Magnetism

• Permanent magnets are ferromagnetic
• Magnetic moment is an unbalanced propertry of an electron which causes magnetism
• Iron, Nickel, Cobalt, Gadolinium and Dysporium are the only natural ferromagnets.
• When a ferromagnet is placed in a magnetic field, its domains line up and it becomes permanent
• A soft magnet is easily magnetised/demagnitised
• A hard magnet contains impurities which prevents it from loosing their magnetism, eg. Steel
• Induced magnetism is when unmagnitised material's domains align with those of a permanent magnet
• Magnetic field lines travel from North to South
• Viewed from the front, clockwise current in a solenoid causes a South pole.
• Two parallel wires, carrying current in the same direction, are attracted
• 1 Ampere is current flowing when 1 Coulomb of e- passes a point in 1 second
• Magnetic flux, phi, is the amount of magnetic field lines passing through a region of space. Unit = Weber.
• Magnetic flux density (B) is a measure of magnetic flux per unit area. Unit : Tesla
• Magnetic flux = A (area) * B (flux density) * cos@
• Magnetic field lines pointing in the same direction add.
• Force on a current carrying wire is the result of interaction magnetic fields.
• Due to F= BIL(sin0), if the wire and magnetic field are parallel, there will be no force acting on it.
• Fleming's left hand rule = F(newtons) B(flux density) I (current)
• When a changing magnetic field passes through a conductor an emf is generated between its ends
• Faradays law of EM induction - magnitude of induced emf is proportional to rate of change of flux linkage or cutting magnetic flux.
• Lenz's Law - The direction of the induced emf is always opposite to the direction of the current it
• Neumanns law is the combination of Faraday's and Lenz's
• When a conductor is moved in a magnetic field, the electrons in it causes a current opposite direction of motion
• V (rms) = V (peak) ÷ √2
• When a capacitor is connected in DC, it acquires charge on one side and stops current
• When a capacitor is connecected in AC, it never becomes fully charged so it conducts AC
• Self inductance (L) is caused when an AC current flows through a coil. It will produce a changing magnetic field, which will cut thru the coil generating a back emf. Due to Lenz's law, the emf opposes the current.
• When DC passes through a coil, it causes self inductance momentarily.
• When a coil is in an AC supply, a back emf permanantly reduces current. Thus the coil reduces I both due to resistance and self inductance
• Application of self inductance ~

  1: reduce current spikes.

  2: used in tuning curcuits for radios

  3: dimmer switches
• When AC passes through one coil beside another, mutual inductance occurs.