cy bringing a positively charged rod near it .in this case the electrons will flow from the ground to the sphere when the sphere is connected to the ground with a wre .can you explain why?

The earth is actually a very enormous pool of many positive and negative charges. When we ground the sphere (earthing) , it depends on what charge the sphere has.

 

If it is positively charged, then electrons (negative charges) from earth will be flowing upwards to the  sphere since unlike charges attract.

 

 However, if it is negatively charged on the sphere, earthing it will cause electrons from the sphere to flow downwards to the earth since electrons are attracted to the earth which contains many positive charges.