inductance is the characteristic of an electrical conductor that opposes a change in current flow. The symbol for inductance is l. An inductor is a device that stores energy in a magnetic field.
Inductance exhibits the same effect on current in an electric circuit as inertia does on velocity of a mechanical object. It takes more work to start a load moving than it does to keep it moving because the load possesses the property of inertia. Inertia is the characteristic of mass that opposes a change in velocity. Once current is moving through a conductor, inductance helps to keep it moving. The effects of inductance are sometimes desirable and other times undesirable.
As noted in Chapter 18, the basic principle behind inductance states, when a current flows through a con- ductor, it generates a magnetic field around the conductor. As the magnetic flux lines build up, they create an opposition to the flow of current.
When the current changes direction or stops, or the magnetic field changes, an electromotive force (emf) is induced back into the conductor through the collapsing of the magnetic field. The opposition to the changes in current flow is identified as a counter electromotive force (counter emf). lenz’s law sum- marizes this effect—an induced emf in any circuit is always in a direction to oppose the effect that produced it. The amount of counter emf is in proportion to the rate of change: The faster the rate of change, the greater the counter emf.
All conductors have some inductance. The amount of inductance depends on the conductor and the shape of it. Straight wire has small amounts of inductance, whereas coils of wire have much more inductance.
The unit by which inductance is measured is the henry (H), named for Joseph Henry (1797–1878), an American physicist. A henry is the amount of inductance required to induce an emf of 1 volt when the current in a conductor changes at the rate of 1 ampere per second. The henry is a large unit; the millihenry (mH) and microhenry (H) are more commonly used.
Questions
1. Define inductance.
2. What is the unit for measuring inductance?
3. Define a henry.
4. What letter is used to represent inductance?
5. Describe what happens when the current stops flowing in a conductor.
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