Practice Circuit Problem Figure 355 is a typical combination circuit. To make sure you understand the techniques of solving for the unknown quantities, solve for E_{R1}.
Figure 355.  Combination practice circuit.
It is not necessary to solve for all the values in the circuit to compute the voltage drop across resistor R_{1} (E_{ R1}). First look at the circuit and determine that the values given do not provide enough information to solve for E_{R1} directly.
If the current through R_{1} (I_{R1}) is known, then E_{R1} can be computed by applying the formula:
The following steps will be used to solve the problem.
>The total resistance
(R_{T}) is calculated by the use of equivalent resistance.
Given:
Solution:
Redraw the circuit as shown in figure 355(B).
Given:
Solution:
Solution:
Redraw the circuit as shown in figure 355(C).
Given:
Solution:
The total current (I_{T}) is now computed.
Given:
Solution:
>Solve for the voltage dropped across R_{eq2}. This represents the voltage dropped across the network R_{1}, R_{2}, and R_{3} in the original circuit.
Given:
Solution:
>Solve for the current through R_{eq1}. (R_{eq1} represents the network R_{1} and R_{2} in the original circuit.) Since the voltage across each branch of a parallel circuit is equal to the voltage across the equivalent resistor representing the circuit:
Given:
Solution:
Solve for the voltage dropped across R_{1} (the quantity you were asked to find). Since R_{eq1} represents the series network of R_{1} and R_{2} and total current flows through each resistor in a series circuit, I_{R1} must equal I_{Req1}.
Given:
Solution:
Refer to figure 355(A). If the following resistors were replaced with the values indicated: R_{1} = 900Ω, R_{3} = lkΩ, what is the total power in the circuit? What is E_{R2}?

