Why the test charge is taken to be very small in the definition of electric field intensity?

The SI unit of electric field intensity is

In vacuum, three small spheres are placed on the circumference of a circle of radius r in such a way that an equilateral triangle is formed. If q be the charge on each sphere, determine the intensity of electric field and potential at the centre of the circle.

A parallel plate capacitor is charged to a potential V and is then disconnected from the source. When the distance between the two plates is d, the electric field intensity is E . If the distance between the plates is doubled, what would be the change is this electric field intensity?

The electric force on an electron is equal to its weight in an electric field of intensity E. The charge of electron is e and mass is. m Then the value of E be—

Two charges, + q and -q are separated by a distance d. At which points will the resultant electric field intensity be directed parallel to the line joining the charges ?

If a charge is displaced against the Coulomb force in an electric field,

Two point charges of 10 esu and 40 esu are located at points A and B seperated by a distance 4 cm. Find the electric field intensity at a point halfway between the charges.

The electric field due to a small dipole of length 2l at distance r ( rgtgtl) from the centre of the dipole on the axial line is E. What is the distance of the point on the perpendicular bisector of the dipole from its centre in which the electric field intensity is E ?

Under what condition a charged circular loop behaves as a point-charge in respect of electric field due to it?