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Homework Help: Resistance help

  1. Mar 24, 2007 #1
    I've no idea how to start the following question so any help or guidance would be really appreciated.

    A potential divider circuit is set up with a LDR and a fixed resistor, R. The LDR has a resistance of 200 Ω when it is light and 2500 Ω when it is dark. A motor is set so that it comes on if 4.0 V is applied across it. A 6.0 V battery provides the emf (assume it has no internal resistance).

    When it is dark, does the motor come on if R is (a) 3000 Ω (b) 4000 Ω (c) 5000 Ω?

    When it is light, does the motor come on if R is (a) 300 Ω (b) 400 Ω (c) 500 Ω?

    If the motor were moved so that it is across the LDR instead, what would R have to be so that the motor came on in the light?


    Thank you.

    Cathy
     
  2. jcsd
  3. Mar 24, 2007 #2
    need to post at least some thought about this: maybe you don't have a diagram, and that is the issue: I read it as such. The diffference between
    v1 and v3 is just the applied EMF of 6Volts.

    V1-----R------v2------Rx------v3. where Rx=variable depending on whether dark or light. So for the potential difference between v1 and v2, to be >=4v,
    what must R be. Does this help at all?
     
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