الفيزياء العامة الجزء الثاني#موقع الفيزياء.كوم

Direct Current Circuits
8.7.3 The potentiometer
This circuit is used to measure potential differences by comparison with a
slandered voltage source. The circuit is shown in figure 8.26 where the
working emf is ξ w and the unknown emf is ξ x . The current flow in the
circuit is I in the left branch and I x is the current in the right branch and I-I x
is the current flow in the variable resistor. Apply second Kirchhoff’s rule
on the right branch abcd we get,
I
I x
I - I x
a b
ξ
x
ξ w
R x
G
d
c
Figure 8.26
-(I-I x )R x +ξ x = 0
When the variable resistance adjusted until the galvanometer reads zero, this
mean that I x =0.
ξ x = IR x
In the next step the emf ξ x is replaced with standard emf ξ s , and adjusted
the resistance until the galvanometer reads zero, therefore,
ξ s = IR x
Where the current I remains the same, divide the two equations we get,
x
Rx
ζ
s
Rs
ζ = (8.24)
This shows how unknown emf can be determined using known emf.
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