Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Basic Electrical Concepts
55
present equal resistances. To calculate the voltage drops across the individual
resistors, you might use Ohm’s law as in the previous example. Another
way of calculating the same drops is by using the ratio method. To use the
ratio method, calculate R total
by adding all of the individual resistive values.
Make a division problem with R total
the divisor, and the dividend the resistive
value of the resistor for which the voltage drop is being calculated. Perform
this division, and then multiply the quotient by the value of the
source voltage. The answer is the value of the unknown voltage drop.
Referring to Fig. 2-19, assume you desire to calculate the voltage drop
across R1 using the ratio method. Start by finding R total
:
R total
R 1
R 2
R 3
2 ohms 3 ohms 5 ohms 10 ohms
Next, make a division problem with R total
as the divisor and R 1
as the
dividend and perform the division:
2 ohms
0.2
10 ohms
Finally, multiply this quotient by the value of the source voltage:
(10 volts)(0.2) 2 volts
The voltage dropped across R1 in Fig. 2-19 will be 2 volts. As an exercise,
calculate the voltage drop across R1 using Ohm’s law. You should come
up with the same answer.
Series-Parallel Circuits
In many situations, series and parallel circuits are combined to form
series-parallel circuits (Fig. 2-20). In this circuit, R2 and R3 are in parallel,
Figure 2-19
Series circuit with
unequal resistances.
–
+
E 10 Vdc
R 1 2
R 2 3
R 3 5