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When an electric current flows through a circuit, it is opposed by two things: resistance and reactance. Resistance, found in both alternating current (AC) and direct current (DC), comes about when the electrons that carry the current collide with each other, with the conductor material itself, or with a resistor designed to do just that in the circuit. Reactance is associated only with AC or other form of varying current. The form of reactance called inductive reactance (involving inductors to induce magnetic fields) is explained in the tutorial below, which uses a type of AC dimmer as an illustration. (This is not, by the way, how your AC dimmers work at home, but serves as a handy way of explaining the concept of inductive reactance).
In the circuit above a bank of small lamps serves as a resistor, while a copper wire coil makes up the inductor. An iron core can be moved in and out of the coil with the Iron Core Position slider to strengthen the magnetic field of the coil inductor. Either AC or DC may be selected by clicking the appropriate radio button. To throw the knife switch in the circuit and apply the current, click the blue Turn On button; click the red Turn Off button to lift the switch and halt the current.
Notice that when the DC power supply is selected and the circuit is on, moving the iron core does not affect the brightness of the light bank. However, this same action produces a dimming of the lamps or their complete shut down when the AC power supply is chosen. The further inside the coil the core penetrates, the greater the magnetic field produced, and the greater the reactive inductance. This phenomenon can be explained by Lenz’s Law, which states that the direction of the induced current is such as to oppose the change causing it. In other words, this is what happens:
- The current-carrying wire generates around it a circular magnetic field.
- When the current changes direction (as it does dozens of times a second with AC), the magnetic field it produces also changes.
- This changing field creates a voltage which (according to Lenz’s Law) opposes the direction of the primary current.
So the alternating current induces an alternating magnetic field in the coil that increases the opposition to the flow of current. This magnetic field, and the inductive reactance that arises from it, is amplified by the presence of the iron core in the coil, so that the electrical reactance (combined with the resistance) becomes so great that an inadequate current supply reaches the lamps.
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