Chapter 18 Magnetism Magnets and Magnetic Fields Magnets got their name from a region of magnesia, which is now part of present- day Greece Lodestone: which is a naturally occurring magnetic rock (composed of an iron- based mineral called magnetite) Magnets and Magnetic Fields Magnetic forces are similar to electrical forces Like electrical charges / magnetic forces repel Unlike electrical charges / magnetic forces attract Two like poles repel each other Two unlike poles attract each other Magnets and Magnetic Fields Some material can be made into permanent magnets Iron close to a magnet or rubbed with a magnet can become magnetized Magnets and Magnetic Fields Magnetic Fields: is a region where a magnetic force can be detected Magnets repel or attract each other because of the interaction of their magnetic fields Magnetic field lines can be used to represent magnetic fields Magnetic field lines always form closed loops Magnets and Magnetic Fields Earths Magnetic Field Magnets and Magnetic Fields For historical reasons, the poles of magnets are named for the geographic pole to which they point N is the north-seeking pole of the magnet S is the south-seeking pole of the magnet Magnets and Magnetic Fields Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earth's magnetic field is not completely understood, but is thought to be associated with electrical currents produced by the coupling of convective effects and rotation in the spinning liquid metallic outer core of iron and nickel. This mechanism is termed the dynamo effect. Magnets and Magnetic Fields Earth's Magnetic Field The reason why a compass works is more interesting. It turns out that you can think of the Earth as having a gigantic bar magnet buried inside. In order for the north end of the compass to point toward the North Pole, you have to assume that the buried bar magnet has its south end at the North Pole, as shown in the diagram at the right. If you think of the world this way, then you can see that the normal "opposites attract" rule of magnets would cause the north end of the compass needle to point toward the south end of the buried bar magnet. So the compass points toward the North Pole. Magnets and Magnetic Fields Rocks record the direction of the ambient magnetic field as they form, allowing us to reconstruct the history of these reversals. In the next figure, periods when the field is normal (the same as the present day) are in black, and periods when it is in the opposite, reversed polarity are in white. The field last flipped over about 780,000 years ago (0.78 million years); previous reversals occurred about 0.99, 1.07, 1.19, 1.2, 1.77 and 1.95 million years ago. Magnets and Magnetic Fields How quickly do the poles 'flip'? We have no complete record of the history of any reversal, so any claims we can make are mostly on the basis of mathematical models of the field behavior and partly on limited evidence from rocks that retain an imprint of the ancient magnetic field present when they were formed. For example, the mathematical simulations seem to suggest that a full reversal may take about one to several thousand years to complete. This is fast by geological standards but slow on a human time scale. Magnets and Magnetic Fields Is there any danger to life? Almost certainly not. The Earth's magnetic field is contained within a region of space, known as the magnetosphere, by the action of the solar wind. The magnetosphere deflects many, but not all, of the high- energy particles that flow from the Sun in the solar wind and from other sources in the galaxy. Sometimes the Sun is particularly active, for example when there are many sunspots, and it may send clouds of high-energy particles in the direction of the Earth. During such solar 'flares' and 'coronal mass ejections', astronauts in Earth orbit may need extra shelter to avoid higher doses of radiation. Therefore we know that the Earth's magnetic field offers only some, rather than complete, resistance to particle radiation from space. Indeed high-energy particles can actually be accelerated within the magnetosphere. Magnets and Magnetic Fields At the Earth's surface, the atmosphere acts as an extra blanket to stop all but the most energetic of the solar and galactic radiation. In the absence of a magnetic field, the atmosphere would still stop most of the radiation. Indeed the atmosphere shields us from high-energy radiation as effectively as a concrete layer some 13 feet thick. Magnets and Magnetic Fields Human beings have been on the Earth for a number of million years, during which there have been many reversals, and there is no obvious correlation between human development and reversals. Similarly, reversal patterns do not match patterns in species extinction during geological history. Magnets and Magnetic Fields Some animals, such as pigeons and whales, may use the Earth's magnetic field for direction finding. Assuming that a reversal takes a number of thousand years, that is, over many generations of each species, each animal may well adapt to the changing magnetic environment, or develop different methods of navigation. Magnets and Magnetic Fields The source of earths magnetism is not yet fully understood Core is mostly iron Too hot to have any magnetic properties Belief is that the circulation of ions or electrons in the liquid layer of Earths core may be the source of the magnetism Magnets and Magnetic Fields Northern and Southern Lights: collision between charged particles emitted from the Suns magnetic field and Earths atmospheric atoms Magnetism from Electric Currents When a wire carries a strong, steady current, all of the compass needles move to align with the magnetic field created by the electric current Magnetism from Electric Currents The right-hand rule is used to find the direction of the magnetic field produced by the current: If you hold a wire in your right hand and point your thumb in the direction of the positive current, the direction that your fingers curl is the direction of the magnetic field Magnetism from Electric Currents Solenoid:A solenoid (from the French solnode, derived in turn from the Greek solen "pipe, channel" + combining form of Greek eidos "form, shape"[1]) is a coil wound into a tightly packed helix. The term was invented by French physicist Andr- Marie Ampre to designate a helical coil.FrenchGreek[1]helixFrenchphysicistAndr- Marie Ampre Magnetism from Electric Currents In physics, the term refers specifically to a long, thin loop of wire, often wrapped around a metallic core, which produces a uniform magnetic field in a volume of space (where some experiment might be carried out) when an electric current is passed through it. A solenoid is a type of electromagnet when the purpose is to generate a controlled magnetic field. If the purpose of the solenoid is instead to dampen changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet. Not all electromagnets and inductors are solenoids; for example, the first electromagnet, invented in 1824, had a horseshoe rather than a cylindrical solenoid shape.physicsmetallicmagnetic field electric current electromagnetinductor Magnetism from Electric Currents In a solenoid, the magnetic field of each loop of wire adds to the strength of the magnetic field of any neighboring loops. The result is a strong magnetic field similar to the magnetic field produced by a bar magnet. Like a magnet, a solenoid has a north and south pole. Magnetism from Electric Currents Solenoid in car: causes a magnet to push the gear on your starter into your flywheel, thus turning and starting your car. If you turn your key and the only thing that happens is the starter spins, you need a new solenoid! Magnetism from Electric Currents Solenoids in Car: There are many of them used in a car including the starter solenoid. They have an electromagnet which pulls a steel cylinder in when you connect your battery to the electromagnet coil wire. So the mechanical motion of the cylinder can be used to close switch contacts or valves of various sorts. The electric door locks are another example of their use. An electrical relay is similar to a solenoid but is specifically for the purpose of connecting and disconnecting various switch contacts. So the starter solenoid can really be thought of as an electrical relay since the coil magnet is used to close high current electrical contacts that hook the starter motor to the car's battery. Magnetism from Electric Currents Starter solenoid The starter solenoid works as a powerful electric relay - when activated, it closes the electric circuit and sends the battery power to the starter motor. At the same, the starter solenoid pushes the starter gear forward to mesh with the engine flywheel. A typical starter solenoid has one small connector for the control wire (the white connector in the photo) and two large terminals: one for the positive battery cable and the other for the starter motor. Magnetism from Electric Currents Battery cables A starter motor requires a very high current to crank the engine, that's why it's connected to the battery with thick (large gauge) cables (see the diagram). The negative (ground) cable connects the "-" battery terminal to the engine cylinder block close to the starter. The positive cable connects the "+" battery terminal to the starter solenoid. Magnetism from Electric Currents The strength of a solenoid can be increased Increase the number of loops: increase magnetic field Increase current: increase magnetic field Insert a rod of a magnetic metal in the coil: increases the magnetic field This device is called an electromagnet This is stronger magnet than a solenoid Magnetism from Electric Currents Electric motors: are machines that convert electrical energy into mechanical energy Magnetism from Electric Currents Galvanometers are devices that are used to measure current Detects current or movement of charges in a circuit Consist of a coil of insulated wire wrapped around an iron core that can rotate between the poles of a permanent magnet When attached to a circuit, a current exits in the coil of wire The coil and iron core act as an electromagnet and produce a magnetic field This magnetic field interacts with the magnetic field of the permanent magnet The resulting force moves the needle Can be used with other circuit elements to measure current (ammeter) or volts (voltmeter) Magnetism from Electric Currents Motors use a commutator to spin in one direction; Used to make the current change direction every time the flat coil makes a half revolution Two half rings of metal Brushes connect the commutator to the wires from the battery Because of the slits in the commutator, charges must move through the coil of wire to reach the opposite half of the ring The magnetic field of the coil changes direction as the coil spins Magnetism from Electric Currents The coils is repelled by both the north and south poles of the magnet surrounding it Because the current keeps reversing, the loop rotates in one direction Magnetism from Electric Currents The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator. Magnetism from Electric Currents Electric Currents from Magnetism Faradays Law: an electric current can be produced in a circuit by changing the magnetic field crossing the circuit Moving a magnet into and out of a coil of wire causes charges in the wire to move The process of creating a current in a circuit by changing a magnetic field is called electromagnetic induction Electric Currents from Magnetism Moving a wire perpendicular to the magnetic field causes the charges to be at maximum and thus a current in the wire Moving a wire parallel to the magnetic field causes no current in the wire Electric Currents from Magnetism Generators are similar to motors but convert mechanical energy into electrical energy Loop of wire inside turns in a magnetic field and produces a current For each half rotation of the loop, the current produced by the generator reverses direction A current that changes direction at regular intervals is called an alternating current (AC) Electric Currents from Magnetism Most students of electricity begin their study with what is known as direct current (DC), which is electricity flowing in a constant direction, and/or possessing a voltage with constant polarity. DC is the kind of electricity made by a battery (with definite positive and negative terminals), or the kind of charge generated by rubbing certain types of materials against each other. Electric Currents from Magnetism As useful and as easy to understand as DC is, it is not the only kind of electricity in use. Certain sources of electricity (most notably, rotary electro- mechanical generators) naturally produce voltages alternating in polarity, reversing positive and negative over time. Either as a voltage switching polarity or as a current switching direction back and forth, this kind of electricity is known as Alternating Current (AC) Electric Currents from Magnetism Whereas the familiar battery symbol is used as a generic symbol for any DC voltage source, the circle with the wavy line inside is the generic symbol for any AC voltage source. Electric Currents from Magnetism One might wonder why anyone would bother with such a thing as AC. It is true that in some cases AC holds no practical advantage over DC. In applications where electricity is used to dissipate energy in the form of heat, the polarity or direction of current is irrelevant, so long as there is enough voltage and current to the load to produce the desired heat (power dissipation). However, with AC it is possible to build electric generators, motors and power distribution systems that are far more efficient than DC, and so we find AC used predominately across the world in high power applications. To explain the details of why this is so, a bit of background knowledge about AC is necessary. Electric Currents from Magnetism If a machine is constructed to rotate a magnetic field around a set of stationary wire coils with the turning of a shaft, AC voltage will be produced across the wire coils as that shaft is rotated, in accordance with Faraday's Law of electromagnetic induction. This is the basic operating principle of an AC generator, also known as an alternator Electric Currents from Magnetism Notice how the polarity of the voltage across the wire coils reverses as the opposite poles of the rotating magnet pass by. Connected to a load, this reversing voltage polarity will create a reversing current direction in the circuit. The faster the alternator's shaft is turned, the faster the magnet will spin, resulting in an alternating voltage and current that switches directions more often in a given amount of time. Electric Currents from Magnetism The amount of current produced by an AC generator changes with time Electric Currents from Magnetism 0: loop is perpendicular to the field-current is zero No magnetic force 90: loop turns and is parallel to the field-current is maximum A magnetic force is experienced Electric Currents from Magnetism 180: loop is perpendicular to the field-current is zero No magnetic force 270: loop turns and is parallel to the field-current is maximum A magnetic force is experienced Direction of current is reversed Electric Currents from Magnetism Generators produce the electrical energy that you use in your home Power plants produce electrical energy from mechanical energy Coal Oil Nuclear Water Wind Electric Currents from Magnetism Electromagnetic (EM) Force: Both the electric and magnetic fields in an EM wave are perpendicular to the direction in which the wave travels EM waves are transverse waves (light, X-rays, etc.) As an EM wave moves along, the changing electric field generates the magnetic field and the changing magnetic field generates the electric field Because each field regenerates the other, EM waves are able to travel through empty space Electric Currents from Magnetism Transformers: Devices that increase or decrease the voltage of alternating current Consist of two coils of wire wrapped around opposite sides of a closed iron loop Can increase or decrease current Step-up transformers: the primary coil has fewer turns than the secondary coil (voltage across the secondary coil is greater) Step-down transformer: the secondary coil has fewer turns than the primary coil (voltage across the secondary coil is lower) Electric Currents from Magnetism Most of the electronic devices that we use need a transformer if they are to run when they are plugged into the mains. Microchip circuitry does not need a big voltage to operate, in fact a big voltage will simply cause the chip circuitry to burn out. Such circuits run on voltages of between 5V and 12V. Therefore a transformer is necessary to 'step-down' mains voltage (230V) to this level. Any device you have that can run off batteries or via a mains connector will have a transformer incorporated into that connector. Sometimes they 'hum' and they always get warm after being switched on for a while. Electric Currents from Magnetism How a transformer works An alternating voltage (VP) is applied across the primary coil. This causes a changing magnetic field to be formed around the primary coil. The magnetic domains inside the soft iron core line up in response to the magnetic field from the coil. The secondary coil experiences the changing magnetic field produced by the primary and the core. It responds to this changing magnetic field by producing a voltage (VS) across its ends (an induced EMF) by electromagnetic induction. Electric Currents from Magnetism A step-up transformer is used at or near a power plant to increase the voltage to about 120,000 V. This high voltage limits the loss of energy that the resistance of the transmission wires causes. A step-down transformer like the ones near your homes is used to reduce voltage to about 120V. This low voltage is much safer to use in homes.