Magnetic fields & electromagnetic induction Learning outcomes Teaching challenges Permanent magnets Defining magnetic flux density
Hymer, Dian, Freelance Columnist has reference to this Academic Journal, PHwiki organized this Journal Magnetic fields & electromagnetic induction Learning outcomes describe magnetic fields in terms of magnetic flux & flux density use Flemings left in addition to right h in addition to rules to describe interactions between magnetic field & current quantitatively describe B fields around a straight current-carrying wire in addition to a solenoid quantitatively describe the as long as ce on a charged particle moving at right angles to a uni as long as m B field explain electromagnetic induction using Faradays & Lenzs law use the concept of flux linkage to explain how trans as long as mers work describe how B fields are used in circular particle accelerators recall the postulates in addition to key consequences of special relativity solve related quantitative problems Teaching challenges fields are abstract involves 3-D thinking but generally illustrated in 2-D involves rates of change different concepts have similar names some physical quantities have a variety of equivalent units students may need simple trigonometry to find the magnetic flux, or magnetic as long as ce, correctly identifying angle .
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Permanent magnets Magnetic field lines start in addition to finish at poles. Physicists picture this as a flow in magnetic circuit. magnetic flux (phi), unit Weber magnetic flux density B, unit Weber m-2 or Tesla Carl Gauss & Wilhelm Weber investigated geomagnetism in 1830s, made accurate measurements of magnetic declination in addition to inclination, built the first electromagnetic telegraph. Defining magnetic flux density Typical magnetic field strengths: Flemings left-h in addition to rule: Force on the wire is perpendicular to both l in addition to B. Electromagnetism Electric currents have loops of B flux around them. Current-turns produce flux.
Magnetic fields near currents long straight wire long solenoid, N turns in addition to length l is the permeability of free space Forces on parallel currents parallel – attract anti-parallel – repel Forces on parallel currents At the top wire in the diagram, Defining the ampere (straight wires of infinite length) If the current in each wire is exactly 1 A, in addition to the distance between the wires is 1 m, then the as long as ce on each metre length of the wires will be 2 x 10-7 N. Practice questions: TAP Forces on currents
Demonstration: fine beam tube uni as long as m B-field at right angles to an electron beam with v F is perpendicular to v, so the beam travels in a circular path. Force on a moving charge Fluxes in addition to as long as ces Michael Faraday (experimenting in 1830s at the Royal Institution) pictured magnetic field lines as flexible in addition to elastic magnetic attraction: field lines try to get shorter & straighter magnetic repulsion: field lines cannot cross Faradays law of induction Induced emf is proportional to rate of cutting field lines. N is number of turns on the secondary coil. N is its flux linkage. Induced emf is proportional to rate of change in coils flux linkage. NOTE: Eddy currents are induced in iron core linking primary in addition to secondary coils. These can be reduced by laminations in core.
1 the flux cut by a moving wire 2 the change in flux due to a magnet moving 3 the change in flux due to a stationary electromagnet which is changing in strength No relative motion means no induced emf. Under what conditions is there an induced current can be: Experiments Force on a current-carrying wire Current balance Investigating fields near currents (using a Hall probe) Investigating electromagnetic induction Faradays law Jumping ring Practice questions (Adv Physics) Changes in flux linkage (Adv Physics) Flux or flux linkage TAP Rates of change (Adv Physics) Graphs of changing flux in addition to emf
Endpoints rotating coil (AC) generator: motors produce a back emf
Hymer, Dian Freelance Columnist
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