|MadSci Network: Physics|
Hi, You haven't specified what type of information you need for Voltmeters. Ill assume you want information about how they work. Whenever electric current passes through an electric conductor a magnetic field is set up around it. (Note: Curl the fingers of your right hand with your thumb pointing outwards. If the direction of your thumb points in the direction of the flow of current. Then your fingers will point in the direction of the magnetic field.) The higher the current the higher is the magnetic field setup. This principle is used in many electrical appliances like motors, generators, solenoids and of course voltmeters and ammeters (Ammeters are used to measure electric current). To see this phenomenon hold a magnetic compass close to a motor or generator, or even an electric wire. Make sure you dont touch the equipment and have an adult nearby. Another simple experiment could be done by turning a wire many times around a iron nail. Connect the ends of the wire to a battery. This will deflect the magnetic compass. The higher the number of turns the higher will be the deflection of the compass. Getting closer to your questions, voltmeters contain a coil of high resistance suspended between 2 magnets. The ends of the coil are connected in parallel to the device across which voltage is to be measured. When current passes through the coil, it deflects since the magnetic field setup in the coil repulses the magnetic field of the magnets. The coil is suspended on springs (usually made of phosphor bronze ) which restrict its movement. The springs deflect to balance the force exerted by the change in magnetic field in proportion to the current that passes through the coil. What follows is some theory on magnetic field effects on current carrying conductors. This will help you do some calculations for your project if needed. Consider the ends of a magnet (say a horseshoe magnet) kept facing each other at some distance. Put a coil of wire between the ends such that it can rotate freely about an axis perpendicular to the magnet. If you pass a current through this coil you will find that the coil will try to turn (ie: The current will set up a magnetic field opposing that of the magnet. This will cause a torque to be setup.) This torque can be calculated by the equation: T = niAB sinO Where, T: Torque n: Number of turns of the coil i: Current in the coil A: Area of each turn B: Strength of the magnetic field. O: The angle between the direction of the magnetic field (ie: The direction of the 2 poles of the magnet from north to south) and a normal (perpendicular line) to the area of the coil. For further information you can refer to most physics textbooks. You can also look up topics like Galvanometers and magnetic effect of electric current. References: 1) The Usborne book of Science seems to have good explanations of electromagnetic effects which you will understand easily. 2)Fundamentals of Physics by Resnick and Halliday seems to have explained theories in detail, if you really require want to learn all about electromagnetics. 2) Progressive Physics by Bapat and Mahajan too have covered this topic. Hope the above information helps. If you need more information please feel free to email me. Nauzad Tantra
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