A p-n junction diode is connected to series resistor of 100 ohm and a 4.5 V D.C battery. The barrier potential at the junction is 0.5 V .Find the current

Here E=4.5V 

R=100ohm

Voltage drop across pn junction diode=0.5V………………………..(as the barrier potential in the given question is 0.5V)

Effective voltage in the circuit, V=4.5-0.5=4.0V

We should keep in mind here that the positive side of the terminal is connected to p side of the diode, as a result the voltages are subtracted.

Current in the circuit therefore will be:

I=V/R

=4/100

=0.04A