Electric Charges and Fields

Flashcards for review

Coulomb's Force

Force between two point charges kept in a medium of dielectric constant K.

Electric field strength E

Force experienced by a test charge q₀ at a point.

Electric field strength due to infinite line charge

Electric field strength due to infinite line charge having linear charge density (λ) coulomb/metre.

Electric field strength near an infinite thin sheet of charge

Electric field strength near an infinite thin sheet of charge.

Electric Dipole moment

Product of either charge and distance between them.

Total electric flux

Net charge enclosed by the closed surface.

A body can be negatively charged by

Giving excess of electrons to it

The unit of permittivity of free space (ε₀) is

C² N⁻¹m⁻²

Which of the following is not a property of field lines?

They form closed loops

Gauss's law is valid for

Any closed surface

Spatial distribution of the electric field due to two charges (A, B)

A is +ve and B is – ve and |A| > |B|

When air is replaced by a medium of dielectric constant K, the force of attraction between two charges separated by a distance r

decreases K times

A point positive charge is brought near an isolated conducting sphere. The electric field is best given by

Fig (i)

The Electric flux through the surface

is the same for all the figures.

A hemisphere is uniformly charged positively. The electric field at a point on a diameter away from the centre is directed

perpendicular to the diameter

Figure shows electric field lines in which an electric dipole p is placed as shown. Which of the following statements is correct?

the dipole will experience a force towards left.

A point charge +q, is placed at a distance d from an isolated conducting plane. The field at a point P on the other side of the plane is

directed perpendicular to the plane and away from the plane.

There are two kinds of charges-positive charge and negative charge. The property which differentiates the two kinds of charges is called

polarity of charge

A method for charging a conductor without bringing a charged object in contact with it is called

electrostatic induction

If ∫E.ds=0 over a surface, then

both (b) and (c)

A cup contains 250 g of water. The number of negative charges present in the cup of water is

1.34 x 10⁷ C

When the distance between two charged particles is halved, the Coulomb force between them becomes

four times.

Two charges are at distance d apart in air. Coulomb force between them is F. If a dielectric material of dielectric constant K is placed between them, the Coulomb force now becomes

F/K

Two point charges q₁ and q₂ are at separation r. The force acting between them is given by F= K (q1q2/r²). The constant K depends upon

both on (a) and (b)

Three charges +4q, Q and q are placed in a straight line of length l at points at distance 0, l/2, and l respectively. What should be Q in order to make the net force on q to be zero?

-q

An electron falls from the rest through a vertical distance h in a uniform and vertically upward directed electric field E. The direction of electric field is now reversed, keeping its magnitude the same. A proton is allowed to fall from rest in it through the same vertical distance h. The time of fall of the electron, in comparison to the time of fall of the proton is

smaller

Which of the following is the unit of electric charge?

All the above

A body is positively charged. It has

deficiency of electrons

A proton at rest has a charge e. When it moves with a high speed, its charge

= e

Two charges 3 x 10⁻⁵ C and 5 x 10⁻¹ C are placed at a distance of 10 cm from each other. Find the value of electrostatic force acting between them.

13.5 x 10¹¹ N

What is the value of minimum force (in N )acting between two charges placed at 1 m apart from each other?

ke²

A glass rod acquires charge by rubbing it with silk cloth. The charge on glass rod is due to

friction

Find the thickness of a dielectric material which has relative permittivity εᵣ, when two charges experience the same force as in air by a distance r.

(√r/√εr)

What will be the value of electric field at the centre of the electric dipole?

Twice the electric field due to one charge at centre

Which physical quantity have unit newton/coulomb?

Electric field

In the process of charging, the mass of the negatively charged body

increases

Charge on a body is integral multiple of ±e. It is given by the law of

quantisation of charge

Electric field intensity due to a short dipole remains directly proportional to (r is the distance of a point from centre of dipole)

r⁻³

Electric field lines contracts lengthwise, It shows

attraction between opposite charges

Force F between charges Q₁ and Q₂ separated by r is 25 N. It can be reduced to 5 N if the separation between them is made

√5r

Which of the following is the unit of electric field intensity?

NC⁻¹

The unit of electric dipole moment is

C-m

A slab of dielectric is introduced between two equal positive charges with a fixed separation. As a result

the force between the two charges decreases

Two like point charges separated by a certain distance exert a force of 0.04 N on each other. When the distance of separation between them is halved, the force exerted by each on the other will be

0.16 N

When a glass rod is rubbed with a dry silk cloth, the glass rod is positively charged due to the transfer of

electrons from glass rod to silk cloth

The unit of electric permittivity ε of a medium is

C²/Nm²

The dimensions of electric permittivity is

M⁻¹L⁻³T⁴A²

An insulated conical shaped metallic conductor is charged positively. The surface charge density on it is

maximum at the apex

The magnitude of force experienced by an electron placed at a point in the electric field E is equal to its weight mg. The magnitude of E is

mg/e

An electric dipole is placed at an angle of 30° with an electric field intensity 2 × 10⁵ NC⁻¹. It experiences a torque equal to 4 Nm. The charge on the dipole, if the dipole length is 2 cm, is

2 mC

What is the SI unit of electric flux?

N×m²

The dimentional formula of electric flux is

[M¹L³T⁻³A⁻¹]

Which of the following statements is not true about Gauss's law?

Gauss's law is not much useful in calculating electrostatic field when the system has some symmetry.

The surface considered for Gauss's law is called

Gaussian surface

Charge on a conducting metal sphere is present

on the surface of sphere

Charge Q is kept in a sphere of 5 cm first, then it is kept in a cube of side 5 cm, the outgoing flux will be

same in both case

A sphere encloses an electric dipole within it. The total flux across the sphere is

zero

A charge q is placed at the centre of a cube, what is the electric flux passing through one of its faces?

q/6ε₀

According to Gauss law, electric field of an infinitely long straight wire is proportional to

1/√r

A charge q μC is placed at the centre of a cube of side 0.1 m. Then the electric flux diverging from each face of this cube is

(q*10⁻⁶)/6ε₀

An electric charge q is placed at one of the corners of a cube of side a. The electric flux on one of its faces will be

q/24ε₀

Consider a region inside in which there are various types of charges but the total charge is zero. At points outside the region

the work done to move a charged particle along a closed path, away from the region, will be zero.

If electric field is uniform, then the electric lines of forces are

parallel

A plane of surface area S is placed in an electric field such that the direction of normal on surface 'S' makes an angle 'θ' with the direction of electric field E. The electric flux through the surface is

ES cos θ

In which of the following cases the electric field strength is independent of distance?

Due to infinite flat sheet of charge

A cylinder of radius R and length L is placed in a uniform electric field E parallel to cylinder axis. The total flux through the surface of the cylinder is given by

zero

A hemispherical surface of radius R is placed with its cross-section perpendicular to a uniform electric field as shown in figure. The electric flux through the surface is

πR²E

A small metal ball is suspended in a uniform electric field with the help of an insulated thread. If high energy X-ray beam falls on the ball, it will

be deflected in the direction of field

Two point charges +q and -q are held fixed at (-d, 0) and (+d, 0) respectively of a (x, y) coordinate system. Then

the dipole moment is q(2d) along negative X-axis

Six charges, three positive and three negative are to be placed at the vertices of a regular hexagon such that the electric field at O is double the electric field when only one positive charge of same magnitude is placed at R. Which of the following arrangements of charges is possible for P, Q, R, S, T and U respectively?

+, -, +, +, -, -

A charged ball B hangs from a silk thread S, which makes an angle θ with a large conducting sheet P, as shown in fig., the surface charge density σ of the sheet is proportional to

tan θ

Force between two identical charges placed at a distance r in vacuum is F. Now a slab of dielectric of dielectric constant 4 is inserted between these two charges. If the thickness of the slab is r/2 then the force between the charges will become

½F

Electric flux is

scalar quantity

The minimum value of charge on any charged body may be

1.6 x 10⁻¹⁹ coulomb

The number of electrons contained in 1 coulomb of charge is equal to

6.25 x 10¹⁸

A charge Q is placed at each of the opposite corners of a square. A charge q is placed at each of the other two corners. If the net electrical force on Q is zero, then Q/q equals

-2√2

Three concentric metallic spherical shells of radii R, 2R and 3R are given charges Q1, Q2 Q3 respectively. It is found that the surface charge densities on the outer surfaces of the shells are equal. Then the ratio of the charges given to the shells Q1 : Q2 : Q3 is

1:3:5

A thin semicircular ring of radius r has a positive charge 'q' uniformly distributed over it. The net electric field E at centre O is

(-9/2πε₀r) ĵ

For a point charge, the graph between electric field versus distance shows

E ∝ 1/r²

An infinite number of identically charged bodies are kept along the x-axis at points x = 0, 1 m, 2 m, 4 m, 8 m, 16 m and so on. All other charges repel the charge at the origin with a force of 1.2 N. Find magnitude of each charge.

10 μC

If the net electric flux through a closed surface is zero, then we can infer

no net charge is enclosed by the surface.

An electric dipole placed in a non-uniform electric field can experience

always a force and a torque.

A point charge is situated at an axial point of a small electric dipole at a large distance from it. The charge experiences a force F. If the distance of the charge is doubled, the force acting on the charge will become

F/8

The electric flux emerging out from 1C charge is

1/ε₀

An electric dipole consisting of charges +q and -q separated by a distance r, is kept symmetrically at the centre of an imaginary sphere of radius R (> r), Another point charge Q is also kept is also kept at the centre of the sphere. The net electric flux coming out of the sphere will be

Q/ε₀

Two large conducting spheres carrying charges Q₁ and Q₂ are kept with their centres r distance apart. The magnitude of electrostatic between them is not exactly (1/4πε₀) (Q1Q2/r²) because

charge distribution on the spheres is not uniform.

The electric flux through a closed Gaussian surface depends upon

net charge enclosed and permittivity of the medium

Coulomb's Force Law

(1/4πε₀)(q1q2/r²)

Electric field

force a unit positive test charge would experience if placed at that point

Coulomb field at a point

continuous if there is no charge at that point

a point charge at a distance from isolated conducting plane

directed perpendicular to the plane and away from the plane

Coulomb field at origin

4.8x10⁴ N/C

Type of physical quantity electric flux

scalar, [ML³T⁻³A⁻¹]

electric flux thru one face if total electric flux through gaussian surface is q/ε₀

q/6ε₀

Electric flux of flat square of 10 m² in uniform electric field of 8000 N/C

8x10⁴ Nm²/C

Gauss's is valid for

any closed surface

Electric charge inside the surface will if electric flux entering and leaving are Φ₁ and Φ₂

ε₀ (Φ₂ - Φ₁)

Charge given to a metallic sphere does not depend on whether it is hollow or solid. Since the charge resides only on the surface of the conductor.

Both A and R are true and R is the correct explanation of A.

Charge is quantized because only integral number of electrons can be transferred. There is no possibility of transfer of some fraction of electron.

Both A and R are true but R is not the correct explanation of A.

Period of oscillation of a simple pendulum remains same if the bob is kept in a horizontal electric field.

A is true but R is false.