1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.

  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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook