点\(M( \sqrt {2},1)\)在椭圆\(C\)：\( \dfrac {x^{2}}{a^{2}}+ \dfrac {y^{2}}{b^{2}}=1(a > b > 0)\)上，且点\(M\)到椭圆两焦点的距离之和为\(2 \sqrt {5}\)
\((1)\)求椭圆\(C\)的方程；
\((2)\)已知动直线\(y=k(x+1)\)与椭圆\(C\)相交于\(A\)，\(B\)两点，若\(P(- \dfrac {7}{3},0)\)，求证：\( \overrightarrow{PA}\cdot \overrightarrow{PB}\)为定值．