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Electric field

Electric field due to a charge is the space around the charge, in which any

other charge is acted upon by an electrostatic force.

Electric field intensity

The electric field intensity at a point due to a source charge may be

defined as the force experienced per unit positive test charge placed at that point

without disturbing the source charge. The electric field is also known asstrength of electric field or simply electric field.

Consider that a positive test charge q0 experiences a force , when placedat the observation point (the point, where the electric field is to be

determined).Then, electric field at the observation point is given by

........ (1)

The idea of taking is that on placing the test charge at the observationpoint, the source charge will not be disturbed.

The electric field at a point is a vector quantity.

SI unit of electric field intensity is newton coulomb-1 (NC-1).

Another unit of electric field is volt metre-1 (Vm-1).

Force on a charge inside an electric field

If is the electric field at a point, then b definition, a charge q placed at

that point will experience a force given by

Electric field due to a point charge

Consider that a point charge +q is placed at the origin O of the co-

ordinate system. Let p be the point, where electric field due to the point charge

+q is to be determined. Let be the position vector of the point P.

To find the electric field at point P, place a vanishingly small positive test

charge q0at point P. According to Coulombs law, force on the test charge q0

due to charge q is given by

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Where is unit vector along OP. If is the electric field at point P, then

Or .......... (1)

The magnitude of electric field at point P is given by

.......... (2)

From the above expression for electric field, it follows that electric field isspherically symmetric as seen from the charge q and its magnitude decreases

inversely as the square of the distance from the charge.

Electric Lines of Force

Electric field in any space is represented by the electric field lines or lines

force. In general an electric field line is a curve drawn in such a way that thetangent to it at each point is in the direction of the net field at that point. The

strength of electric field at any point is measured as the number of electric lines

of force crossing a unit area held normal to the lines of force at that point.

Properties of Electric lines of force

1. Field lines start from positive charges and end at negative charges. Ifthere is single charge, they may start or end at infinity.

2. In a charge-free region, electric field lines can be taken to be continuouscurves without any breaks.

3. Two field lines can never cross each other. In case, two lines of forceintersect at a point, then at that point, electric field will not have a uniquedirection. As it is not possible, the lines of force can never intersect each

other.

4. The relative closeness of lines of force in different regions of space givesthe idea about the relative strengths of the electric field in the different

regions. The region, where lines of force are closer, the electric field isstrong and the region, where the lines of force are farther apart, the field

is weak.5. Electrostatic field lines do not form any closed loops. This follows from

the conservative nature of the electric field.

6. The lines of force never pass through a conductor. This indicates that theelectric field inside a conductor is always zero.

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Electric field lines of charge configurations.

I. Electric field lines due to isolated positive and negative charge.

II. Electric field lines due to two positive charges

III. Electric field lines due to two unlike charges of equal magnitude

Electric Dipole

An electric dipole is a pair of equal and opposite

point charges q andq separated by a distance of 2a.

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If charges have magnitude q each and the distance from the centre of the

negative charge to that of the positive charge is 2a, the product isdefined as the electric dipole moment. The electric dipole moment is a vector

quantity, is directed along the line joining the two charges from negative charge

towards positive charge. It is given by

The SI unit of electric dipole moment is coulomb-meter (Cm). The dimensional

formula is [LTA].

The centre of the dipole lies at the midpoint of the line joining the two

charges q and +q. The total charge of an electric dipole is zero. Water and

hydrogen chloride molecule have electric dipole moments.