The portion underlined in black
Why potential difference across zener diode don't change when the resistance and zener diode is connected in parallel?
Ok this is characteristics of zener diode that after breakdown potenial difference across it remain constant.
1.R and zener diode are in series not in parallel (also mentioned in first line)
2.under the condition of reverse breakdown the voltage drop across the diode does not increase since the resistance offered goes to zero.i think the author means this condition is achieved making the voltage across it constant and all the voltage drop is across R if we ignore RL.
by the way which book is this?
And plz don't follow this book it seems it is very confusing and does not have very good explanation.
ok, i think the theory is not explained well you can refer to some other book to get better clarity.
It may help you
I agree they in series but doesn't it look like parallel? I have the similar series parallel doubt in other circuit also, can anyone explain why they are not in parallel because potential difference about both resistance and zener diode looks same
i think you are confused between RL and R , they are 2 different resistances.R and diode in series because one end of R is connected to the +ve end of the voltage source while the end of diode is connected to the negative one so they cannot be in parallel.
I am confused between the connection of zener diode with Rl(load resistance)
Think of Rl(Load) as an just any device connected across which the potential is same as the potential across the diode.During the analysis treat RL as open-circuited and then we observe the voltage across the zener diode (which remains constant) as shown in figure in reverse breakdown region.this is how the characteristics are obtained by ignoring or open circuiting the load.
whereas R here can represent internal resistance of the voltage source or some resistance offered by junction connecting the diode and the voltage source
Watch is video for better understanding: