Alternating Current Circuit containing Resistance (RCircuit): Let us consider, An alternating current circuit containing resistance $R$ only. This resistance $R$ is connected with an alternating EMF i.e electromotive force source i.e.
$E=E_{\circ}sin\omega t\qquad(1)$

The potential difference across the circuit
$E=iR$
Then from equation $(1)$
$iR=E_{\circ}sin\omega t $
$i=\frac{E_{\circ}}{R}sin\omega t $
$i=i_{\circ}sin\omega t \qquad(2) $

Where $i_{\circ}$ is the peak value or amplitude of the current in the circuit which has value $i_{\circ}=\frac{E_{\circ}}{R}$.
Now compare the equation $(1)$ and equation $(2)$ which shows that if a circuit is containing a resistor only then the current is always in phase with the applied EMF i.e electromotive force. The phase diagram between EMF and the current of resistance is shown below
The phasor diagram between the EMF and current of resistance is also shown in the given figure below