Electrostatics

Question 0

What factors do tubes of force originating from a charge depend on?

Answer 0

Magnitude of charge

Permitivity of medium surrounding the charge

Question 1

Define Electric tubes of force.

Answer 1

Electric lines of force from a small surface area of positively charged conductor encloses a tube like structure and are called electric tubes of force.

Question 2

Define TNEI

Answer 2

Total number of tubes of induction passing normally through a given surface area is known as the total normal electric induction.

Question 3

Define NEI

Answer 3

Number of tubes of induction passing normally through a unit area drawn round a uniform electric field is called Normal Electric Induction.

Question 4

State Gauss’ theorem

Answer 4

According to Gauss’ theorem, the TNEI over any surface is equal to the sum of charges enclosed by that surface.

Question 5

Electrical intensity at a point outside a charged conducting sphere- derivation.

Answer 5

Gaussian sphere of radius r.
E= (1/4πε)* (q/r2)
= ( RRσ/(εrr))

Question 6

Electric field intensity at a Point outside infinitely infinitely long charged conducting cylinder - derivation

Answer 6

Construct Gaussian cylinder
E = (1/2πε)*(q/r)
q= charge per unit length

Question 7

Electric field intensity at a point outside and near surface of charged conductor of any shape- derivation

Answer 7

Construct small cylinder
Equate , no integration
E= σ/ε

Question 8

Electric field intensity due to uniformly charged infinite plane sheet- derivation

Answer 8

Construct an imaginary cylinder around P with axis perpendicular to plane sheet and ends having area of cross section ds.
Equate, no integration
E= σ/(2*ε)

Question 9

Electric field intensity due to uniformly charged infinite plane sheet - Expression

Answer 9

E= σ/(2*ε)

Question 10

Electric field intensity at a point outside and near surface of charged conductor of any shape- expression

Answer 10

E= σ/ε

Question 11

Electric field intensity at a Point outside infinitely infinitely long charged conducting cylinder - expression

Answer 11

E = (1/2πε)*(q/r)
q= charge per unit length

Question 12

Electrical intensity at a point outside a charged conducting sphere- expression

Answer 12

E= (1/4πε)* (q/r2)

= ( RRσ/(εrr))

Question 13

Derivation for mechanical force acting per unit area on the surface of a charged conductor.

Answer 13

E1=E2=(σ/(2*ε))
Mechanical force= charge on ds * electric intensity due to charge of remaining area of conductor

f= F/ds = σσ/(2ε) = (1/2)εE*E

Question 14

What is the direction of mechanical force per unit area?

Answer 14

Always directed outwards (σ*σ always positive)

Question 15

Define energy density

Answer 15

The electrostatic energy per unit volume is called energy density.

Question 16

Derivation for energy density of a medium

Answer 16

dW=F.dl

du/dv = (1/2)εE*E

Question 17

Define dielectrics

Answer 17

Dielectrics are non-conducting substances with no free charges or electrons.

Question 18

Define Polar molecule

Answer 18

A polar molecule is a molecule in which “centre of gravity” of positive nucleus and revolving electrons do not coincide.

Question 19

Define non-polar molecule

Answer 19

A non-polar molecule is a molecule in which “centre of gravity” of positive nucleus and revolving electrons coincide.

Question 20

Define Polarisation (electrostatics)

Answer 20

Polarisation is defined as dipole moment per unit volume.
Or
Polarisation may be defined at amount of induced surface charge per unit area or the surface density of Polarisation charges appearing at right angles to external electric field.

Question 21

Give and expression for Polarisation

Answer 21

P= χe*E
χe—> electric susceptibility of dielectric material
Or
P= (Polarization charges (qp) / area of cross section of dielectrics) - charge density of Polarisation charges (σ p)

Question 22

Linear isotropic substances

Answer 22

Linear isotropic substances are substances in which induced dipole moment is induced in the di erection of field and proportional to field strength.

Question 23

How do polarized dielectrics work?

Answer 23

A polarized dielectric is equivalent to two charged surfaces with induced charges (polarization charges). These induced surface charges oppose the external electric field and so weaken the original field within the dielectric.

Question 24

Define capacitor

Answer 24

Capacitor is a system of conductors having equal and opposite charges separated by an insulator (insulating medium) or dielectric.

Question 25

Define capacitance of a capacitor

Answer 25

The capacitance of a capacitor is defined as the ratio of magnitude of charge on either of the conductors to the magnitude of potential difference between two conductors.

Question 26

Give the expression for capacitance of a capacitor

Answer 26

C= Q/V

Question 27

What is capacitance?

Answer 27

The ability of a conductor to store electric charge and electric potential energy is known as capacitance of the capacitor.

Question 28

Why is condenser a misnomer?

Answer 28

Capacitor doesn’t condense energy but stores it.

Question 29

Applications of capacitor

Answer 29

Tuning of radio circuit.
Smoothing rectified current in power suppliers.
Elimination of sparking points when they are open or close in and ignition system of automobile engine.
Storage of large amount of charge for use in nuclear reactors.

Question 30

Define the SI unit of capacitance

Answer 30

A capacitor is said to have a capacitance of one farad if the potential difference across it rises by one volt when one coulomb of charge is given to it.

Question 31

Explain the concept of a parallel plate capacitor

Answer 31

Voltage decreases, capacity increases when p2 bought near p1

Question 32

What are the types of capacitors? Draw diagrams.

Answer 32

Parallel plate capacitor
Cylindrical capacitor
Spherical capacitor
199

Question 33

Capacitance of a parallel plate capacitor - derivation

Answer 33

Pg 199-200
E= σ/ε
E= Q/(A*ε)
E= V/d
Equate
C= A*ε / d

Question 34

Capacitance of a parallel plate capacitor - expression

Answer 34

C= A*ε/ d

Question 35

What does the capacity of a parallel plate condenser depend on?

Answer 35

Directly : area of plates , dielectric constant of medium

Inversely : distance of separation between the two plates

Question 36

Define dielectric constant (with ref. to capacitor)

Answer 36

Dielectric constant is the factor by which the capacitance increases from its value in air when the dielectric material is fully inserted between the two plates of the parallel plate condenser.

Question 37

Explain the effect of dielectrics on capacity

Answer 37

Cd= k* Cair
Surface charge on conducting plate does not change but induced charge of opposite sign appears on each surface of dielectric.
Pg 200
Cd=Co(Eo/Ed)
Capacitance of a capacitor is increased in proportion to the ratio of electric field without dielectric and with dielectric in the space between the plates of capacitor.

Question 38

Energy of a charged capacitor - derivation

Answer 38

v=q/c (at intermediate stage during process if charging)
dW= v.dq
Integrate

W= QQ / 2C = (1/2)CVV = (1/2)QV

Question 39

Energy of a charged capacitor - expression

Answer 39

W= QQ / 2C = (1/2)CVV = (1/2)QV

Question 40

When are capacitors said to be connected in series?

Answer 40

Capacitors are said to be connected in series if they are connected one after another in the form of a chain.

Question 41

Derive a relation for capacitors connected in series

Answer 41

V= V1+ ….. +Vn
Q constant

1/Cs = (1/C1) + …… + (1/Cn)

Question 42

When are capacitors said to be connected in parallel?

Answer 42

If capacitors are connected between two common points or junctions, they are said to be connected in parallel.

Question 43

Derive a relation for capacitors connected in parallel.

Answer 43

Q= Q1 + …… + Qn
V constant
Cp= C1 + …… + Cn

Question 44

Define Artificial Transmutation

Answer 44

Artificial transmutation is the process in which bombardment of highly energetic particles on nucleus of and element causes it to be transformed into some other element.

Question 45

What is a Van de Graaff generator?

Answer 45

It is a electrostatic generator that can produce high potential of the order of millions of volts and is used to accelerate charged particles such as protons, deuterons, alpha-particles which are then further used for artificial transmutation.

Question 46

Draw a diagram of Van de Graaff generator. Explain construction and working.

Answer 46

Pg 203 - 204

Question 47

Why is a Van de Graaff generator enclosed in and Earth connected steel tank filled with air under pressure?

Answer 47

The potential dome continuously increases due to accumulation of positive charges. Charges are crowded on outer surface of dome which leaks to the surroundings in the form of a spark. Two avoid such a leakage, Van de Graaff generator is enclosed in an earth connected steel tank filled with air under pressure.

Question 48

Uses of Van de Graaff generator

Answer 48

Produce High potential of the order of few million volts
Accelerate charged particles such as protons, deuterons, alpha-particles.
Used in nuclear physics for study involving collision experiments for probing nuclear structure, producing different radioactive elements / isotopes for carrying out nuclear reactions, nuclear disintegration.
In medicine such beams are used to treat cancer.