why current will not flow from battery if I connect positive terminal of battery to ground even when there is potential difference
I connect positive terminal of battery to ground...
[The] negative terminal is not connected to anything.
If the negative terminal of the battery is not connected to anything, then no current can flow through the battery. Current can only flow around a complete circuit (i.e., it can only flow around a closed loop).
Since the negative terminal is very small, the capacitance between the terminals does not change much, even when the negative terminal is connected to ground. So, to maintain the same voltage between the terminals, no current would need to flow.
If the negative terminal was also connected to a large object, preferably close to ground, the capacitance between the terminals would increase and the current would flow.
When you connect the positive terminal to ground, the potential difference, if there was any at all, vanishes. The terminal becomes part of the ground.