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Why does the potential drop in this case?

  1. Apr 4, 2006 #1
    8 A voltage of 3.0 V is required to operate an alarm
    circuit. Only a 12 V supply is available, together with a
    number of resistors. The resistance of the alarm circuit is
    about 1000 Ω. A student selects four resistors of value
    10 kΩ and sets them up in a series circuit. The voltmeter
    indicates a potential drop across each resistor of 3.0 V.
    However, as soon as the alarm circuit is connected, the
    potential drops to near zero.
    a Comment on why this happens.
    b Check the student’s calculations.
    c Should the student have selected resistors of higher
    or lower value?
  2. jcsd
  3. Apr 4, 2006 #2


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    You should really show some working and thoughts
  4. Apr 4, 2006 #3

    Andrew Mason

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    I think you mean that the four resistors have a value of 10 Ω, not kΩ.
    a. With just the 4 10 Ω resistors, what is the current? When you add the 1000 Ω load, what is the current? Since voltage drop, V = IR what is the voltage across the 1000 Ω alarm?

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