الفيزياء العامة الجزء الثاني#موقع الفيزياء.كوم

Direct Current Circuits
Note that the quantity RC in the equation has a unit of time (sec). Therefore
it is called the time constant of the circuit.
Unit of RC is Ohm . Farad=Sec
Coulomb Amp.
Sec Volt.
Sec
Ohm.Farad= Ohm . = Ohm.
= = Sec
Volt
Volt Volt
8.6.2 Discharging a capacitor
When the switch S is connected to point (b) the battery is disconnected and
now the charged capacitor plays the role of emf. Therefore the capacitor
will force a charge q to move through the resistor R this called discharging
process of the capacitor.
Apply the first Kirchhoff’s rule in a direction from the resistor to the
capacitor we get,
- IR - C
q = 0 (8.17)
The current I and the charge q are varying with time. Substitute for
dq
I = dt
(8.18)
dq q
R + = 0
(8.19)
dt C
By solving the differential equation to find the q as a function of time we
get,
q
−t
RC
= Qe
(8.20)
The current I during the discharging process is
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