Electric Field

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Introduction :- 

If I gave a charge ( either positive or negative) to a particular particle and kept it in a room and what we observed that the charge is moving in a particular direction without being in any contact with anyone or without applying any kind of force. Despite of not applying any kind of force or a contact (by human or by other) to that charge it is moving in a particular direction.

But If I took that same charge to a different place or in a different room what we observed? that charge is not moving at all like it was moving in other room instead it is static (not moving). 

In some place the charge is moving very rapidly and at some it is very slower; At some places it is moving from left to right at some it  is moving from right to left. But we can't see anything. So what is that force which is driving or moving a charge or applying a force to that charge? What is that unseen thing which is applying force on the charge ? What is that thing in presence of which the charge is moving fastly at some places and slowly at the other; left to right at some places but right to left at other and remains static at some? What is that thing?

To take a practical example :- 

Consider a person standing at his gallery which is on the third floor of his apartment having a ball in his hand and he had hold his hand outside the gallery and immediately took his hand off the ball. What will happen? 

Obviously ball will fall down towards earth due to the gravitational force but when the person was about to take his hands off the ball was the earth in contact with the ball ? No the ball was in air so it can be concluded that to apply a force didn't need any physical contact. But can humans apply force without any physical contact? No 

So this concludes that there are two types of forces present: 

1) Force generated by making physical contact 

2) Force generated without making any physical contact. 

So earth applies force on ball without making any physical contact with it. But if we change the place i.e. If we try this experiment in space then will earth be able to pull that ball towards it ? No. But as soon as we take ball near earth it will pull the ball towards it.

To study the that force applicable without making any physical contact we made an concept out of it called as field.

To study this kind of forces which can be applied towards an object without any physical contact with each other we made an concept out of it which is called as field.

If the cause for a force is Gravity then we call it as Gravitational Field.

If the cause of force is Electricity then we will call it as Electric Field.

If charge experiences an electric force then we can say that there is presence of electric field. 


ELECTRIC FIELD

The region around a charged body where another charged body experiences a mechanical force is called an electric (or electrostatic) field.


ELECTRIC LINES OF FORCE

As with the magnetic field, stress distribution in the electric field surrounding the charged body is represented by imaginary lines called electric (or electrostatic) lines of force.

Also, similar to a magnetic line of force, an electric line of force can be considered as a line along which a small positive charge (which is free to move) would travel when placed in an electric field.

A tangent drawn at a given point on the line indicates the direction of the resultant force which this free positive charge would experience at the point of tangency.

A bunch containing a fixed number of electric lines of force is sometimes called a tube of force. Thus, the electric field can also be represented by number of such tubes of force.

The mapping of the electric field into a series of electric lines of force can be done by sprinkling gypsum crystals on a glass plate placed over the highly charged body. Figure 1 shows a few typical electric field patterns.


Electric Fields

               (a)                                    (b)

      (a) Isolated positive charge                                           (b) Isolated negative charge

Why is electric field lines away from (+) and toward ...

                (c)                                 (d)

   (c) Two equal unlike charges                                         (d) Two equal like charges

Figure 1 : Electric field patterns in a few typical cases


Properties of Electric Lines of Force :- 

 The properties of electric lines of force are almost identical with those of magnetic lines of force. Some of the important properties of electric lines of force are as follows:

  1.  The lines of force originate on a positive charge and terminate on negative charge (Figure 1)
  2. They always leave or enter a conducting surface at right angles to it.
  3. They never touch or cross one another.
  4. Parallel lines of force acting in the same direction repel one another, while lines acting in the           opposite directions attract one another.
  5. They always try to contract in length and thus behave like stretched elastic bands.
  6.  These lines adjust themselves so that the dielectric reluctance of the field is a minimum.
  7.  The lines of force become highly concentrated at the places where radius of curvature of the charged body becomes small.
  8.  They pass only through the insulating (dielectric) medium between the charges but do not enter the charged bodies. Therefore, they are not in the form of closed loops (magnetic lines of force on the other hand, pass through magnetic material as well as through the field and thus form closed loops).
  9. The direction of field lines at any point in space indicates the direction of electric field at that point.
  10. The number of electric lines of force per unit cross-sectional area at a point is directly proportional to the magnitude of electric field at that point.
  11. While representing electric field by field lines, the number of lines are drawn in proportional to magnitude of electric charge so that the density of lines truly represent magnitude of field at given point. 

Advantages of using Electric Field over Coulomb's Law to determine Electric Force between Charges

  1. Electric field is associated with the position. Hence the value of electric field at a given point in space due to some given charge or group of charges is fixed. Electric field depends only upon charges producing it and the distance of the point where it is measured. It is easy to determine electric force experienced by an electric charges in that region as electric field of given region in space is known.
  2. When charges are moving, the electric field and Coulomb's law do not describes the electric forces in similar manner. The Coulomb's law tells that the effect of motion of a charge is felt immediately by other charges. But this is not supported by  experimental observations. The effect is actually felt after some time. This limitation of Coulomb's law is taken into account by electric field.

ELECTRIC FLUX

The total number of lines of force (or tubes of force) in any particular electric field is called the electric flux.

It is represented by the symbol Ѱ.

Similar to charge, unit of electric flux is also coulomb (C).

It is defined as that flux which emanates from a positive charge of one coulomb.

Thus, for a charge of Q coulomb,

                                                

  Electric flux ψ = Q coulombs.


ELECTRIC FLUX DENSITY

The flux per unit area (a) measured at a right angle to the direction of the electric flux is known as electric flux density.

It is represented by the symbol D and its unit is coulomb per square metre (C/m^2).    

         Thus,   D = ψ/ a      coulombs/m^2


ELECTRIC FIELD STRENGTH

The mechanical force experienced by a unit positive charge (i.e. a charge of one coulomb) placed at any point in the electric field is known as electric field strength at that point.

Higher the value of this force, stronger is the field.

The direction of the electric field strength at any point is given by the direction of the mechanical force on a positive charge situated at that point.

Or, it is also given by the direction of the line of force passing through the point under consideration.

The symbol for electric field strength is E and its unit is newton per coulomb (N/C) or volt per meter (V/m).




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