## ELECTRIC FIELD

## ELECTRIC FIELD

Two charges kept at a finite distance apart exert forces on each other. Let's concentrate on only one charge presently.

A charge Q placed in the vicinity of another charge Q2 will experience some force. Suppose if I remove the charge Q from its place. Question. What will remain at the previous position of Q? There must be something there that was exerting a force on Q. That place can't just be 'empty'. We call this something as the electric field.

Mathematically the electric field intensity (E) at a point is written as

E = F/Q 1)

(Important Note: Letters written in bold face are vectors. This is true for all articles of this website. Click here to know more.)

where F is the force experienced by the charge Q when placed at the point at which E is required. If you draw a vector of E

Note electric field and electric field intensity at a point are two completely different concepts. Electric field is the region in which any charged object experiences a force. Electric field intensity at a point is numerically equal to the force experienced by a unit charge placed at that point.

Strictly speaking the E defined by equation 1 above is the electric field intensity but some authors prefer to call it as simply the electric field. So you should be careful about its usage.

Electric field intensity is a vector quantity. If Q is positive then its direction is in the same direction as F. If Q is negative then it is opposite to that of F.

Its unit is Newton/ Coulomb.

From coulomb's law we know the force between two point charges. Therefore the magnitude of electric field intensity due to a point charge can be written as follows.

ELECTRIC FIELD VECTOR

If I draw a vector whose length is proportional to the magnitude of E and its direction is in the direction of E then such a vector is known as an electric field vector.

Every point of the electric field can be associated with an electric field vector. Hence there will be infinitely many electric field vectors in any electric field.

ELECTRIC FIELD LINES

Latitudes and longitudes do not really exist but you know how useful they are. A chart in any junior level science class contains the picture of our solar system showing planets revolving around the sun in definite orbits. If you see a real picture of our solar system then you'll find that no such orbits exist in reality. They are drawn only to help you visualize what's happening. The case of electric field lines is not too different from these concepts.

Electric field lines were first thought of by Michael Faraday. He had misconceived the space around any charged object to be filled by 'lines of force' - the old name of electric field lines.

Electric field lines do not really exist. They are drawn only to help you understand the concepts of electric charges and their interactions in a better way.

These lines are drawn with the following considerations:

- Electric field lines begin from positive charge and terminate on negative charge. Since they end on negative charge therefore unlike magnetic field lines, electric field lines do not make closed curves.

- The tangent to any point on the field line gives the direction of electric field at that point. But the tangent can show two directions. So you must select the direction that is away from the positive charge and/or is towards the negative charge.

tangents that will imply two directions. Obviously electric field at one point can be only in one direction.

- Draw an area of unit magnitude perpendicular to the field lines. The number of lines intersected by that area will be proportional to the magnitude of E at that point.

Below you can see electric field lines of some common charge configurations.

The variation of field lines of an electric dipole as the two charges move towards and away from each other.

CONSERVATIVE AND NON CONSERVATIVE FIELDS

Electric field is not the only field that exists in nature. There are others too like magnetic field and gravitational field.

Broadly speaking fields are divided into two categories - conservative and non conservative. What's the meaning of these two words?

DEFINITION OF CONSERVATIVE AND NON CONSERVATIVE FIELD

Take any two points in the field. Take a test particle and move it from one point to another along any arbitrary path. Repeat the process again and again each time changing the path of the particle. If in all cases the work done by the field gives you the same value then the field is said to be conservative. Otherwise its called a non - conservative field.

It follows directly from the above definition that the work done in moving a particle in a closed curve in a conservative field is always equal to zero. (Try to figure it out yourself before reading further.)

Look at the figure below.

Take a charged particle. Move it from A to B in a straight line. Let's say that the work done by the field is W. Bring it back along the same path. Obviously it should be - W. Therefore the total work done is

W + (-W) = 0.

Since the field is conservative therefore we could have brought the charge from any arbitrary path and could have taken it back along any arbitrary path and the work done should be equal in all cases. Hence the proof is general.

The force exerted by the conservative field is known as the conservative force and similarly the force exerted by a non - conservative field is known as a non conservative force.

It is found that the electric field is a conservative field.(I haven't proved it here.)

The path independence of work means that one can assign a specific number to each point of the concerned region and the difference of these two numbers can be used to find the work done in moving the particle from one point to another along any path.

This property of conservative fields resulted in inventing a new scalar function which was named as the field potential or simply the potential.

Next article is about Electric Potential and Potential Energy.

You may buy it by clicking on the link to the left.

Michael Faraday - Courtesy of John Cochran (Dibner Library) [Public domain], via Wikimedia Commons

Field lines for positive and negative charges - Courtesy of Nein Arimasen (Own work) [GFDL (www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (www.creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons

Field lines of a dipole shown in red - Courtesy of Sharayanan (Own work) [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0-2.5-2.0-1.0 (www.creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Animated field lines - Courtesy of Geek3 [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons