LIGHTBULB PUT WHERE ?

yiha093 SG

power source is 240v , with 2 parallel branches, branch 1 has 2 3000ohms conductor

2nd branch has a 1200 ohm and a LDR of 1200~3600 ohms

so where am i supposed to place ONLY ONE lightbulb, to ensure day during will WONT LIGHT UP, and during night WILL LIGHT UP

* no degree of dimness specified*

* lightbulb is assumed to been lit if any amt of current passes through it *

I am thinking like making the branch go short circuit, but then.... X.X

eagle SG

You must take note that for LDR, when it is bright, resistance goes low, but when it is dark, resistance goes high, so high that it can be like an open circuit.

But before I continue, was that how the question was phrased? I'm assuming that's how the circuit was given to you.

Basically, you can ignore the 3k resistor. Just take note of the 1.2k ohms resistor and LDR branch. You have to connect your light bulb in parallel to one of the components in the following manner. I leave it for you to derive it first.

sensor circuits

Btw... this is under O levels physics... A levels should know this already, especially by J2 at this time... Jiayou jiayou!

yiha093 SG

er. i 16 this year : D so..

my cher give, he say uni lvl ._.

erm. btw the branch is the 2nd 1 .

eagle SG

O levels ah... Paiseh I thought you As liao

Okok I go lunch first... come back explain to you...

Do look at the 2 circuits I gave you... 1 will light up when it is bright, the other when it is dark... It's not uni level, don't worry.... It's in past prelim papers

yiha093 SG

i can understand the right circuit might work if its dark, and to all the current going to the bulb. however. during the day. resistance low, then current would also go to bulb, hence not answering the qn ?

btw btw. is only 1 circuit, not 2 circuit to show . is just 1 circuit to put 1 put. the arrange of the 2nd branch is the right circuit

eagle SG

k back

Correct, right circuit will work when dark. Basically, this functions like a potential divider circuit.

I put 2 circuits to illustrate 2 scenarios... Just guiding you to choose the correct one as the answer... You can't take both as the answer ;)

During the day, resistance of LDR is low, so p.d. across is very much lower as compared to the 1200 ohms. Since your bulb is in parallel to the LDR, during the day, the p.d. across the bulb is _______________? Fill in the blanks :D

yiha093 SG

i got ans liao

cher say is between the space between 3k ohm and anther branch between the 2 resistors.

cause diff in P.D then got current.