Magnetic Quantities and Units of Measurement
Date: 2015-10-07; view: 640.
Magnetomotive force is the quantity of magnetic field force, or "push." Analogous to electric voltage (electromotive force).
Field flux is the quantity of total field effect, or "substance" of the field. It is analogous to electric current.
Field intensity is the amount of field force distributed over the length of the electromagnet. Sometimes it is referred to as magnetizing force.
Flux density is the amount of magnetic field flux concentrated in a given area.
Reluctanceis the opposition to magnetic field flux through a given volume of space or material. It is analogous to electrical resistance.
Permeability is the specific measure of a material's acceptance of magnetic flux, analogous to the specific resistance of a conductive materia, except inverse (greater permeability means easier passage of magnetic flux, whereas greater specific resistance means more difficult passage of electric current).
We have several different systems of measurement for each of these quantities: one is called the cgs, which stands for Centimeter-Gram-Second, denoting the root measures upon which the whole system is based; SI, or International System of Units, and English system (Table 1) (from ‘Physics for Scientists and Engineers').
Table 1. Magnetic Quantities and Units of measurement.
| Quantity | Symbol | Units of Measurement and Abbreviation | ||
| CGS | SI | English | ||
| Field force | mmf | Gilbert (Gb) | Amp-turn | Amp-turn |
| Field flux | Φ | Maxwell (Mx) | Weber (Wb) | Line |
| Field intensity | H | Oersted (Oe) | Amp-turns per meter | Amp-turns per inch |
| Flux density | B | Gauss (G) | Tesla (T) | Lines per square inch |
| Reluctance | R | Gilberts per Maxwell | Henry per meter, Amp-turns per Weber | Amp-turns per line |
| Permeability | µ | Gauss per Oersted | Tesla-meters per Amp-turn | Lines per inch-Amp-turn |
Exercise 64. Match the terms in the list with the appropriate definition.
Pole Magnetomotive force To repel Compass
Flux density To deflect Magnet To attract
Permeability Electromagnet Reluctance Polarity
| Term | Definition |
| the property of a magnetic circuit of opposing the passage of magnetic flux lines, equal to the ratio of the magnetomotive force to the magnetic flux. | |
| a quantity measuring the influence of a substance on the magnetic flux in the region it occupies. | |
| a magnet that consists of a piece of iron or steel surrounded by a coil. | |
| the state of an object or system in which some measure has opposite value or sign at two different points. | |
| each of the two points or regions of a magnet to and from which the lines of magnetic force are directed. | |
| a quantity representing the sum of the magnetizing forces along a circuit; it is considered analogous to the electromotive force in an electric circuit. | |
| the amount of magnetic, electric, or other flux passing through a unit area. | |
| an instrument containing a magnetized pointer which shows the direction of magnetic north and bearings from it. | |
| a piece of iron or other material which has its component atoms so ordered that the material exhibits properties of magnetism, such as attracting other iron-containing objects or aligning itself in an external magnetic field. | |
| cause (something) to change direction; turn aside from a straight course. | |
| exert a force on (an object) which is directed towards the source of the force. | |
| to drive or force back or away. |
Exercise 65. Answer the following questions.
1. What did Pierre de Maricourt find in 1269?
2. What can you tell about magnet poles?
3. What did William Gilbert suggest about the earth?
4. How did Hans Christian Oersted discover the relationship between magnetism and electricity?
5. When is magnetic field produced around the conductor?
6. What does the left-hand rule state?
7. What magnetic field force is stronger: generated by a straight-line wire or a coiled wire? Why?
8. What is an electromagnet?
9. What is magnetomotive force? What is its unit of measurement?
10. What is field flux? What is its unit of measurement?
11. What is field intensity? What is its unit of measurement?
12. What is flux density? What is its unit of measurement?
13. What is reluctance? What is its unit of measurement?
14. What is permeability? What is its unit of measurement?
15. What systems of measurement do you know?
Exercise 66. Are the following statements True or False?
1. The unit of magnetic flux density is the weber.
2. Magnetic flux flows from the north pole to the south pole within the magnet.
3. A voltage is induced across a conductor if it remains absolutely stationary within a magnetic field.
4. 
When the south poles of two bar magnets are brought close together, there will be a force of attraction.
5. A magnetic field is made up of flux lines.
6. The tesla is the unit of magnetic flux.
7. Ampere-turn is the unit of magnetomotive force (mmf).
8. Reluctance in a magnetic circuit is analogous to current in an electrical circuit.
9. A relay is an electromagnetically controlled mechanical device in which electrical contacts are opened or closed by the magnetizing current.
Video watching ‘Introduction to Magnetic Fields'.
Exercise 67. Watch the video and answer the following questions.
1. What technologies do magnetic fields allow operate?
2. When and where were magnetic fields first documented?
3. What is the origin of the name ‘magnet'?
4. What is a lodestone?
5. Who invented a magnetic compass?
6. What was it look like?
7. What did H. Oersted notice during his experiment?
8. What did Michael Faraday try to do unsuccessfully?
9. What did he discover?
10. Can you describe Faraday's experiment?
11. What is magnetic flux? How does increase of the magnetic flux influence the strength of the magnet?
12. What does the right hand rule state?
13. Which direction will the conductor on the picture be motivated to move?
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