REVISION MOTORS & GENERATORS ELECTRODYNAMICS Study of the relationship between electricity, magnetism and mechanical phenomena ELECTROMAGNETIC INDUCTION When a conductor and a magnet move relative to each other, it causes a changing magnetic field around the conductor. The changing magnetic field causes (induce) an emf and the emf causes charge to flow FARADAYS LAW The induced emf in a conductor is directly proportional to the rate of change in die magnetic flux in the conductor. MAGNETIESE VLOED The number of magnetic field lines cutting through an area LENZS LAW The direction of the induced current is such that it withstands cause thereof. WHY IS CURRENT INDUCED? When the magnet is moved close to the coil, the magnetic field lines come into contact with the electrons in the coil. This creates a potential difference. This potential difference causes the electrons to move (what we call current) The coil now has its own magnetic field which opposes the applied magnetic field MOTORS convert electric energy into mechanical energy FLEMINGS LEFT-HAND RULE FOR MOTORS Your middle finger points in the direction of the current Your index finger points in the direction of the magnetic field Your thumb represents the direction of the force FLAT RIGHT-HAND RULE FOR MOTORS Your thumb points in the direction of motion of a POSITIVE charge (or conventional current) your fingers point in the direction of the magnetic field and the direction of the force is given by the direction your palm would push something ELECTRODYNAMICS AC MOTORS SLIP RINGS & CARBON BRUSHES ELECTRODYNAMICS DC MOTORS COMMUTATORS & CARBON BRUSHES GENERATORS Convert mechanical energy into electrical energy FLEMINGS RIGHT-HAND RULE FOR GENERATORS Your middle finger points in the direction of the current Your index finger points in the direction of the magnetic field Your thumb represents the direction of the force FLAT RIGHT-HAND RULE FOR GENERATORS Your thumb points in the direction of the force Your fingers point in the direction of the magnetic field The direction of the current comes out of your palm IMPROVING A GENERATOR ELECTRODYNAMICS AC GENERATORS SLIP RINGS & CARBON BRUSHES Most generators are AC generators AC is preferred, due to the fact that the power can easily be increased through the use of transformers Power is delivered over power lines with AC, because less energy is lost USES OF AC GENERATORS Electrical energy is sent through power cables so that we can use it The equations for power (rate of work) is So to increase the power, we need to increase the current and/or potential difference WHY ALTERNATING CURRENT?? but So when current increases, the amount of energy lost as heat also increases. Large current also means thicker cables, because thin cables will melt therefore Potential difference is increases at the power stations (with step-up transformers) And when it reaches substations in our neighborhood, it is decreases to safe magnitudes by step-down transformers WHY ALTERNATING CURRENT?? ELECTRODYNAMICS DC GENERATORS COMMUTATORS & CARBON BRUSHES Electroplating and electro-refining of materials Charging battery & normal lighting purposes Generators for welding and arc welding. USES OF DC GENERATORS ORDINARY CIRCUITSvs ALTERNATING CURRENT EQUATIONS FOR ALTERNATING CURRENT Loudspeaker 1 Loudspeaker 2 36V rms