(1) *Applications of Kirchhoff's Voltage Law.*

Kirchhoff's voltage law

can be written as an equation, as shown below:

Ea + Eb + Ec + ...

En = 0

where Ea, Eb, etc., are the voltage drops around any closed circuit loop.

To set up the equation for an actual circuit, the following procedures are

used.

(a) Assume a direction of current through

the

circuit.

(The

correct direction is desirable but not necessary.)

(b) Using the assumed direction of current, assign polarities to

all resistors through which the current flows.

(c) Place the correct polarities on any sources included in the

circuit.

(d) Starting at any point in the circuit, trace around the

circuit, writing down the amount and polarity of the voltage across each

component in succession.

The polarity used is the sign after the assumed

current has passed through the component. Stop when the point at which the

trace was started is reached.

(e) Place these voltages, with their polarities, into the equation

and solve for the desired quantity.

Example: Three resistors are connected across a 50 volt source. What is the

voltage across the third resistor if the voltage drops across the first two

resistors are 25 volts and 15 volts?

Solution: First, a diagram (such as figure 28 on the following page) is

drawn.

Next, a direction of current is assumed (as shown).

Using this

current, the polarity markings are placed at each end of each resistor and

also on the terminals of the source. Starting at point A, trace around the

circuit in the direction of current flow, recording the voltage and polarity

of each component. Starting at point A and using the components from the

circuit:

(+Ex) + (+E2) + (+E1) + (-EA) = 0

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