Let an AC source of EMF E be connected to a pure resistance R. The instantaneous EMF from the source is given by
E = Eo sin wt
From Ohm's law
I = E/R = Eo/R . sin wt
I = Io sin wt
The variation of E and I shows that both reach maximum and zero value at the same instant of time. We therefore say the current is in phase with applied voltage.
The time varying quantities E and I can be represented geometrically by phasors.
