Electrostatics

Question 1

What is the force between two point charges given as?

What is it commonly called?

Answer 1

Coulombs law

Question 2

Coulombs law in vector form

Answer 2

Question 3

What is the superposition principle?

Answer 3

The superposition principle states that the net force acting on q₁ is the resultant of the two forces due to q₂ and q₃ separately. These forces can be resolved into components and the components added.

Question 4

Eq for the force on a charge in a electric field?

Answer 4

F = Eq

Question 5

Eq for the electric field due to an electric charge Q?

Answer 5

Question 6

How do you find the electric field due to a number of electric charges?

Answer 6

It is the sum of all the unit vectors formed by all the charges. Times Q_n and hen multiplied by k

Question 7

Finding the electric field due to a charged conducting wire ?

Derivative expression

Answer 7

Remember, vector CP over PC² is the unit vector. Equation is based on eq to calc field due to a number of electric charges.

Question 8

How do you calc the parallel and perpendicular components of the Electric field due to a charged conducting wire?

Answer 8

You multiply dE by sinθ or cosθ, the respective components.

Question 9

Finally, with all the equations, how do you calc the Electric field due to a charged conducting wire:

Answer 9

Since the electric field is directed along the direction CP, we only want the perpendicular component.

Question 10

What happens when an insulator is placed in an electric field?

Answer 10

The material can be polarised and dipoles can be induced.

Question 11

What is a dipole defined as ?

Answer 11

A dipole is defined as a pair of oppositely charged charges, q, separated by a small distance d. The dipole moment is defined as:

p = qd

Question 12

How do you measure the intensity of polarisation of the total dipole moment p?

Answer 12

The dipole as a whole is electrically neutral since it is made up of charges of opposite sign, but equal strength. As a measure of intensity of polarisation the total dipole moment p per unit volume v is used

P = p/v

Question 13

Electric Flux density, why do we use it? and what is it?

Answer 13

In order to remove the dependency on the medium or material a quantity known as the electric flux density is introduced.

Question 14

What is the Electric flux density of a single charge?

Answer 14

Question 15

This is the equation for the electric field due to a number of charges. How does the equation work?

Answer 15

It takes the sum of all the electric field formed between all the charges and multiplies it by k.

k being equal to 1/(4piε)

Question 16

What is the electric field due to a charged conducting wire?

Answer 16

a_r is a unit vector at right angles (radial) to the wire

Question 17

What is the equation for electric flux density, and in the case of a isotropic material?

Answer 17

First equation is general expression for electric flux density.

Second, is for isotropic materials.

Question 18

Why is the total electric flux radiating out of spherical charge equal to Q?

Answer 18

Since the surface is spherical both D and dS point in the same direction, as such D and dS are parallel.

Because they are parallel, the dot product becomes multiplication between the two.

As a result, D can be factored out of the integral, and the integral of ds becomes S, which represents the area of the sphere.

The expression obtained is D*4piR².

This can be simplified to Q from the electric flux density.

Question 19

Definition: What is Gauss’s law?

Answer 19

The total electric flux, emerging from any closed surface S is equal to the charge within S.

Question 20

What is the equation that represents Gauss’s law?

Answer 20

ρ is the conventional symbol for charge density, or in other words charge per unit volume.

The first integral is a surface integral over the closed surface S.

The second integral is a integral over the volume v.

Question 21

What equation represents the electric potential (or potential difference between two points)?

Answer 21

Question 22

With a positive charge, when is work done in moving a charge in a electric field?

Answer 22

Work is done in moving a positive charge against the direction of the electric field.

Question 23

What is the potential of point charge at a distance r?

Answer 23

Question 24

What is electric flux?

Answer 24

The amount of electric field lines that penetrate a surface.

Question 25

For Gauss’s law, what does the circle represent?

Answer 25

Represents an integral over a closed surface.