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Electricity & Magnetism Michael Faraday discovered this idea in 1831 and it has been the basis for all generators & dynamos. Whenever a magnet moves into a current carrying coil, or a current carrying wire moves through a magnetic field, it induces a voltage, causing a current to flow. The strength of the induced current depends upon: The speed of movement The magnetic field strength The number of turns on the coil Suppose a magnet is moved at a uniform speed into a current carrying coil of N turns. Fleming’s RIGHT HAND RULE tells us the direction of the induced current. FARADAY’S LAW: “The induced e.m.f. in a circuit is equal to the rate of change of flux linkage (i.e. change of total magnetic flux cut through, d= d(BA) )”. In symbols: = -Nd/dt measured in Volts Gradient = The – sign indicates that the direction of the induced voltage acts so as to oppose the change which produces it. This is known as LENZ’S LAW. V V This idea is used in ELECTROMAGNETIC BRAKES on trains: As the disc rotates a current flows to/from the edge of the disc, inducing an e.m.f. This is fed through the cable, producing a magnetic field that opposes the original one, slowing the disc down. If we have a straight conductor, such as a metal rod, moving through a magnetic field, at a speed, v, then the area it cuts through in a time, dt, is equal to the length of the rod (l) multiplied by the distance it travels (d), so: A = lvdt. Movement V = -d/dt = -BA/dt = -B(lvdt)/dt = -Blv for a moving, straight conductor. When an aeroplane flies across the Earth’s magnetic field it actually generates a small e.m.f. across its wingtips. Magnetic Field V V V V V V V Induc ed Current

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