DC Electric Circuits

Inductance

20 questions By Tony R. Kuphaldt

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Question 10 of 20

Suppose you wished to build a component with no other purpose than to provide inductance in an electric circuit (an inductor). How might you design such a device to perform this function, and how could you maximize its inductance?

Reveal answer

I’ll let you determine how an inductor is constructed, from your own research.

To increase inductance:

Increase number of “turns” in coil

Increase diameter of coil

Decrease length of coil

Increase permeability of core material

Notes:
These factors are important to understand for comprehending the function of variable inductors. Be sure to bring up the subject of variable inductors in your discussion with students.
Question 11 of 20

Magnetic fields, like all fields, have two fundamental measures: field force and field flux. In an inductor, which of these field quantities is directly related to current through the wire coil, and which is directly related to the amount of energy stored?

Based on this relationship, which magnetic field quantity changes when a bar of iron is brought closer to a wire coil, connected to a source of constant current?

Reveal answer

Field force is a direct function of coil current, and field flux is a direct function of stored energy.

If an iron bar is brought closer to a wire coil connected to a constant current source, the magnetic field force generated by the coil will remain unchanged, while the magnetic field flux will increase (and along with it, the amount of energy stored in the magnetic field).

Notes:
The concept of a field is quite abstract, but at least magnetic fields are something within most peoples’ realm of experience. This question is good for helping students distinguish between field force and field flux, in terms they should understand (constant current through a coil, versus the attractive force produced by a magnetic field flux).
Question 12 of 20

Suppose an inductor is connected directly to an adjustable-current source, and the current of that source is steadily increased over time. We know that an increasing current through an inductor will produce a magnetic field of increasing strength. Does this increase in magnetic field constitute an accumulation of energy in the inductor, or a release of energy from the inductor? In this scenario, does the inductor act as a load or as a source of electrical energy?

Now, suppose the adjustable current source is steadily decreased over time. We know this will result in a magnetic field of decreasing strength in the inductor. Does this decrease in magnetic field constitute an accumulation of energy in the inductor, or a release of energy from the inductor? In this scenario, does the inductor act as a load or as a source of electrical energy?

For each of these scenarios, label the inductor’s voltage drop polarity.

Reveal answer

As the applied current increases, the inductor acts as a load, accumulating additional energy from the current source. Acting as a load, the voltage dropped by the inductor will be in the same polarity as across a resistor.

As the applied current decreases, the inductor acts as a source, releasing accumulated energy to the rest of the circuit, as though it were a current source itself of superior current. Acting as a source, the voltage dropped by the inductor will be in the same polarity as across a battery, powering a load.

Notes:
Relating the polarity of voltage across an inductor to a change of applied current over time is a complex concept for many students. Since it involves rates of change over time, it is an excellent opportunity to introduce calculus concepts ([d/dt]).

Vitally important to students’ conceptual understanding of an inductor exposed to increasing or decreasing currents is the distinction between an electrical energy source versus a load. Students need to think “battery” and “resistor,” respectively when determining the relationship between direction of current and voltage drop. The complicated aspect of inductors (and capacitors!) is that they may switch character in an instant, from being a source of energy to being a load, and visa-versa. The relationship is not fixed as it is for resistors, which are always energy loads.