Definition In a DC field the reluctance is the ratio of the magnetomotive force

# Definition in a dc field the reluctance is the ratio

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Definition In a DC field, the reluctance is the ratio of the " magnetomotive force ” (MMF) in a magnetic circuit to the magnetic flux in this circuit. In a pulsating DC or AC field, the reluctance is the ratio of the amplitude of the " magnetomotive force ” (MMF) in a magnetic circuit to the amplitude of the magnetic flux in this circuit (see phasors ). The definition can be expressed as follows: where ("R") is the reluctance in ampere-turns per weber (a unit that is equivalent to turns per henry ). " Turns " refers to the winding number of an electrical conductor comprising an inductor. ("F") is the magnetomotive force (MMF) in ampere-turns Φ ("Phi") is the magnetic flux in webers. Reluctance is sometimes known as Hopkinson's law and is analogous to Ohm's Law with resistance replaced by reluctance, voltage by MMF and current by magnetic flux. Magnetic flux always forms a closed loop, as described by Maxwell's equations , but the path of the loop depends on the reluctance of the surrounding materials. It is concentrated around the path of least reluctance. Air and vacuum have high reluctance, while easily magnetized materials such as soft iron have low reluctance. The concentration of flux in low-reluctance materials forms strong temporary poles and causes mechanical forces that tend to move the materials towards regions of higher flux so it is always an attractive force (pull).
~ Page 35 of 79 ~ The reluctance of a uniform magnetic circuit can be calculated as: or where l is the length of the circuit in metres is the permeability of vacuum, equal to 4π × 10 −7 henry per metre is the relative magnetic permeability of the material (dimensionless) is the permeability of the material ( ) A is the cross-sectional area of the circuit in square metres The inverse of reluctance is called permeance . Its SI derived unit is the henry (the same as the unit of inductance , although the two concepts are distinct).
~ Page 36 of 79 ~ EXERCISE 1. Explain why most metallic materials used in engineering products are alloys. 2. Sort the following materials into groups of ferrous metals, non-ferrous metals and non-metals: carbon, sulphur, low-carbon steel, bronze, polystyrene, brass, stainless steel, zinc, PVC, silicon, nickel-chrome alloy steel, duralumin, aluminium, titanium, melamine-formaldehyde, phosphorus, and grey cast iron. 3. State, for the manufacture of steels, which of the following are alloying elements and which are impurities: sulphur, chromium, silicon, nickel, manganese, carbon, phosphorus, and molybdenum. 4. List the essential criteria for selecting a material for a specific application. 5. List the factors that can affect the properties of a material. 6. Select suitable materials for each of the following applications and discuss the reason for your selection in each case in terms of cost and availability, suitability for the manufacturing processes required and suitability for the applications listed below.

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