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How do resistance and speed of a coil in pendulum motion in a magnetic filed affect

  1. Mar 4, 2008 #1
    How do resistance and speed of a coil in pendulum motion in a magnetic filed affect the induced voltage and the decay of Potential Difference?


    Q1 When the speed of a coil is kept constant and a 3.3ohm resistor is added to the circuit of which the coil is a part of, the induced voltage drops. Why is this so? When the resistor is replaced with a 5.0ohm resistor the induced voltage for some reason increases and when the 5.0ohm resistor is replaced with a 6.8ohm resistor the induced voltage further increases. Why is this so? The induced voltage is considerably low when resistors are added to when compared to what the induced voltage is when a resistor is not added to the circuit.

    Q2 why and how is the rate of decay of potential difference affected by resistance? The rate of decay of potential difference is highest when no resistor is added to the circuit followed by when a 6.8ohm resistor is added and then by a 5.0ohm and 3.3ohm resistor.

    Note According to the data collected the induced current remains the same even when the resistance is changed.

    I included the diagram of the setup. This is not a homework question.

    Attached Files:

    Last edited: Mar 4, 2008
  2. jcsd
  3. Mar 5, 2008 #2

    Refer to the above thread recently active. My comments in that thread cover what you're observing. Let me know if you need further detail. The induction issue was discussed at length. The induced voltage and current change as the resistance varies, to a point. As the resistance is lowered, a "constant current" mode of operation is eventually attained. Reducing the resistance further will not increase the current as it approaches a max limit.

    If the resistance is increased indefinitely, the induced voltage will increase until it reaches a max limit. This becomes "constant voltage" behavior.

    The key to understanding all going on here is CEL (conservation of energy law), AL (Amperes law). FL (Faradays law), OL (Ohms law). and LL (Lenz law). All 5 laws must be simultaneously upheld under all loading conditions.

    What is happening in your experiment is perfectly consistent with the laws of physics. It can't be any other way. You just empirically proved what I covered in the above ref post. You got it right.
    Last edited: Mar 5, 2008
  4. Mar 5, 2008 #3
    Thanks for helping out but i still don't get why there is a drop in voltage when a 3.3ohm resistor is added to the circuit. In the original post i mentioned that the speed is kept constant and this done by displacing the pendulum bob by a certain distance, but this is not so. The speed is constant when there is no magnetic field, but when there is a magnetic field it changes depending on the resistance. the initial speed increases as resistance increases but is below the initial speed when there is no magnetic field. Although at higher resistance the speed decreases at a faster rate. can you explain why this is so.
  5. Mar 5, 2008 #4
    Your diagram is not very detailed, so I can't give a detailed answer. I need to know how the magnetic field is generated, its direction, etc. Also, the orientation of the coil wrt the mag field is important. A more detailed diagram would help.

    As far as the drop in voltage when 3.3 ohms is "added" to the circuit, is this 3.3 ohm *across* the coil terminals? Before the resistor is added, is the coil open circuited? Under open circuit conditions, maximum voltage will appear at the coil terminals. As the loading resistors are added, the voltage will decrease. If the resistor values are large, let's say in the megohm range, the voltage hardly changes, but the current changes inversely with resistance.

    But, if the resistance is lowered enough, the voltage drops substantially. Constant voltage operation no longer takes place. If the resistance is lowered down to a low enough value, ohms in your case, a constant current region of operation is observed.

    The link to the post from last week should explain it in detail. BR.

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