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

78 Chapter Two
R4 is dropping 1 volt (a total of 3 volts), the remaining portion of the
total 4.6 volts must be dropped across R2. Therefore, R2 is dropping 1.6
volts (4.6 volts 3 volts 1.6 volts). Now you know two electrical variables
pertaining specifically to R2: the voltage across it (1.6 volts) and the
current flow through it (10 milliamps). Its resistance value can be calculated
from Eq. (2-3):
E 1.6 volts
R
160 ohms
I 10 milliamps
The resistance value of R1 can be determined in the same manner.
Since 10 milliamps is flowing through the R3-D1-R1 parallel leg, the
current flow through R3 will be 10 milliamps. The voltage drop across
R3 can be calculated using Eq. (2-1):
E IR (10 milliamps)(68 ohms) 0.68 volts or 680 millivolts
The sum of the 2 volts dropped across D1 and the 680-millivolt drop
across R3 is 2.68 volts. Remembering that 4.6 volts is dropped across the
entire R3-D1-R1 leg, you can find the voltage drop across R1 by subtracting
the combined voltage drops of D1 and R3. 4.6 volts minus 2.68
volts provides a difference of 1.92 volts, which is the voltage drop across
R1. Now that you know the voltage drop across R1 (1.92 volts) and the
current flow through it (10 milliamps), you can calculate its resistance
value using Eq. (2-3):
E 1.92 volts
R
192 ohms
I 10 milliamps
Now that you have gone through this exercise involving Fig. 2-24a, refer
to Fig. 2-24b. Note that this circuit is electrically identical to the Fig. 2-24a
circuit, even though its visual appearance is quite different. Prove this to
yourself by following the current flow from the negative terminal to the
positive terminal of the source. Finally, refer to Fig. 2-24c. You’ll discover
that this circuit is also electrically identical to the Fig. 2-24a circuit diagram.
Again, prove this to yourself by following the current flow paths.
What is the practical purpose of this exercise Again, this exercise is
primarily directed toward helping you improve your ability of conceptualizing
circuits. It may be wise for you to repeat all of the previous
exercises several times. Remember, conceptualizing electronic circuitry is
a learned process—as you continue through this book, your abilities in
this regard will improve greatly.