Electric charges and fields

Question 1

What is static electricity?

Answer 1

It is the build up of charges on an isultator, that produced sparks when discharged

Question 2

Electrostatics:

Answer 2

Is the study of forces, feilds and potentials of charges at rest.

Question 3

Electricity comes from the Greek word:

Answer 3

Elektron, which means ‘amber’

Question 4

What device is used to detect charge on a body?

Answer 4

A gold-leaf electroscope

Question 5

How does one electrify a neutral body?

Answer 5

By transferring electrons on/off the body.

Question 6

Initially, all objects in the universe are:

Answer 6

Electrically neutal

Question 7

What are the two methods of charging?

Answer 7

Induction and Conduction

Conduction = with contact/like charges
Induction = without contact/unlike charges

Question 8

What are the two types of materials that exist around us?

Answer 8

Conductors and Insulators

Question 9

What are the two types of charges/polarity?

Answer 9

+ve and -ve

(Like = repel)
(Unlike = attract)

Question 10

Quantisation of charge can be represented as:

Answer 10

Q = ne

Where e = ± 1.6 * 10 ^ -19 c = charge of 1 electron or proton

Question 11

How many electrons make up ± 1 C ?

Answer 11

6 * 10^18 proton/electrons

Question 12

Coulomb’s law:

Answer 12

F = k q1 q2 / r ^ -2

k = 9 * 10^9
ε0 = 8.8 * 10 ^ - 12

Question 13

What is ε0?

Answer 13

ε0 is the permittivity of free space.
It represents the efficiency of a medium in the transmission of field lines.

Question 14

Relative permittivity/ Dielectric constant =

Answer 14

ε /ε0

For a conductor, K = infinity

Question 15

What device did Coulomb use to experimentally arrive at his law?

Answer 15

A torsion balance

(Cavendish used this device to later prove Newton’s law of gravitation)

Question 16

What is the relation b/w Gravitational and Electrostatic force?

Answer 16

Electrostatic force > Gravitation force

(However, gravitational forces are the leading forces in the universe)

Question 17

What does the slope of the F and 1/r 2 give?

Answer 17

Slope = y/x = kq1q2 = tan θ

Question 18

What is the electrical field of a charge ?

Answer 18

The region around a charge within which its influence can be left is called its field.

Question 19

Electric field =

Answer 19

Force / test charge (q0)

Question 20

Field lines were first introduced by:

Answer 20

Michael Faradary. He called them ‘lines of focre’.

Question 21

Charge to mass ratio (emf) ∝ ??

Answer 21

the displacement/degree of deflection

Question 22

What are field lines?

Answer 22

They are imaginary lines that represent the motion of a unit + ve charge when placed in the field of another charged, when allowed to move freely.

They go from the +ve charge to the -ve charge.

Question 23

Field lines are:

Answer 23

Continuous curves and NOT closed loops. The tangent at any point represents the direction of the electric field. They DO NOT cross each other at any instant at time.

Question 24

The magnitude of the field is represented by:

Answer 24

The density of the field lines. More crowed the field lines at a point, stronger the electrical field.

Question 25

An uniform electric field has:

Answer 25

a constant strenght and magnitude throughout

Question 26

Electric dipole:

Answer 26

A pair of equal and opposite charges seperated by a very short distance.

The total charge in a dipole = 0

Its direction is conventionall from -ve to +ve

Question 27

Dipole moment =

Answer 27

2l * q

2l = total distance between the charges

Question 28

Rel. b/w E(axial) and e (equitorial) =

Answer 28

e (equitorial) = e (axial) /2
OR
e (axial) = 2 * e (equitorial)

Question 29

Torque in an electric field =

Answer 29

P X E = PESinθ

Question 30

Work done to rotate a dipole =

Answer 30

• P. E = - PE Cos θ

Question 31

Surface charge density (sigma) =

Answer 31

q / area

Question 32

Linear charge density (lambda) =

Answer 32

q/l

Question 33

Volume charge density/charge density =

Answer 33

q/v

Question 34

Electric flux =

Answer 34

E. ds or EdsCos θ

Question 35

Electric flux as per Gauss’ law =

Answer 35

q net / ε0

Question 36

E for an infinitely charged rod =

Answer 36

λ/2 pi r ε0

Question 37

E for an infinite charged thin plane sheet=

Answer 37

σ / 2 ε0

Question 38

E for a thick charged sheet =

Answer 38

σ/ε0

Question 39

E for a uniformly charged spherical shell =

Answer 39

σ R^2 / r^2 ε0

E inside the shell = 0
E on the surface = σ /ε0