Conceptual Questions Based On Capacitance

Q.1. Where is the knowledge of dielectric strength helpful?
Answer: Dielectric strength is the maximum value of electric field that can be applied across an insulator
before its breakdown or the magnitude of electric field intensity across insulator at which it starts
conducting. This value helps us in determining the value of maximum potential, which can be applied
across a capacitor.

Q.2. The safest way to protect you from lightning is to be inside a car. Why?
Answer: The body of the car is metallic, thus it provides electrostatic shielding to the person inside it. Even if
there is charge on the body of the car, electric field inside it remains zero.

Q.3. A sensitive instrument is to be shielded from the strong electrostatic field in is environment. Suggest
a possible way.

Answer: To shield an instrument it must be placed inside a metallic body, because the electric field intensity
inside it will be zero.

Q.4. When it is lightning it is safe to be inside a house than under a tree. Why?
Answer: When we are standing under a tree we provide easy passage for the charge to flow through our body to the ground. But when we are inside a house, the charge can pass to the ground through lightning
conductor present in the building.

Q.5. The discharging current in the atmosphere due to small conductivity of air is known to be 1800A on
an average over the globe. Why then does the atmosphere not discharge itself completely in due
course and become electrically neutral?

Answer: The atmosphere does not gets charged because it gets the same amount of charge by lightning all
around the globe and net charge on it almost remains same.

Q.6. How can whole charge of a body be transferred to another isolated conductor?
Answer: This can be done by placing a charge conductor inside hollow isolated conductor and connecting
them by a wire. As charge resides on the outer surface thus whole charge from the inner body will be
transferred to the outer body.

Q.7. Can we give as much charge to the capacitor as we wish?
Answer: No, the maximum charge that be given to the capacitor depends upon its capacity or the dielectric
strength of medium between the plates of capacitor.

Q.8. A metal foil of negligible thickness is introduced between the plates of capacitor at the center. What
will be new capacitance of the capacitor?

Answer: When a metal sheet is introduced then this system is equivalent to two capacitors each of thickness
d/2 connected in series and the equivalent capacity is C.

Q.9. The separation between the plates of charged capacitor is to be increased. What will be the effect on
energy in case battery is removed after charging the capacitor or battery is removed?

Answer: When distance between the plates of capacitor is increased the capacitance of parallel plate capacitor decreases.[a] With battery disconnected, Q remains constant thus energy will increase. [b] With
battery in connection V remains constant thus if C decreases energy will alo decreases.

Q.10. The battery remains connected to parallel plate capacitor and dielectric slab is inserted between the
plates. What will be the effect on [a] capacity [b] charge [c] potential energy [d] electric field intensity.

Answer: If dielectric slab is inserted with dielectric constant K between the plates, then the capacitance of
capacitor will increase K times. With battery in connection V remains constant. Q =CV, will increase
K times. Potential energy [ ] will also increase K times. Electric field intensity [Q/Aε0] will decrease
K times.

Q.11. What happen when the plates of charged capacitor are suddenly connected by metallic wire?
Answer: When we connect two plates of capacitor, charge begins to flow from higher to lower potential plate and the energy stored in the capacitor gets converted into heat energy.

Q.12. Why does electrical conductivity of earth’s atmosphere increases with altitude?
Answer: The electrical conductivity of earth’s atmosphere increases with altitude because of increase in number density of charged particles and decrease in water vapours.

Q.13. Two copper spheres one hollow and other solid are charged to same potential. Which one carries
more charge?

Answer: Charge on both the spheres will be same as capacity depends only on the radius. This happens
because charge resides only on the outer surface of conductor and not inside it.

Q.14. Why capacitors should be handled carefully even when there is no current?
Answer: A capacitor once charged takes infinite time to discharge completely even when battery is disconnected
or there is no current. Thus if we touch the plates of capacitors we will gets shock.

Q.15. If battery is disconnected and distance between the plates of capacitor is increased, how will the
charge, electric field, potential energy be affected?

Answer: If the distance between the plates of capacitor is increased, then the capacitance is going to decrease. As battery is disconnected thus charge on the plates and electric field remains constant, whereas
potential energy will increase.

Q.16. What will be effect on capacity of parallel plate capacitor when the distance between the plates is
doubled and plate area is also doubled?

Answer: As both A as well as d are doubled capacitance remains unaffected.

Q.17. Given a battery, how would you connect the two capacitors, in series or parallel for them to store the
greater [a] total charge [b] total energy?

Answer: As V is constant for a given battery therefore Q = CV and U =1/2 CV2
As both energy and charge are
directly proportional to capacitance thus they will be more in parallel combination.

Q.18. When a battery is connected across the capacitor, are the charges on the plates always equal and
opposite even for plates of unequal size?

Answer: Yes, because of charge conservation the charges on the plates of capacitor are always and opposite in nature.

Q.19. Can there be potential difference between two conductors of same volume carrying equal positive
charges?

Answer: Yes, there can be potential difference, because although there volumes are equal they can have different shapes and thus different capacities.

Q.20. Is it possible for metal sphere of radius 1cm to hold a charge of 1C?
Answer: No, At such a high voltage electric breakdown of air occurs and charge will leak to the surroundings.

Q.21. Why does the electric field inside the dielectric decrease when it is placed inside an external field?
Answer: The electric field inside the dielectric decreases because external field will create an induced field
inside the dielectric whose direction will be opposite to the external field. Thus, net field inside it
will decrease.

Q.22. A parallel plate capacitor has a capacity of 6µF and when dielectric is inserted it is 60µF. What is the
dielectric constant of the medium?

Answer: The dielectric constant K = Cm/Ca, thus it is equal to 10.

Q.23. Two spheres of different capacities are charged to different potential are then connected by conducting wire. Will the total energy increase or decrease?
Answer: When two spheres are connected by wire, the charge flows from sphere having higher potential to
lower potential till their potential becomes same. In the process of sharing of charges energy will be
lost in the form of heat.

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