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Flashcards in Electrostatics Deck (25):

What is the force between two point charges given as?

What is it commonly called?

Coulombs law


Coulombs law in vector form


What is the superposition principle? 

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. 


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

F = Eq


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


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

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


Finding the electric field due to a charged conducting wire ? 

Derivative expression

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


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

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


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

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


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

The material can be polarised and dipoles can be induced


What is a dipole defined as ? 

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 


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

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 


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

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



What is the Electric flux density of a single charge? 


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

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)


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

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


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

First equation is general expression for electric flux density. 

Second, is for isotropic materials. 


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

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. 


Definition: What is Gauss's law? 

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


What is the equation that represents Gauss's law? 

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.  


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


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

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


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


What is electric flux? 

The amount of electric field lines that penetrate a surface. 


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

Represents an integral over a closed surface.