photonics question

[cranberry]

just a question on lux with a reverse biased LDR:

Question 6
The circuit as shown in Figure 4.39 is set up with the fi xed resistor
and the LDR reversed. Using the characteristic curve as shown in
Figure 4.38 for the LDR, fi nd:
a the voltage across the resistor when the intensity is 30 lux
b the potential difference across the resistor when the intensity
is 500 lux
c the voltage across the LDR when the intensity is 30 lux
d the potential difference across the LDR when the intensity
is 500 lux

ty

[luffy]

Sorry, I could be wrong in saying this, but:

Resistors do not have polarity - you are thinking of diodes. Therefore, a reverse biased LDR does not make sense... i.e. Reverse Bias Resistor is the same as Forward Bias resistor.

[cranberry]

i have another q:

Question 9
A low-current LED has a forward-bias voltage of 2.0 V. It is to be
operated from a 6 V DC supply. Determine the value of the resistor
required if the LED is to allow a current of 2 mA in the circuit.

Question 10
Calculate the series resistor required to operate the LED in
Question 9 from a 6 V AC supply.

what effect does an AC supply have on a resistor? (i know AC supply can make current go through multi-biased diodes at the same time..)