Skip to main content

Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 29.4 Inductors

Definition 29.4.1. Inductor.

An inductor is a circuit element that provides a voltage given by
\begin{equation*} V_{\text{inductor}} = -L \frac{dI}{dt} \end{equation*}
🔗
  • The sign of the voltage can be found using Lenz’s Law. The induced voltage always opposes the change in flux through the inductor (this is denoted by the minus sign in the equation above).
    🔗
    🔗
  • \(L\) is the inductance: a physical property measured in henries (H). It depends on the size and shape of the inductor.
    🔗
    🔗
  • Any circuit element can have an inductance. Inductance tends to be most significant for wire coils and solenoids.
    🔗
    🔗
Left: a picture of a solenoid. Right: a curling line representing an inductor in a circuit.
Figure 29.4.2. The circuit diagram representation for an inductor.
🔗
🔗

Definition 29.4.3. Potential Energy Stored in an Inductor.

The potential energy stored in an inductor is given by
\begin{equation*} U_{\text{inductor}} = \frac{1}{2} L I^2 \end{equation*}
🔗
🔗

Subsubsection Activities

Activity 29.4.1. Sensemaking: Direction.

Consider the inductor shown in the Inductor Circuit Diagram above. Suppose at some instant in time the current is flowing downward and increasing in magnitude. Use an area vector for the inductor that points upward.
🔗
(a)
Use the right-hand rule to determine whether the current is positive or negative for the given area vector.
🔗
Answer.
Positive!
🔗
🔗
🔗
(b)
Use the right-hand rule to determine whether the change in the current is positive or negative for the given area vector.
🔗
Answer.
The current is increasing, so the change is in the same direction as the current, which is positive!
🔗
🔗
🔗
🔗

Activity 29.4.2. Inductor Behavior.

Provide a physical argument to justify the generalized statement below about inductors.
🔗
🔗
🔗
🔗
PrevTopNext