Download Kirchhoff*s Laws

Kirchhoff’s Current Law
For a series circuit:
In a series circuit there is only one path so the current must
be…
For a parallel circuit:
In a parallel circuit the charge can take different paths.
Therefore the amount of charge at any point…
V T = V 1 + V 2 + V 3…
V T = V 1 = V 2 = V 3…
RT = R1 + R2 + R3…
1=1+1+1…
RT R1 R2 R3
Example
A series circuit is shown to the left.
a) What is the total resistance?
R(series) = 1 + 2 + 3 = 6W
b)
What is the total current?
DV=IR
V1W=(2)(1)= 2 V
12=I(6)
I = 2A
c)
What is the current across EACH
resistor? They EACH get 2 amps!
d)
What is the voltage drop across
each resistor?( Apply Ohm's law
to each resistor separately)
V3W=(2)(3)= 6V
V2W=(2)(2)= 4V
Notice that the individual VOLTAGE DROPS add up to the TOTAL!!
Example
To the left is an example of a parallel circuit.
a) What is the total resistance?
1 1 1 1
=  
RP 5 7 9
2.20 W
1
1
= 0.454  RP =
=
Rp
0.454
b) What is the total current? DV = IR
8 = I ( R ) = 3.64 A
c) What is the voltage across EACH resistor?
8 V each!
d) What is the current drop across each resistor?
(Apply Ohm's law to each resistor separately)
DV = IR
8
8
8
I 5W = = 1.6 A I 7 W = =1.14 A I 9W = = 0.90 A
5
7
9
Notice that the
individual currents
ADD to the total.