(a) Voltage at P side is less than voltage at N side of the diode. So, it is in “Reverse Bias”.
(b) Circuit diagram of a full wave rectifier

Working of full wave rectifier:
(a) A simple full way rectifier consists of two transformers mutually inducted and two diodes which filter the negative cycle of alternating current. At any instant the voltage at A (input voltage of diode 1) and end B (input voltage of diode 2) of the secondary with respect to the centre tap will be out of phase.
(b) Suppose during a positive half cycle of Ac input, the end A is positive and end B is negative with respect to the centre tap. Then diode 1 gets forward biased (allows the flow of current) and diode 2 gets reversed biased (does not allow the flow of current). Hence the current flows through the diode 1 towards the centre tap along the path AXY as shown in the above diagram.
(c) Similarly, during the negative half cycle of Ac input the end B becomes positive and end A becomes negative. Hence diode 1 gets reversed biased and diode 2 gets forward biased resulting in the current to flow from diode 2 to the centre tap along the path BXY as shown in the above diagram. (d) As during both the half cycles the input a.c. the current through the load resistor will flow from X to Y and will keep on pulsating.
Given below is the waveform for the full wave rectifier indicating inputs of AC and the output voltage.