Introduction To Engineering
Electrical Circuits - 2
Agenda
Kirchhoff’s Law
Resistors & Resistance
Wheatstone Bridge
Kirchhoff’s Laws
Voltage Law: The sum of the voltage
rises around a closed loop in a circuit
must equal the sum of the voltage drops.
Current Law: The sum of all currents
into a junction (node) must equal the sum
of all currents flowing away from the
junction.
Resistors in Series
Applying Kirchhoff’s
voltage law gives us:
V = I R1 + I R2 + I R3
Resistors in Parallel
voltage law: V = I1R1 = I2R2 = I3R3
current law: Ix = I1 + Iy and Iy = I2 + I3
Equivalent Resistance
If desired, several resistors can sometimes
be replaced by a single “equivalent” resistor:
For resistors in series: Req = R1 + R2 + R3 + …
R1
R2
R3
Req
Equivalent Resistors (cont)
1
1
1
1



 ...
Req R1 R2 R3
For
Resistors in
Parallel
R1
R2
R3
Req
Student Problem:
 If each of the R’s were 60 W light bulbs, (R =240
ohm), what would be the equivalent resistance
for all three bulbs in parallel.
 What would be the total power draw for the three
bulbs
1 1 1 1
    ...
Req R1 R2 R3
R1
R2
R3
V=IxR
P=VxI
Wheatstone Bridge
Circuit with resistors in
both series and parallel
configurations. Used in
strain gages, alarm
circuits, and many other
systems. The bridge is
“balanced” when im = 0,
at which point:
R1/R2 = R3/R4
Assignment #29
Do problem set 2 on electrical circuit
drawings found in the assignment packet.