Electrostatics

Question 1

Mass of electron

Answer 1

9.1× 10^-31 kg

Question 2

Coulomb’s law

Answer 2

F=K q1q2/r²

Question 3

Value of “K” in coulomb’s law

Answer 3

1/4πε0

OR

9 × 10⁹ Nm²/C

Question 4

Electrostatic force in terms of electric field

Answer 4

F = qE

Question 5

Electric field, E=

Answer 5

E=Kq/r²

Question 6

Value of ε0 =

Answer 6

ε0 = 8.85 × 10^-12

Question 7

K depends on…

Answer 7

Medium

Question 8

Minimum value of permittivity ε

Answer 8

Is in vacuum or free space

Question 9

Relative permittivity εr =

Answer 9

εr = εm / ε0

• εm - permittivity of any medium
• ε0 - permittivity of free space

Question 10

For +ve charge electric field is

Answer 10

Radially outwards

Question 11

For -ve charge electric field is…

Answer 11

Radially inwards

Question 12

What is electric flux Φ?

Answer 12

No: of electric field lines passing perpendicular to a plane surface

• Φ = E.A
• Φ = EA cosθ
• scalar quantity

Question 13

What is Gauss law?

Answer 13

The net electric flux through a closed surface is 1/ε0 times the net charge enclosed by it

Φ = q/ε0

Question 14

Charge on proton or electron

Answer 14

1.6 × 10^-19

Question 15

Electric field due to plane sheet of charge

Answer 15

E= q/2ε₀

• E is independent of distance

Question 16

Electric field due to uniformly charged wire

Answer 16

E= 2Kλ/r

Question 17

Electric field inside solid/ hollow conducting cylinder

Answer 17

zero

There’s no charge inside a conducting cylinder…solid or hollow

Question 18

Electric field outside solid/ hollow conducting cylinder

Answer 18

E = σR/ε₀r

• r: distance from the axis
• R: radius of cylinder

Question 19

Electric field INSIDE due to solid (conducting) / hollow sphere (ie, charge at surface)

Answer 19

Zero

Question 20

Electric field OUTSIDE due to solid (conducting) / hollow sphere (ie, charge at surface)

Answer 20

E= KQ/r²

• r: distance from centre of sphere

Question 21

Torque acting on a dipole

Answer 21

τ= PE sinθ

Question 22

Work done in rotating a dipole in uniform electric field

Answer 22

τ= PE (cosθ1- cosθ2)

• θ1 initial angle of dipole
• θ2 final angle of dipole

Question 23

What is potential diff b/w two points?

Answer 23

Work done in moving a charge from one point to another

• ∆V = W/q0
• scalar quantity

Question 24

Electric potential V due to point charge

Answer 24

V = KQ/r

Question 25

Electric potential due to uniformly charged ring

Answer 25

- V axis: KQ/ √(R²+ x²) - V centre: KQ/R *R: radius of ring* *x: distance from centre*

Question 26

Potential due to dipole

Answer 26

- V axis = KP/x² - V equi = 0

Question 27

Electric field due to dipole

Answer 27

- E axis = 2KP/x³ - E equi = -KP/x³

Question 28

Electric field and potential relationship

Answer 28

E= -dV/dr *Potential decreases in the direction of electric field .... So electric field is the -ve of potential gradient*

Question 29

Capacitance of spherical capacitor

Answer 29

C= 4πε₀R - R: radius of sphere

Question 30

Energy stored in a capacitor

Answer 30

- U= Q²/2C - U= CV²/2 - U = QV/2

Question 31

Capacitance of parallel plate capacitor

Answer 31

C= ε₀A/d - A: area of plate - d: distance b/w plate

Question 32

Force on one plate of capacitor due to other plate

Answer 32

F = Q²/2Aε₀

Question 33

Capacitance formula with dielectric

Answer 33

C= ε₀AK/d

Question 34

Value of 'K' for metals/conductors?

Answer 34

K= ♾️

Question 35

Capacitor with partially filled dielectric

Answer 35

C=ε₀A/ (d-t + (t/k)) -t: thickness of dielectric slab -d: distance b/w plates of capacitor

Question 36

Third charge in eqm for like charges

Answer 36

*Eqm point is somewhere in b/w charges* X = [√q1 / (√q1 + √q2) ] L - X: distance from smaller charge - L: distance b/w charges

Question 37

Third charge in eqm for unlike charges

Answer 37

*Eqn point is somewhere outside near the smaller charge* X = [√q1 / (√q1 - √q2) ] L - X: distance from smaller charge - L: distance b/w charges - q1: smaller charge