36. Two point charges are separated by 6 cm. The attractive force between them is 20 N. Find the force between them when they are separated by 12 cm. (Why can you solve this problem without knowing the magnitudes of the charges

The force between two charges when they are separated by distance 12 cm is    

Explanation:

Here lets us consider two charges and .

Let  be the force between them when they are separated by distance 6 cm

      be the force between them when distance between them = 12 cm

             =20 N     ..................1

                            ...................2

 Dividing both equations , we get

    .

Here we need not to know the values of charges , since they get cancelled while dividing.

answer: 2×10^-2kg

explanation: density= mass per volume

vol=mass/density

converting to s.i units we have;

3÷1000kg = 0.003

1.5÷100m = 0.15

0.003/0.15 = 0.02kg