| Contents for this page | Related topics |  |
|---|---|---|
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A circuit causing induction Inductance Inductance in series and parallel Additional questions |
Heating effects of electric currents Electric machines Alternating currents Resistive, capacitive and inductive AC circuits Alternating currents and safety |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will be familiar with the basic principles underlying electric motors and generators. | ||
Induction takes place when a current is caused to flow in a conductor when that conductor experiences a change in the ambient magnetic field. Commonly, this occurs when the conductor moves across a magnetic field, or the magnetic field moves across a conductor.
The presence of a coil in a circuit cause induction in that circuit whenever there is a change in the current flowing through the conductor. A circuit in which a change in current causes the generation of an induced electromotive force is said to be an INDUCTIVE CIRCUIT. Such a circuit possesses a property called SELF-INDUCTION or INDUCTANCE.
The circuit shown above will enable us to understand the properties of a simple DC inductive circuit. A wire is wound around one arm of a soft iron ring, forming a coil C, with a resistance RC. This is connected in parallel with a resistance R, and connected to a battery delivering a voltage V volts. Ammeters A1 and A2 eneble one to measure the currents flowing in R and C respectively. A switch S is provided to either close or open the circuit. Initially, when the switch S is open, no current is detected by the ammeters.
This tells us that a current flows in R when the swicth is opened, that is, when R is no longer connected to the voltage source.
When the switch was closed, an emf was induced in the coil C, the polarity of which was such as to oppose the current that caused it (Lenz's law). As soon as the switch is opened, there is a decrease in the current shown by A2, which in turns induces an emf in the coil that opposes the change in current, and so the current through R, measured by A1, is in a direction opposite to the current shown by A2.
A coil in a circuit produces INDUCTANCE in that circuit, a property that describes how the emf induced in an inductive circuit is dependent on the rate of change in the current that produces that emf.
INDUCTORS, that is circuit elements that introduce inductance in a circuit are indicated in circuit diagrams as |
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It can be shown experimentally that the induced emf is directly proportional to the rate of change of current, that is,
The factor L is called the inductance of the circuit. The unit of inductance is the HENRY, H, which is defined as follows:
A circuit has an inductance of 1 henry if an emf of 1 volt is induced in the circuit when the current changes uniformly at a rate of 1 ampere per second.
The inductance, L, of a straight coil depends on a number of parameters, as shown in the formula below
where Km is the magnetic constant of the material placed inthe coil (Km = 1 if the coil is "empty"), n is the number of turns per metre, A is the cross-sectional area of the coil, and l the length of the coil. We see therefore that the inductance depends on
When inductances are in series, the total inductance is the sum of the individual inductances (in the same way that resistances in series behave), Lcombined = L1 + L2 + L3...
When inductances are in parallel, the reciprocal of the total inductance is the sum of the reciprocals of the individual inductances (in the same way that resistances in parallel behave), 1/Lcombined = 1/L1 + 1/L2 + 1/L3...