * The amount of voltage induced depends on how fast the magnetic field lines are entering or leaving the coil. Faraday’s Law The induced voltage in a coil is proportional to the number of loops multiplied by the rate at which the magnetic field changes within those loops. * Three ways in which voltage can be induced in a loop of wire: (changing magnetic field in the loop) 1. Moving the loop near a magnet 2. Moving the magnet near the loop 3. Changing a current in a nearby loop * Examples of Electromagnetic induction: • Activation of traffic lights (car drives over buried coils of wire) • Airport Security System (metal you carry alters the magnetic field in the coils) • Magnetic strip on the back of a credit card is scanned (induced voltage pulses identify the card) LEFT HAND RULE (Fleming’s Rule) The direction of Induced Current in a motor will always move in a direction according to the LEFT HAND RULE. If the thumb, forefinger, and middle finger of the left hand are held at right angles to each other Thumb is pointing in the direction the wire is moving Forefinger is pointing in the direction of the magnetic field (north to south) Middle finger will point in the direction of the induced electron flow Power Production * A generator is used to convert mechanical energy into electrical energy by electromagnetic induction. The generator has armatures made up of bundles of copper wires. The armatures are forced to spin within strong magnetic fields by a turbine, which is spun by a form of energy (falling water, steam, wind, heat, etc.) * The rotating loops of wire in the armature cut through the magnetic field of the surrounding electromagnets, inducing alternating voltage and current. Fossil Fuels Hydro Dam Hydroelectric power Nuclear power Wind Power Tidal Power Geothermal Solar Solar water heating Photovoltaic cells Solar furnace Transformers * Transformer – device in which alternating current in one coil of wire induces a current in a second coil * a transformer consists of two coils of wire around a magnet the first coil, called the primary coil, is connected to the power source the second coil, called the secondary coil, is connected to the load (something that uses electricity) * a step-up transformer increases voltage it consists of more coils of wire in the secondary coil than the primary coil * a step-down transformer decreases voltage it consists of more coils of wire in the primary coil than the secondary coil