Magnetism and magnetic forces

Current off coil Molecular magnets aligned randomly N S

Current on coil Magnetic field lines (Representing magnetic flux Φ) Magnetic field lines (Representing magnetic flux Φ) Molecular magnets aligned North to South N S

Flux density Flux lines Flux density (B) is the amount of flux (Φ) (represented by flux lines) passing perpendicularly through a given area (A) B = Φ ÷ A Φ = B x 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 N S Field Direction Current coming out to page direction of Force (movement) up

Force on a conductor N S Field Direction Current going in to page Force (movement) Down

Electric motor effect S N 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)

Force on a conductor Force = Flux density (B) x current (I) x length of conductor in magnetic field (L) F = B x I x L

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 N S Field Direction Current (from induced EMF) going into page Movement up through the field

EMF induced in a conductor N S Field Direction Current (from induced EMF) coming out to page 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