Determination of Voltage Sign
In applying Kirchhoff's laws to specific problems, particular attention should be paid to the
algebraic signs of voltage drops and e.m.fs.
(a)
Sign of Battery E.M.F.
A
rise
in voltage should be given a + ve sign and
a fall
in voltage a -ve sign. That is, if we go from
the -ve terminal of a battery to its +ve terminal there is a
rise
in potential, hence this voltage
should be given a + ve sign.
And on the other hand, we go from +ve terminal to -ve terminal, then there is
a fall
in potential,
hence this voltage should be preceded by a -ve sign.
The sign of the battery e.m.f is independent of the direction
of the current through that branch.
(b)
Sign of
IR
Drop
Now, take the case of a resistor (Fig. 2.4). If we go through a resistor in the
same
direction as
the
current, then there is a fall in potential because current flows from a higher to a lower
potential..
Hence, this voltage fall should be taken -ve. However, if we go in a direction opposite to that of
the
current, then there is a
rise
in voltage. Hence, this voltage rise should be given a positive sign.
Consider the closed path
ABCDA
in Fig .
As we travel around the mesh in the clockwise direction, different voltage drops will have the
following signs :
I
1
R
1
is - ve (fall in potential)
I
2
R
2
is - ve (fall in potential)
I
3
R
3
is + ve (rise in potential)
I
4
R
4
is - ve (fall in potential)
E
2
is - ve (fall in potential)
E
1
is + ve (rise in potential)
Using Kirchhoff's voltage law, we get
-I
1
R
1
–
I
2
R
2
–
I
3
R
3
–
I
4
R
4
–
E
2
+ E
1
= 0
Or
I
1
R
1
+ I
2
R
2
–
I
3
R
3
+ I
4
R
4
= E
1
–
E
2
Dostları ilə paylaş: |