Electromagnetism – Terms to Know

     

  1. Magnetic pole or pole – a region of a magnetic material where the effect of magnetic forces is large. (Usually found at the ends of the material.)

     

  2. North pole of a magnet - the end of a magnet that will point towards the northern end of the Earth if the magnet is free to rotate. Sometimes called a "north-seeking pole".

     

  3. South pole of a magnet - the end of a magnet that will point towards the southern end of the Earth if the magnet is free to rotate. Sometimes called a "south-seeking pole".

     

  4. Law of magnetic poles - like poles repel and opposite poles attract.

     

  5. Compass - a device consisting of a small magnetized needle or bar that is capable of rotating to point towards the northern and/or southern poles of the Earth.

     

  6. North geomagnetic pole - the magnetic pole located near the north geographic pole. (Note: Since this pole attracts north-seeking poles it is, in fact, a south-seeking pole.)

     

  7. Ferromagnetic materials - those materials capable of producing very strong magnetic fields. (Includes the elements iron, cobalt, and nickel and other materials.)

     

  8. Magnetic field - the region of space surrounding a magnet (or the region or space around a current carrying conductor) that produces magnetic forces on magnetic materials.

     

  9. Magnetic field or magnetic field strength or magnetic flux density or magnetic induction - the strength or intensity of a magnetic field at a point in the field. Symbolized by the letter B. Measured in units called tesla (T). By definition 1 T = 1 N/(A•m). (Analogous to g for gravitational fields or E for electric fields.) (V)

     

  10. Direction of a magnetic field at a point in the field - the direction that a free north pole would move initially if released in the field. Also the direction the north pole of a compass points when placed at the point in the field.

     

  11. Magnetic field line - the line or path that a free magnetic north pole would follow if released in the field.

     

  12. Domain - tiny magnetic regions found in magnetic materials. They are approximately one millimetre in length and width and act like tiny bar magnets. Note that in a magnetized material most of the domains are pointing in the same direction to produce a large magnetic effect. Ferromagnetic materials that are unmagnetized have their domains pointing in random directions to produce little or no net magnetic effect.

     

  13. Solenoid - a long coil of wire.

     

  14. Magnetic flux - the product of the component of the magnetic field strength perpendicular to the face of coil and the area of the coil. Defining equation F = B A cosq where B is the magnetic field strength, A is the area of the coil and q is the angle between the magnetic field lines and a line drawn perpendicular to the face of the coil. Note that the magnetic flux is proportional to the number of lines passing through the coil. Obviously the flux is zero if the coil is orientated so that no field lines pass through the coil - that is, its face is parallel to the field lines. Measured in units of Weber (Wb). 1 Wb = 1 T•m2. (S)

     

  15. Faraday's Law of Induction - when the magnetic flux through a coil changes an emf is induced and the magnitude of this induced emf is proportional to the rate of change of flux-linkage. Defining equation:     e = - N∆F/∆t  where N is the number of coils or turns. Note that the negative sign is a consequence of Lenz's law.

     

  16. Flux-linkage - the product of the flux and the number of turns in a coil. Defining equation:flux-linkage = NF. Measured in Weber. (S)

     

  17. Lenz's law - the direction of an induced current (if one were to flow because of a closed circuit) is such that the magnetic field produced by the current opposes the original change in flux. (This law is a consequence of the law of conservation of energy.)

     

  18. Eddy current - a current set up in a conductor that is moving across a magnetic field or through which the flux is changing. Usually considered undesirable in electrical appliances since eddy currents cause heat due to the ohmic resistance of the metal in which they are flowing. This implies less useful energy is available in such devices as transformers, armatures, loudspeakers,etc. The effect of eddy currents can be reduced by laminating the metal components of the device so that each metal layer is insulated from the others.(V)

     

  19. Back or counter emf - the emf that is produced in such devices as motors when a coil is moving across a magnetic field. The back emf is produced as a consequence of Lenz's Law. The back emf is always opposite in sign to that of the input emf that actually causes the motion of the moving coil.(S)

     

  20. Transformer - an electrical device used to increase or decrease a.c. voltages. Transformers are an application of Faraday's Law. They usually contain laminated iron cores that can produce large magnetic fields but very small eddy currents (transformers are typically 99% efficient). The main parts of a transformer are the primary and secondary coils. The transformer equations is as follows: Vs/Vp = Ns/Np, where the subscripts "p" and "s" mean "primary" and "secondary" respectively, the N means the number of turns or windings on the coil, and V is the voltage. Note that if a transformer steps up the voltage by a certain factor, the current will be stepped down by the same factor. This is a consequence of the law of conservation of energy.