CAPACITOR & DIELECTRIC
Q-Define the capacitance of a conductor. What is its SI unit?
A-The capacitance (C) of a conductor is a measure of its ability to store electric charge. It is defined as the ratio of the charge (Q) given to the conductor to the rise in its electric potential (V).
C=Q/V
SI unit of the capacitor is Farad or coulomb/volt
Q-Why does a capacitor store much more charge than an isolated conductor of the same size?
A-In a capacitor, an uncharged or earthed grounded plate is placed near a charged plate. This neighboring plate lowers the electric potential of the charged plate due to induction. Since C = Q/V a lower potential (V) means a significantly larger amount of charge (Q) can be given to the capacitor before its potential reaches the breakdown limit.
Q-A parallel plate capacitor with air between the plates has a capacitance C. What will be its new capacitance if the distance between the plates is halved?
A- Watch video solution. click below.
Q-What is the net charge on a charged capacitor? Explain.
A-The net charge on a charged capacitor is zero. A capacitor consists of two identical parallel plates. When charged, one plate acquires a positive charge (+Q) and the other acquires an equal and opposite negative charge (-Q). Therefore, the total net charge is:
Q ( total)= (+Q) + (-Q) = zero
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Q-How does the energy stored in a capacitor change if the plates are pulled further apart while it remains connected to a battery?
A-The energy stored decreases.
Reasoning: 1. Since the capacitor remains connected to the battery, its potential difference (V) stays constant.
2-Pulling the plates further apart increases the distance (d), which decreases the capacitance (C).
3-C decreases, so energy stored is also decreases.
U=1/2 C V^2
Q-What is the function of a secondary earthed plate in a parallel plate capacitor?
A-The primary function of earthing the second plate is to drain away the induced positive charge on its outer surface to the ground. This leaves only the negative charge on its inner face, which effectively lowers the potential of the first plate even further, thereby maximizing its capacity to store more charge.
Q-Why is water not used as a dielectric in capacitors despite having a very high dielectric constant
A-Water cannot be used as a dielectric for two main reasons:
Water has a very low dielectric strength, meaning it will easily undergo electrical breakdown under relatively small electric fields, causing the capacitor to discharge (short-circuit).
It is a liquid with highly polar molecules, leading to exceptional electrical conductivity if even minor impurities are present. A dielectric must act as an insulator.
Q-Define dielectric constant (K) in terms of capacitance. What is its unit?
A-The dielectric constant (K) of a substance is defined as the ratio of the capacitance of a capacitor with the dielectric filling the space between its plates (C) to its capacitance with vacuum between the plates (C0).
K=C/C0
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Q-When a dielectric slab between the plate of capacitor inserted , How capacitance increases?
A- Watch video solution.