L/R Time Constant.

Electronic Principles
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because the rate of change of current is maximum. Thus the

battery voltage is equal to the drop across the inductor, and

the voltage across the resistor is zero. As time goes on, more

of the battery voltage appears across the resistor and less

across the inductor. The rate of change of current is

approached, the drop across the inductor approaches zero, and

all of the battery voltage is used to overcome the resistance of

the circuit.

Thus, the voltages across the inductor and the resistor change

in magnitude during the period of growth of current in the same

way the force applied to the boat divides itself between the

inertia and friction effects. The force is developed first.

across the inertia/inductive effect and finally across the

friction/resistive effect.

When switch S2 is closed (source voltage Es removed from the

circuit), the flux that has been established around the inductor

(L) is essentially equal to Es in magnitude. The induced voltage

causes decay current (id ) to flow in resistor R in the same

direction in which current was flowing originally (when S1 was

closed). A voltage (eR) that is initially equal to source

voltage (Es) is developed across I. The voltage across the

resistor (eR) rapidly falls to zero as the voltage across the

inductor (eL) falls to zero due to the collapsing flux.

Just as the example of the boat was used to explain the growth

of current in a circuit, it can also be used to explain the

decay of current in a circuit. When the force applied to the

boat is removed, the boat. continues to move through the water

before eventually coming to a stop. This is because energy was

being stored in the inertia of the moving boat. After a period

of time, the friction of the water overcomes the inertia of the

boat and the boat stops moving. Just as inertia of the boat

stored energy, the magnetic field of an inductor stores energy.

Because of this, even when the power source is removed, the

stored energy of the magnetic field of the inductor tends to

keep the current flowing in the circuit until the magnetic field

collapses.

(1) L/R Time Constant. The L/R TIME CONSTANT is a variable

tool for use in determining the time required for current in an

inductor to reach a