الفيزياء العامة الجزء الثاني#موقع الفيزياء.كوم

Direct Current Circuits
8.6 RC Circuit
In the previous section we studied either circuits with resistors only or with
capacitors only, now we will deal with circuits contains both the resistors
and capacitors together, these circuits are time dependent circuit where the
current in the is varying with time.
In the circuit shown in figure 8.18 we have connected an emf with resistor R
and uncharged capacitor C using a switch S.
R
ξ
a s
b
C
I
Figure 8.18
8.6.1 Charging a capacitor
When the switch S is connected to point (a) the battery will force charges to
move to the capacitor this called charging process of the capacitor. Note
that the current will not flow through the capacitor since there is no way for
the charge to jump from one plate to the other. However a positive charge
will accumulate on the plate connected with the positive terminal of the
battery. The same number of a negative charge will accumulate on the other
plate.
The current must stop after the capacitor will become fully charged and its
potential difference equals the emf.
نلاحظ هنا في هذه الحالة أن التيار يكون ذا قيمة عظمى عند غلق المفتاح وبمرور الزمن يتناقص
التيار إلي أن يصل إلى الصفر.‏ وعندها يكون شحن المكثف قد اآتمل.‏
To analyze this circuit let’s assume that in time dt a charge dq moves
through the resistor and the capacitor. Apply the first Kirchhoff’s rule in a
direction from the battery to the resistor to the capacitor we get,
ξ - IR - C
q = 0 (8.11)
Where IR is the potential difference across the resistor and q/C is the
potential difference across the capacitor.
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