Electromagnetic induction

• When a conducting rod moves relative to a magnetic field, the electrons in the rod will experience a force and build up on one side of the rod, causing an emf to be induced in the rod.
• Faraday's law states that the magnitude of induced emf is equal to the rate of change of flux linkage.
• If there is a complete circuit, a current will also be induced when an EMF is induced.
• Lenz's law states that the direction of induced current is such to oppose the motion causing it.
• The general equation for induced EMF is E=-N delta flux/delta t.
• The equation for induced EMF for a straight conductor is E=BLv.
• The equation for induced EMF in a rotating coil is E=BANw sin(wt).
• An oscilloscope shows the variation of voltage with time.
• On an oscilloscope, an alternating current is shown with a sinusoidal waveform with the time-base on, and a vertical line with the time-base off.
• On an oscilloscope, a direct current is shown as a horizontal line with the time-base on and a dot with the time-base off.
• Peak voltage is the maximum magnitude of voltage supplied.
• Peak to peak voltage is the voltage between the maximum and minimum voltage, and for a regular voltage is double the peak voltage.
• Root mean square (rms) voltage is the average of all the squares of the possible voltages, giving the average voltage output which can be used with alternating currents to find the equivalent direct current. To find rms voltage without directly measuring it, divide the peak voltage by root 2.
• The electricity supplied in UK homes is 230V rms (330V peak) and 50Hz.
• Transformers are used with alternating currents to change the size of their voltage.
• Transformers are made from a primary coil attached to the input voltage, a secondary coil attached to the output, and an iron core.
• The primary coil provides an alternating magnetic field which induces an EMF in the secondary coil.
• The ratio of voltage in the primary coil to the secondary coil in a transformer is equal to the ratio of the number of turns on the primary coil to the number of turns on the secondary coil.
• A step up transformer has more turns on the secondary than primary and increases voltage and decreases current.
• A step down transformer has more turns on the primary than secondary and decreases voltage and increases current.
• Transformer efficiency is the power (P=IV) of the secondary divided by the power of the primary coil.
• One of the main causes of energy loss in a transformer is the production of eddy currents in the core by the alternating magnetic field. These can be reduced by adding layers of insulation within the core.
• Other than eddy currents, energy is lost in transformers through heating due to resistance in the coils and if the core is not easily magnetised. These can be reduced by using wider wires and soft iron in the core.
• When transferring electrical power, the power lost due to resistance is P=I^2R, and therefore current is decreased to a minimum during transfer.
• In the national grid, current is stepped up from a generating station, and then stepped down in stages to factories needing higher voltages and then to the general population.