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Thread: answer a few questions?

  1. #1 answer a few questions? 
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    1.) A proton traveling due west in a region that contains only a magnetic field experiences a force vertically upwards away from the surface of the earth. What is the direction of the magnetic field?

    A) West
    B) East
    C) North
    D) South
    E) Up


    2.) A proton traveling due north enters a region that contains both a magnetic field and an electric field. The electric field points due east. It is observed that the proton continues to travel in a straight line due north. In which direction must the magnetic field lines point?

    A) North
    B) Down
    C) Up
    D) South
    E) West
    F) East


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  3. #2  
    Administrator KALSTER's Avatar
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    What do you think are the answers and why? After that we would love to help you out. We encourage students to try and do their own research and studying and at least give it a try first. That is the best way to learn :wink:


    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
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  4. #3  
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    For the first one my only thought was that the magnetic field and force are perpendicular to each other but that doesnt help me out...
    For the second one If the proton continues to go north that means that the magnetic field must also be north isnt it?
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  5. #4  
    Forum Ph.D. william's Avatar
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    F=q(E + v x B) the Lorentz force
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  6. #5  
    Forum Masters Degree bit4bit's Avatar
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    ...the vector cross product in William's equation leads you to use the 'right hand rule' for the magnetic field. This doesn't apply for the electric field. Have you done the right hand rule?
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  7. #6  
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    number 1 is south because of the right hand rule correct?
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  8. #7  
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    Ok so for number 2, the proton is moving north and the electric field points due east so that would force the proton to to move into the direction of the electric field which is east.... the magnetic field causes the proton to keep moving north but im not really sure how this works...i know it isnt up or north but i dont know how to eliminate the others
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  9. #8  
    Forum Masters Degree bit4bit's Avatar
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    Yeh number 1 is right.

    For number two, the electric field facing east, causes the proton to curve eastwards as it moves forwards. To balance this out, and keep it moving in a straight line, the magnetic force must be acting westwards.

    You know the direction of velocity and force, so you can work out the required direction of the magnetic field lines by the right hand rule. (same as in number 1)
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