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Physics 8463 - 4.7 Magnetism and electromagnetism

4.7.3. Induced potential, transformers and the National Grid			Helpful pages
4.7.3.1	Induced potential	If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect. An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field. You should be able to recall the factors that affect *the size* of the induced potential difference/induced current. You should be able to recall the factors that affect the direction of the induced potential difference/induced current. You should be able to apply the principles of the generator effect in a given context.
4.7.3.2	Uses of the generator effect	The generator effect is used in an alternator to generate ac and in a dynamo to generate dc. You should be able to: explain how the generator effect is used in an alternator to generate ac and in a dynamo to generate dc draw/interpret graphs of potential difference generated in the coil against time.
4.7.3.3	Microphones	Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits. You should be able to explain how a moving-coil microphone works.
4.7.3.4	Transformers	A basic transformer consists of a primary coil and a secondary coil wound on an iron core. Iron is used as it is easily magnetised. Knowledge of laminations and eddy currents in the core is not required. The ratio of the potential differences across the primary and secondary coils of a transformer V_p and V_s depends on the ratio of the number of turns on each coil, n_p and n_s. V_p /V_s = n_p/n_s This equation is given on the Physics equation sheet - but you need to know what the letters mean and are measured in - so I suggest learn it anyway! Vp and Vs - potential difference, in volts, V In a step-up transformer V_s > V_p In a step-down transformer V_s < V_p If transformers were 100% efficient, the electrical power output would equal the electrical power input. V_s × I_s = V _p × I _p This equation is given on the Physics equation sheet - but you need to know what the letters mean and are measured in - so I suggest learn it anyway! Where V_s × I_s is the power output (secondary coil) and V_p × I_p is the power input (primary coil). Power input and output are measured in watts, W You should be able to: explain how the effect of an alternating current in one coil in inducing a current in another is used in transformers explain how the ratio of the potential differences across the two coils depends on the ratio of the number of turns on each calculate the current drawn from the input supply to provide a particular power output apply the equation linking the p.d.s and number of turns in the two coils of a transformer to the currents and the power transfer involved, and relate these to the advantages of power transmission at high potential differences.