Calculate Current out of Function Generator
Ideally the function generator is a perfect voltage source for any load. A 5V 50Hz sine wave has the form V(t) = 5*sin(2*pi*50Hz*t) [V]. V = IR, giving I(t) = 0.5*sin(2*pi*50Hz*t), or a 0.5A-peak, 50Hz sine wave. It looks like your linked device behaves this way.

The source and load will be in phase so long as the impedance is real (no inductors, capacitors, etc.).