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Induced Solenoid Voltage by external magnetic fields

  1. Nov 7, 2011 #1
    1. The problem statement, all variables and given/known data
    In my physics lab, we were asked to connect an unpowered solenoid to an oscilloscope and then move a magnet through the solenoid and observe the changes in voltage on the oscilloscope. First we were asked to move the north pole of the magnet into the solenoid and then the south pole. I am just confused about what the oscilloscope should show.


    2. Relevant equations



    3. The attempt at a solution
    I know that when the north pole of the magnet is placed in the oscilloscope, that the induced magnetic field points in the opposite direction, so the current would go clockwise around the solenoid. I just don't know how to translate this into what the oscilloscope should show. When i did the experiment with the north pole of the magnet, the graph on the oscilloscope goes from negative to positive. I just don't understand why this intuitively happens. Any help would be appreciated. Thanks!
     
  2. jcsd
  3. Nov 7, 2011 #2

    PeterO

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    Did you put the North ple in then take it out, or did you have the magnet pass all the way through and out the other end?
     
  4. Nov 7, 2011 #3

    PeterO

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    Another thing to be careful of ...

    We are familiar with current flowing from the positive terminal of a battery to negative terminal of the battery in a circuit. Have a think about the current flow inside the battery.
    The battery is the driving part of the circuit.
    When a magnet is introduce to, or removed from, a Solenoid - a voltage is induced and a current can flow if you connect a circuit.
    The Solenoid takes on the role of the battery in the above example so which terminal is the current flowing from and which terminal is it flowing to?
     
  5. Nov 7, 2011 #4
    I go along with Peter O's comment. You need to be very careful about directions of emf (voltage) and direction of current.
    You also need to be clear about the laws of electromagnetic induction..... Faraday's and Lenz's laws.
    Faradays law states that 'whenever a conductor (your solenoid) experiences a CHANGING magnetic field an EMF (voltage ) is induced.'
    You ALWAYS get a voltage but you will only get a current if the conductor (solenoid) is part of a complete circuit. Think about a battery sitting on a table.... there is an emf but no current unless it is made part of a circuit
    The oscilloscope is effectively a perfect VOLTMETER so no significant current flows into the oscilloscope and what it displays is the induced emf NOT a current.
    Lenz's law states that 'the Direction of the induced emf opposes the CHANGE producing the emf'.
    So, if you push the N pole towards the solenoid an emf will be induced in the solenoid in a direction which will try to stop the N pole approaching.
    If you pull the N pole away from the solenoid...... can you see what will happen?
    Does this fit with what you saw on the oscilloscope?
     
  6. Nov 7, 2011 #5
    Ok so, since the induced EMF will be in the direction opposing the change, the induced emf would go from positive to negative so if it were connected in a circuit, the current would be induce a magnetic field in the opposite direction?
     
  7. Nov 7, 2011 #6
    almost there !!
    The emf will try to stop the N pole approaching. This means that it will try to make current flow in the solenoid to repel the N pole.So the end of the solenoid facing the incoming N pole wants to be a N pole to repel the magnet. You should be able to say which way the current would like to flow (do you know how to tell which way the current must flow to make the end of a solenoid N or S?).
    Now comes the really tricky bit mentioned by PeterO..... the solenoid is the source of the emf and is effectively a battery. When a current flows from a battery round a circuit the current flows from the + of the battery, through the circuit back to the - of the battery. BUT IT FLOWS FROM THE - OF THE BATTERY TO THE + OF THE BATTERY when it goes through the battery. If you can apply this to your solenoid you have got it and you should also be able to explain what you see when the N pole of the magnet is moved away from the solenoid.
    If you get it you will jump for joy !!!
    One final point
    Current flows from - to + through a source of emf (battery etc)
    Current flows from + to - through a Potential Difference (PD) ... whatever is connected to the emf.
    This is the difference between an emf and a PD
     
  8. Nov 7, 2011 #7
    Try studying "Lenz's Law" ... its all based on that ...
     
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