Electromagnetic Force

Choose the correct alternative : (a) If the gravitational potential energy of two mass points infinite distance away is taken to be zero, the gravitational potential energy of a galaxy is (positive /negative/zero). (b) The universe on the large is shaped by (gravitational/electromagentic) forces, on the atomic scale by (gravitational/electromagnetic) forces, on the nuclear scale by (gravitational/electromagnetic/strongnuclear) forces.

A: Gravitational force is always attractive but electromagnetic force can be attractive or repulsive: R : Mass comes only in one variety (there is no negative mass) but charge comes in two varieties. (Positive and negative charge)

If a chraged particle at rest experiences no electromagnetic force, (i) the electric field must be zero (ii) the magnetic field must be zero (iii) the electric field may or may not be zero (iv) the magnetic field may or may not be zero

If a charge particle kept at rest experiences an electromagnetic force, (i) the electric field must not be zero (ii) the magnetic field must not be zero (iii) the electric field may or may not be zero (iv) the magnetic field may or may not be zero