Topic 13 - Electromagnetic Induction

• Electromagnetic induction is the process whereby a conductor and a magnetic field are moved relative to each other to induce (generate) a current or electromotive force (emf)
• A changing magnetic field can induce a voltage or potential difference in a wire and this causes current to flow. A voltage can also be induced if a wire is moved in a magnetic field
• A changing magnetic field induces a potential difference in a conductor
• • A potential difference is produced if a conductor (wire) is moved in a magnetic fieldOr a magnet is moved in a coil of wire
• Spinning a coil of wire in between two permanent magnets will cause a current to flow in the wire
• • The factors that affect the direction of an induced potential difference : - Changing direction of magnetic field/magnet and Changing polarity of magnet
• • electromagnetic induction is used in alternators to generate current which alternates in direction : Every half-turn, the current switches direction, as the wire will be in the opposite orientation from the starting position.Produces alternating current
• The size of the induced potential difference depends on the number of turns in a coil of wire, the strength of the magnetic field, and on how fast the magnetic field changes or moves past the coil
• a generator consists of a coil of wire that is rotated inside a magnetic field
• many electrical appliances need direct current, a commutator switches over the connections every half turn and so produces direct current
• microphones convert the pressure variations in sound waves into variations in current in electrical circuits
• loud speakers convert variations in an electrical current into sound waves .
• a transformer can change the size of an alternating voltage
• turns ratio equation for transformers to calculate either missing voltage or missing number of turns

  
• • A primary voltage drives an alternating current through the primary coilThe primary coil current produces a magnetic field, which changes as the current changesThe iron core increases the strength of the magnetic fieldThe magnetic field passes through (cuts) the secondary coil.The changing magnetic field induces a changing potential difference in the secondary coilThis induced potential difference produces an alternating current in the external circuit.
• Transformers only work with alternating current because An alternating current produces a changing magnetic fieldA changing magnetic field is required to induce a potential difference in the secondary coil
• • Step-up:at power stationsincreasing p.d. decreases current, which reduces amount of energy wasted by heating in the transmission lines
• • Step down:in local substations before sent to factories and homes, shops and officesdecreasing p.d. means it is safer and more usable
• V(P) * I(P) = V(s) * I (s)