# 11.4 RL Circuits

#### LEARNING OBJECTIVES

By the end of this section, you will be able to:

• Analyze circuits that have an inductor and resistor in series
• Describe how current and voltage exponentially grow or decay based on the initial conditions

A circuit with resistance and self-inductance is known as an  circuit. Figure 11.4.1(a) shows an  circuit consisting of a resistor, an inductor, a constant source of emf, and switches  and . When  is closed, the circuit is equivalent to a single-loop circuit consisting of a resistor and an inductor connected across a source of emf (Figure 11.4.1(b)). When  is opened and  is closed, the circuit becomes a single-loop circuit with only a resistor and an inductor (Figure 11.4.1(c)).

(Figure 11.4.1)

We first consider the  circuit of Figure 11.4.1(b). Once  is closed and  is open, the source of emf produces a current in the circuit. If there were no self-inductance in the circuit, the current would rise immediately to a steady value of . However, from Faraday’s law, the increasing current produces an emf  across the inductor. In accordance with Lenz’s law, the induced emf counteracts the increase in the current and is directed as shown in the figure. As a result,  starts at zero and increases asymptotically to its final value.

Applying Kirchhoff’s loop rule to this circuit, we obtain

(11.4.1)

which is a first-order differential equation for . Notice its similarity to the equation for a capacitor and resistor in series (See RC Circuits). Similarly, the solution to Equation 11.4.1 can be found by making substitutions in the equations relating the capacitor to the inductor. This gives

(11.4.2)

where

(11.4.3)

is the inductive time constant of the circuit.

The current  is plotted in Figure 11.4.2(a). It starts at zero, and as ,  approaches  asymptotically. The induced emf is directly proportional to , or the slope of the curve. Hence, while at its greatest immediately after the switches are thrown, the induced emf decreases to zero with time as the current approaches its final value of . The circuit then becomes equivalent to a resistor connected across a source of emf.

(Figure 11.4.2)

The energy stored in the magnetic field of an inductor is

(11.4.4)

Thus, as the current approaches the maximum current , the stored energy in the inductor increases from zero and asymptotically approaches a maximum of .

The time constant  tells us how rapidly the current increases to its final value. At , the current in the circuit is, from Equation 11.4.2,

(11.4.5)

which is  of the final value . The smaller the inductive time constant , the more rapidly the current approaches .

We can find the time dependence of the induced voltage across the inductor in this circuit by using  and Equation 11.4.2:

(11.4.6)

The magnitude of this function is plotted in Figure 11.4.2(b). The greatest value of is ; it occurs when  is greatest, which is immediately after  is closed and  is opened. In the approach to steady state,  decreases to zero. As a result, the voltage across the inductor also vanishes as .

The time constant  also tells us how quickly the induced voltage decays. At , the magnitude of the induced voltage is

(11.4.7)

The voltage across the inductor therefore drops to about  of its initial value after one time constant. The shorter the time constant , the more rapidly the voltage decreases.

After enough time has elapsed so that the current has essentially reached its final value, the positions of the switches in Figure 11.4.1(a) are reversed, giving us the circuit in part (c). At , the current in the circuit is . With Kirchhoff’s loop rule, we obtain

(11.4.8)

The solution to this equation is similar to the solution of the equation for a discharging capacitor, with similar substitutions. The current at time  is then

(11.4.9)

The current starts at  and decreases with time as the energy stored in the inductor is depleted (Figure 11.4.3).

The time dependence of the voltage across the inductor can be determined from :

(11.4.10)

This voltage is initially , and it decays to zero like the current. The energy stored in the magnetic field of the inductor, , also decreases exponentially with time, as it is dissipated by Joule heating in the resistance of the circuit.

(Figure 11.4.3)

### EXAMPLE 11.4.1

#### An RL Circuit with a Source of emf

In the circuit of Figure 11.4.1(a), let , , and . With closed and  open (Figure 11.4.1(b)), (a) what is the time constant of the circuit? (b) What are the current in the circuit and the magnitude of the induced emf across the inductor at , at , and as ?

#### Strategy

The time constant for an inductor and resistor in a series circuit is calculated using Equation 11.4.3. The current through and voltage across the inductor are calculated by the scenarios detailed from Equation 11.4.2 and Equation 11.4.10.

#### Solution

a.     The inductive time constant is

b.     The current in the circuit of(b) increases according to Equation 11.4.2:

At ,

At and , we have, respectively,

and

From Equation 11.4.10, the magnitude of the induced emf decays as

At , and as we obtain

and

#### Significance

If the time of the measurement were much larger than the time constant, we would not see the decay or growth of the voltage across the inductor or resistor. The circuit would quickly reach the asymptotic values for both of these. See Figure 11.4.4.

(Figure 11.4.4)

### EXAMPLE 11.4.2

#### An RL Circuit without a Source of emf

After the current in the  circuit of Example 11.4.1 has reached its final value, the positions of the switches are reversed so that the circuit becomes the one shown in Figure 11.4.1(c). (a) How long does it take the current to drop to half its initial value? (b) How long does it take before the energy stored in the inductor is reduced to  of its maximum value?

#### Strategy

The current in the inductor will now decrease as the resistor dissipates this energy. Therefore, the current falls as an exponential decay. We can also use that same relationship as a substitution for the energy in an inductor formula to find how the energy decreases at different time intervals.

#### Solution

a.     With the switches reversed, the current decreases according to

At a time when the current is one-half its initial value, we have

and

where we have used the inductive time constant found in Example 11.4.1.

b.     The energy stored in the inductor is given by

If the energy drops to  of its initial value at a time , we have

Upon canceling terms and taking the natural logarithm of both sides, we obtain

so

Since , the time it takes for the energy stored in the inductor to decrease to  of its initial value is

#### Significance

This calculation only works if the circuit is at maximum current in situation (b) prior to this new situation. Otherwise, we start with a lower initial current, which will decay by the same relationship.

Verify that  and  have the dimensions of time.