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Magnetism and magnetic forces Current off coil S N Molecular magnets aligned randomly Magnetic field lines (Representing magnetic flux Φ) Current on N coil Magnetic field lines (Representing magnetic flux Φ) S Molecular magnets aligned North to South Flux density Flux lines B=Φ÷A Φ=BxA Flux density (B) is the amount of flux (Φ) (represented by flux lines) passing perpendicularly through a given area (A) Force on a conductor • When a current flows through a conductor in a magnetic field a force acts on the conductor The direction of the force depends on the direction of the magnetic field and the direction of the current. These directions can be found using Fleming’s Left Hand Rule This is called the motor effect Direction of current Cross sections of conductor (wire) Current coming out of page(like an arrow coming towards you) Current going into page(like an arrow going away from you) Force on a conductor Current coming out to page Field Direction N S direction of Force (movement) up Force on a conductor Field Direction N Current going in to page S Force (movement) Down Electric motor effect If the conductor is part of a coil with the current going into the coil on the right and out on the left, the coil will spin ( as per an electric motor) N S Force on a conductor Force = Flux density (B) x current (I) x length of conductor in magnetic field (L) F=BxIxL EMF induced in a conductor • If a conductor is moved through a magnetic field an EMF (electro-motive- force) is induced in the conductor which causes a current to flow in the direction of the force. • The directions can be found using Fleming’s right hand rule • This is called the generator effect EMF induced in a conductor Field Direction N S Current (from induced EMF) going into page Movement up through the field EMF induced in a conductor Field Direction N EMF induced in a conductor Current (from induced EMF) coming out to page S Movement down though field EMF induced in a conductor The magnitude of the EMF (hence current) induced depends on the rate at which the conductor ‘cuts‘ through the flux lines or ‘the rate of change in flux: E = -dФ/dt The minus sign means that the induced emf opposes the change in flux (Lenz’s Law) EMF induced in a conductor E = -dФ/dt dФ/dt = dBA/dt (Ф = BA) = dBLxL/dt ( L x L = A) E = BLv (v (velocity) = dL/dt B = flux density L = length of conductor in field v = velocity of conductor through field