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Circuit Components-Potential differencd across LDR (photoresistor)

  1. May 21, 2015 #1
    1. The problem statement, all variables and given/known data
    The following figure shows a potential divider circuit made up of a LDR and a variable resistor R. It is used to activate a buzzer circuit which sounds only at 2.6V and above. The resistance of the variable resistor varies from 100ohm to 10kohm while the resistance of the LDR varies from 110ohm to 3.6kohm.
    1432254436688.jpg

    Switch is now closed. The variable Resistor R is set at 2.4kohm and tje potential divider circuit is placed in a room with no light. Calculate the potential difference across the LDR.
    2. Relevant equations
    V=RI
    Total p.d across a series circuit= add up the p.d of all components.
    Total resistance across a series circuit= add up resistance of all components
    Current is the same throughout the whole series circuit.

    3. The attempt at a solution
    I thought of ratio and proportion but I could not find the total resistance as I do not have the Total current... The final answer of this question is 3.6V.

    Please help, thank you!
     
  2. jcsd
  3. May 21, 2015 #2
    Hey, I got the answer...though I 'm wondering why the resistance of the buzzer is not taken into account. Since there is no light, the resistance of the LDR would be at max value, 3.6kΩ. So just add it with resistance of variable resistor to get and find the ratio of the resistance of LDR to total. Multiply that answer with 6V.
     
  4. May 22, 2015 #3
    Oh I see. Thank you! The resistance of the buzzer is extremely large(stated in part 2 of the question) and I guess that is related to why it is not regarded...
     
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