The flow of current through a conductor is accompanied by magnetic effects, in other words, the current sets up a magnetic field. The transfer of energy to the magnetic field represents work done by the source of e.m.f. Power is equal to current multiplied by voltage, there must be a voltage drop in the circuit during the time in which energy is being stored in the field. This voltage “drop” ( which has nothing to do with the voltage drop in any resistance in the circuit ) is the result of an opposing voltage “induced” in the circuit while the field is building up to its final value. When the field becomes constant the induced e.m.f. or back e.m.f. disappears, since no further energy is being stored.
Since the induced e.m.f. opposes the e.m.f. of the source, it tends to prevent the current from rising rapidly when the circuit is closed. The amplitude of the induced e.m.f. is proportional to the rate at which the current is changing and to a constant associated with the circuit itself, called the inductance of the circuit.
If two coils are arranged with their axes on the same line, a current sent through Coil 1 will cause a magnetic field which “cuts” Coil 2. Consequently, an e.m.f. will be induced in Coil 2 whenever the field strength is changing. This induced e.m.f. is similar to the e.m.f. of self-induction, but since it appears in the second coil because of current flowing in the first, it is a “mutual” effect and results from the mutual inductance between the two coils.
Two coils having mutual inductance constitute a transformer. The coil connected to the source of energy is called the primary coil, and the other is called the secondary coil.
The usefulness of the transformer lies in the fact that electrical energy can be transferred from one circuit to another without direct connection, and in the process can be readily changed from one voltage level to another.
【 添削課題 】
Impedance is the total opposition to the flow of alternating current in a circuit that contains resistance and reactance. In the case of pure inductance, inductive reactance XL is the total opposition to the flow of current through it. In the case of pure capacitance, capacitive reactance XC is the total opposition to the current. In the case of pure resistance, it represents the total opposition. The combined opposition of R, XC and XL in series or in parallel to current flow is called “impedance”, symbolized by the letter Z. Impedance is thus a more general term than either resistance or reactance and is frequently used even for circuits that have only resistance or reactance.
The reactance and resistance comprising an impedance may be connected either in series or in parallel. In the series circuit the current is the same for all the elements, with different voltages appearing across the resistance and reactance. In the parallel circuit the same voltage is applied to all the elements, but different currents flow in the two or three branches.
Since in a resistance the current is in phase with the applied voltage while in a reactance it is 90 degrees out of phase with the voltage, the phase relationship between current and voltage in the circuit as a whole may be anything between zero and 90 degrees, depending on the relative amounts of resistance and reactance. XL and XC oppositely act in the phase shifting of –90°and +90°in the currents when these elements are parallely connected to the voltage.
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