Advertisements
Advertisements
Question
A capacitor of unknown capacitance is connected across a battery of V volts. The charge stored in it is 360 μC. When potential across the capacitor is reduced by 120 V, the charge stored in it becomes 120 μC.
Calculate:
(i) The potential V and the unknown capacitance C.
(ii) What will be the charge stored in the capacitor, if the voltage applied had increased by 120 V?
Advertisements
Solution
(i) Initial voltage, V1 = V volts and charge stored, Q1 = 360 µC.
Q1 = CV1 …(1)
Changed potential, V2 = V − 120
Q2 = 120 µC
Q2 = CV2 ...(2)
By dividing (2) from (1), we get `Q_1/Q_2 = (CV_1)/(CV_1) ⇒360/120 = V/(V-120)⇒ V= 180` volts
`∴C = Q_1/V_1 =( 360 xx10^-6)/180 = 2 xx 10^-6 F = 2μF`
(ii) If the voltage applied had increased by 120 V, then V3 = 180 + 120 = 300 V
Hence, charge stored in the capacitor, Q3 = CV3 = 2×10-6×300 = 600 µC
RELATED QUESTIONS
Find the equivalent capacitance of the network shown in the figure, when each capacitor is of 1 μF. When the ends X and Y are connected to a 6 V battery, find out (i) the charge and (ii) the energy stored in the network.
Define capacitor reactance. Write its S.I units.
A spherical capacitor consists of two concentric spherical conductors, held in position by suitable insulating supports. Show that the capacitance of a spherical capacitor is given by
C = `(4piin_0"r"_1"r"_2)/("r"_1 - "r"_2)`
where r1 and r2 are the radii of outer and inner spheres, respectively.
Three identical capacitors C1, C2 and C3 of capacitance 6 μF each are connected to a 12 V battery as shown.
Find
(i) charge on each capacitor
(ii) equivalent capacitance of the network
(iii) energy stored in the network of capacitors
The plates of a parallel-plate capacitor are made of circular discs of radii 5⋅0 cm each. If the separation between the plates is 1⋅0 mm, what is the capacitance?
Each capacitor shown in figure has a capacitance of 5⋅0 µF. The emf of the battery is 50 V. How much charge will flow through AB if the switch S is closed?
A parallel plate capacitor stores a charge Q at a voltage V. Suppose the area of the parallel plate capacitor and the distance between the plates are each doubled then which is the quantity that will change?
Obtain the expression for energy stored in the parallel plate capacitor.
Derive the expression for resultant capacitance, when the capacitor is connected in parallel.
During a thunder storm, the movement of water molecules within the clouds creates friction, partially causing the bottom part of the clouds to become negatively charged. This implies that the bottom of the cloud and the ground act as a parallel plate capacitor. If the electric field between the cloud and ground exceeds the dielectric breakdown of the air (3 × 106 Vm–1), lightning will occur.
- If the bottom part of the cloud is 1000 m above the ground, determine the electric potential difference that exists between the cloud and ground.
- In a typical lightning phenomenon, around 25 C of electrons are transferred from cloud to ground. How much electrostatic potential energy is transferred to the ground?
