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21.

Reason : The force on unit positive charge at the centre, due to the three equal charges are represented by the three sides of a triangle taken in the same order. Therefore, electric field intensity at centre is zero..

20.

If Assertion is true, but the Reason is false. Electron and proton have same amount of charge so they have same Coulomb force. They have different accelerations because they have different masses (a=Fm).

19.

if Assertion is true, but the Reason is false. In a hollow spherical shield (hollow), the charge is present on its surface but charge is zero at every point inside the hollow sphere. Hence , the metallic shield in from of hollow shell may be built to block an electric field.

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