In: Physics
w much positive and how much negative charge is there in one cup of water (neutral H2O )? Imagine all this positive charge to be concentrated in a point-like particle, and all negative charge in another point-like particle located 250,000 miles away. Note that this is roughly the distance between the Earth and the Moon. By what electrostatic force would these point-like particles attract each other (in vacuum)? What amount of mass would have this weight on Earth?
Since, the charges are of opposite sign they attract each other. And so there is negative sign in force.