Chapter 11 ELECTROSTATICS
Short Questions & Answers (Exercise)
11.1 The potential is constant throughout a given region of space. Is the electrical field zero or non zero in this region? Explain.
Ans: Electric Field may or may not be zero in this region.
As we know that the electric field is established in a region due to the potential
difference. If the potential is constant then there will be no electric field.
From the relation:
E = rV
Δ
− ……………………..(i)
Δ
It is given that the potential is constant, therefore; ΔV = 0
−0= 0
put in (i) E = Δr
It means electric field in the given region is zero.
If the region is an equipotential region e.g (surface of a sphere which has a certain charge located at its centre), then at all points of the surface electric potential will be constant but electric field at the surface will not be zero as the surface is situated with in the electric field.
11.2 If a point charge q of mass m is released in a non-uniform electric field, will it make a rectilinear motion?
Ans: If the source of electric field is a single positive charge the electric field lines will be straight. Hence a point charge release in the field will make a rectilinear motion.
If however the electric field is due to two charges then the field lines will be curved. Hence a point charge released in the field will follow a cured path.
11.3 What is the relationship between voltage and energy? More precisely, what is the relationship between potential difference and electric potential energy?
Ans. Voltage is the energy transferred or dissipated by a charge between two points of the electric field. V=W/q
Energy is the work done that stored in a charge when it moves against electric field. U=qV
Relationship: The potential difference or Voltage gives us the measure of the difference of the electric potential energy of a unit charge b/w two points.
ΔV=ΔU/q
11.4 Voltage is always measured b/w two points. Why?
Ans. Voltage or potential difference between two points is the amount of work done in carrying a unit positive charge from one point to another point against the electric field, so two points are needed to measure voltage. Potential difference b/w two points is:
V A – V B = W AB /q
OR ΔV=ΔU/q
11.5 How are the units of volts and electron volts related? How do they differ?
Ans. An electron volt is the unit of energy which is equal to the kinetic energy acquired by an electron when accelerated through a potential difference of one volt. It is equal to 1.602 x 10-19 Joules. Whereas volt is the unit of potential which is defined as if one joule work is done in carrying a charge from one point to another point.
Difference: Electron volt is unit of energy whereas volt is the unit of potential difference which is change in energy per unit charge.
11.6 In what region of space is the potential due to a uniformly charged sphere the same as that of a point charge? In what region does it differ from that of a point charge?
Ans. Electric potential due to uniformly charged sphere at a point p outside the sphere is like electric potential due to a point charge because in both the cases V =kq/r. But inside the charge sphere potential is constant everywhere and V =kq/R.
11.7 Can the potential of a non-uniformly charged sphere be the same as that of a point charge? Explain.
Ans. The potetential due to point charge is...
V=kq/r ————1
The potential due to point charge is constant for same value of r and uniform field. Also E=V/r
V=Er —————-2
Eq 2 shows that if the field is non uniform means E varies, then V varies
So potential of non uniform sphere and that of a point charge are not same
11.8 What is an equi-potential line and equi-potential surface?
Ans. Equipotential line: An equipotential line is a line joining points having same potential. Equipotential lines are always perpendicular to electric field lines due to charges.
Equipotential surface: A surface with same potential at every point. Electric field lines are always perpendicular the equipotential surface.
Work done in moving a charge b/w two points on equipotential surface and along the equipotential line is zero.
A dashed line represents equipotential line and equipotential surface.
11.9 Can different equipotential lines cross each other? Explain.
Ans. No, different equipotential lines cannot cross each other. If the equipotential lines can cross each other than their corresponding electric field lines also intersect which is impossible.
11.10 Water has a large dielectric constant, but it is rarely used in capacitors. Explain why?
Ans. Pure water is a non polar dielectric. Water molecules are moving randomly. They are not tightly bound to each other. In an electric field water molecules are polarized. But they are not at rest and cannot
induce charges to produce electric field like solid dielectric. Therefore water cannot be used as dielectric in capacitors. It can also produce corrosion to plates as well as can ionize the plates.
11.11 A capacitor is connected in series with a resistor and charged. Explain why the potential difference across the resistor decreases with time during the charging.
Ans. According to law of conservation of energy, the source voltage must be equal to sum of the potential drop (VC) across capacitor and resistor. Initially potential drop across capacitor is minimum but charge stored q will increase then VC will also increase so VR will decrease.
11.12 Sketch the graphs of potential difference against time for
a. discharging of capacitor b. charging of capacitor.
Ans.
11.13 Compare the formula for capacitors in series and parallel with those for resistors in series and parallel. Explain why the pattern is different.
Ans. See the topic combination of capacitors and resistors.
11.14 Explain why capacitors are of little use for storage of energy for normal domestic purposes of lighting heating and so on.
Ans. The little use of capacitors for storage of energy because it can store a limited charges on its plates and when they are connected to a load, potential difference across the capacitor cannot remains constant.
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