如图,已知直线\(l\)与\(⊙O\)相离,\(OA⊥l\)于点\(A\),\(OA=5.OA\)与\(⊙O\)相交于点\(P\),\(AB\)与\(⊙O\)相切于点\(B\),\(BP\)的延长线交直线\(l\)于点\(C\).
\((1)\)试判断线段\(AB\)与\(AC\)的数量关系,并说明理由;
\((2)\)若\(PC=2 \sqrt {5}\),求\(⊙O\)的半径和线段\(PB\)的长;
\((3)\)若在\(⊙O\)上存在点\(Q\),使\(\triangle QAC\)是以\(AC\)为底边的等腰三角形,求\(⊙O\)的半径\(r\)的取值范围.