Transcript Document
Magnetism . 1 Magnetic Fields Magnetic fields are historically described in terms of their effect on electric charges. A moving electric charge, such as an electron, will accelerate in the presence of a magnetic field, causing it to change velocity and its direction of travel. This is, for example, the principle used in televisions, computer monitors, and other devices with CRTs (cathode-ray tubes). In a CRT, electrons are emitted from a hot filament. A voltage difference pulls these electrons from the filament to the picture screen. Electromagnets surrounding the tube cause these electrons to change direction, so they hit different locations on the screen. 2 Magnetic Dipoles The north pole of a magnet is the pole that aligns itself with geographic north. As a result, the geographic north pole of the earth is actually very near the earth's magnetic south pole 3 Lorentz force An electrically charged particle moving in a magnetic field will experience a force pushing it in a direction perpendicular to the magnetic field and the direction of motion. 4 Types of Magnets Pulsed Magnets Pulsed magnets are among the strongest magnets in the world, and come in two forms: destructive and non-destructive. Of these two, nondestructive magnets are more suited towards scientific research, as they can reach some of the highest magnetic fields experimentally possible. The energy used by the short pulse magnet is stored in the capacitor bank. This bank is capable of storing extremely large amounts of energy, and is used to discharge (pulse) that energy through the magnetic coil. This in turn makes the pulse magnet's temperature rise greatly, so it is necessary to pulse the capacitor bank for a few milliseconds at a time in order to keep the magnet's temperature under control. However, if too much energy is sent through a capacitor bank with a very high capacitance, there is a possibility that the stress of the magnetic field on the magnet will cause it to explode. Also take note that the crowbar diode is used to change the flow of current from the capacitor to the magnet, from AC to DC. 5 Types Permanent Magnets Iron- composed of many microscopic regions consisting of numerous atomic dipoles (domains), all pointing in the same direction. A strong magnetic field will align the domains of an iron magnet, or in other words, magnetize it. Once the magnetic field is removed, the domains will remain aligned, resulting in a permanent magnet. This effect is known as hysteresis. 6 Electromagnetic Induction The generation of an electromotive force and current by a changing magnetic field is called electromagnetic induction. Faraday confirmed that a moving magnetic field is necessary in order for electromagnetic induction to occur. 7 Speakers Alternating current, generated by a microphone, a pickup head, amplifier, radio, or another source, flows through the coil of the speaker. The current, alternating at the same frequency as the sound waves that generated it, induces an alternating magnetic field in the coil. As the polarity of the magnetic field of the coil alternates, it is alternatively attracted to and repelled by the permanent magnet. This causes the coil to vibrate. The vibrating coil causes the attached cone shaped diaphragm to vibrate and reproduce the sounds generated by the original source. 8 Metal Detectors The operation of metal detectors is based upon the principles of electromagnetic induction. Metal detectors contain one or more inductor coils that are used to interact with metallic elements on the ground. When the magnetic field of the coil moves across metal, the field induces electric currents (called eddy currents) in the coin. The eddy currents induce their own magnetic field which generates an opposite current in the coil, which induces a signal indicating the presence of metal. 9 Strength of Magnets The strength of a magnetic field is measured in units of Gauss (G), or alternatively, in Tesla (T). In the MKS (metric) system of units, 1 T = 1 kilogram*ampere/second^2 = 10^4 G Strength & Distance – affected by inverse cube of distance from magnet. Earth = 1 Gauss, Neodymium magnet =~ 10^4 Gauss Technically, Gauss and Tesla are units of magnetic induction, also known as magnetic flux density. Quantitatively, the force on a charged particle q moving with velocity v is given by the vector equation F = qv x B, where B is the magnetic induction. 10 Electricity and Magnetism Electric and Magnetic phenomena are intricately described by a collection of physical laws, known as Maxwell's equations. Electromagnetic waves-Light waves are oscillating patterns of electric and magnetic fields, propagating through space at the speed of light (3x10^8 meters/second). 11