101. Electric potential of the earth is taken to be zero. Why?
As the earth is consider to be a good conductor of very large size. If some charge is given to it then its potential does not change. So, potential of the earth is assumed to be zero i.e. ground potential. Hence, electric potential of the earth is taken to be zero.
102. If we touch the high power line, we get fatal shock. Why?
If we touch the high power line then large amount of charge flows through our body to the ground since there is a difference of potential between our feet which is at zero potential and our hand which is at high potential. Hence, due to flow of large amount of current flows from our body which results in getting fatal shock.
103. An electron and a proton are situated independently in the electric field. Will their acceleration be same?
An electron & a proton are situated independently in the electric field. Here, the charges on proton & electron are equal in magnitude and the mass of proton is 1836 times the mass of electron. The electric force (F = qE) on them will be equal in magnitude as they are situated independently in the electric field.
No, there acceleration will not be same due to difference in mass. Here, the acceleration of an electron will be 1836 times that of the proton.
104. A charged particle is free to move in an electric field. What would be its type of motion?
The motion depends upon the initial direction of the charged particle compared to the electric field. Figure (i) shows .
106. Can two equipotential surfaces intersect? Explain.
No, two equipotential surfaces cannot intersect. As equipotential surface is normal to the electric field intensity, if two equipotential surfaces intersect then there will be two directions of electric field intensity at the point of intersection which is not possible. Hence, two equipotential surfaces cannot intersect.
107. What is the function of lightening conductor in tall building?
The function of lightening conductor in tall building is that it is used to save big buildings from the damage by the lightening flashes.
A lightening conductor consists of a number of pointed conductors fixed to the top of big building & connected to thick copper wire. As the surface density of a charge around sharp edges or points of a conductor is very great. Therefore, the strength of the electric field near the point is very great. The intense electric field breaks down the insulation of air, and sends a stream of charged molecules away from the point. When a negatively charged cloud passes over the building, it induces a positive charge on the pointed ends of lightening rod and an equal negative charge at the metal plate. Due to discharging action of sharp points, a positively charged electric wind is set up that neutralizes the negative charge on the cloud. Negative charge on the metal plate is completely neutralized in the earth. If lightening occurs then the conducting rod sends it directly to the earth and save our building from damaging.
108. The atmospheric electric field near the surface of the earth is as high as 100 Vm-1. Why do we not get an electric shock as we move on the earth?
Atmospheric electric field near the surface of the earth is as high as 100 Vm-1. We do not get an electric shock as we move on the earth because the earth surface behaves as equipotential surface. Because of the huge size of the earth, its potential does not change (i.e. its potential throughout the surface is same) even if the atmospheric electric field near the surface of the earth is as high as 100Vm-1. This much high electric field is unable to create potential difference within the earth's surface. There is no potential difference on the earth's surface so, we do not get an electric shock as we move on the earth.
109. Why should circuits containing capacitor be handled cautiously even when there is no current?
As the capacitor stored charge and get itself charged to a high potential & does not discharge itself even when the source of voltage is removed. While handling the charged capacitor we should beware of it because capacitor may discharge & stored charges flows through our body in the form of current which may be fatal. Hence, in order to prevent the discharging of capacitor & save our life from electric shock, we should always handle circuits containing capacitor very carefully even when there is no current.
110. Can a single conductor have any capacitance? Explain.
Yes, a single conductor can have capacitance since a single conductor is a capacitor whose one of plates is at infinity. For example: a single sphere is a single conductor of radius r & has capacitance C. The value of capacitance is given as:
111. Two copper spheres of same radii, one hollow and other solid are charged to same potential. Which if any of the two will have more charge?
The capacitance of a charged sphere whose radius is r is given by:
C = 4ε0pr
As the capacitance of a spherical body (copper) depends only on the radius and independent of mass, both a hollow sphere and a solid sphere of copper having same radii will have the same capacitance. So, both of them will have same charge.
112. A hollow sphere and a solid sphere, both have the same radius. Which will store more charge?
The capacitance of a charged sphere whose radius is r is given by:
C = 4ε0лr
As the capacitance of a spherical body depends only on the radius and independent of mass, both a hollow sphere and a solid sphere having same radii will have the same capacitance. So, both of them store equally charge.
113. Can you think of a situation in which two plates of a capacitor do not have equal magnitude of charge?
No, we can not think of a situation in which two plates of a capacitor do not have equal magnitudes of charge. If the plates of a capacitor will have different magnitudes of charges then it will not follow the principle of conservation of charge and also goes against the conclusion drawn by Faraday in his ice pail experiment.
114. A man fixed outside his house one evening a two-meter high insulating slabs carrying on its top a large metal sheet. Will he get an electric shock if he touches the metal sheet the next morning?
The metal sheet and the ground form a system of a capacitor whose dielectric is the slab. During the night, the discharging current of the atmosphere charges the capacitor and raises its potential. So, when a man touches metal sheet in next mourning, his feet will be at zero (ground potential) whereas his hands will be at high potential of metal. As the capacitor discharges and flows charges through his body to the ground so he gets shock.
115. Can we give any desired charge to a capacitor?
No, we cannot give any desired charge to a capacitor but we can give charge up to a certain value of potential i.e. saturation value of charging. Beyond that value, field between two plates exceeds dielectric strength then dielectric suffers a break down.
116. What will happen if the plates of a charged capacitor are suddenly connected by a metallic wire?
If the plates of a charged capacitor are suddenly connected by a metallic wire then the total charged stored on that capacitor starts flowing through that wire & energy stored in the capacitor is dissipated in the form of heat from that metallic wire.
117. How does a spark discharge occur between two charged objects?
If two charged objects are kept close to each other then air between objects is subjected to electric field. If the electric field in air between objects is high enough to cause ionization of gas molecules, a conducting path is created between the charged object. As a result, the free electrons move across the air to discharge the surfaces of the charged object. in the discharge, the stored electric energy is dissipated as hear, light & sound called as sparking.
118. What is the action of dielectric in a capacitor?
As the capacitance C of the capacitor having parallel plates of area A which are separated by distance d is given as:
where, e0 is permittivity of free space. If a dielectric medium of constant er is introduced between the plates then its capacitance Cmed is given as:
From relation 1 and 2, we get:
Hence, when the space between the parallel plate capacitor is completely filled with a dielectric then electric potential decreases but capacitance increases & becomes er times the original capacitance.
119. A metal slab is introduced between the plates of a charged parallel plate capacitor. What happens to the electric lines of force between the plates?
As there is no electric field possible inside a metal so no lines of force are present inside the metal slab when it is placed between the plates of a charged parallel plate capacitor. Hence, the electric lines of force between the plates of a parallel plate capacitor will be seem to divide into two parts.
120. A chimney made of bricks requires a lightening conductor but not an iron chimney. Explain why?
We know that the bricks are bad conductors of electricity while iron is good conductor of electricity. A chimney made up of bricks is unable to send an oppositely charged electric wind to neutralize the charged cloud while an iron chimney is able to send an oppositely charged wind to neutralize the charged clouds. Hence, a chimney made of bricks requires a lightening conductor but not an iron chimney.
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