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B What is cutting lines of force? Elementary Generator

  1. Aug 16, 2016 #1
    I was reading this article on Wikipedia about single phase generators and the operation of a basic generator, and I'm having a very difficult time understanding what is meant by cutting lines of force and how the rotating armature being perpendicular to the field cuts zero lines. While when it is parallel, it cuts the most.

    How does it manage to be in the field without cutting any lines to begin with? How is it cutting more lines when it's parallel?


    I'm actually a physics major and have already taken physics 2 (basic E&M) as well as calc 3 so I'm familiar with flux integrals and Maxwell's Equations if that helps you to explain it. I'm not sure what I'm missing because I feel like I should understand this already.

    Thank you,
  2. jcsd
  3. Aug 16, 2016 #2


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    THe armature in the diagram is a rectangular loop of wire. The mag field lines run parallel from N to S. The sides of the rectangle that are the far and near ends rotate in a plane that is parallel to the mag field lines, and hence never cut any lines. When the rectangle is parallel (perpendicular) to the mag field lines - ie horizontal (vertical), as shown in the picture - the straight 'out' and 'in' sides of the rectangle are moving respectively directly up (to the left) and directly down (to the right), and hence are moving perpendicular (parallel) to the mag field lines, and hence cutting the maximum (minimum - ie zero) number per second of them.
  4. Aug 16, 2016 #3
    Thank you. I think I understand now. So from the initial zero degree position, when the armature is set in motion, the long arms of the conductor are moving mostly parallel to the field lines and is going along with the same line or surface more so than cutting into different ones. Then as it approaches 90 degrees it is moving with a greater perpendicular component to the field lines and is thus cutting through different ones more than dragging along the same one. In effect, you want movement perpendicular to the field direction.

    If I understand this right, this would imply that moving the wire left and right in the zero position would generate no emf as it is being dragged along the same surfaces instead of cutting through new ones. Whereas moving up and down from the 90 degree position would generate the greatest emf because it is cutting through a different surface with every bit of movement. Do you agree?
  5. Aug 17, 2016 #4


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    It depends what is marked as the zero position. If that is the position when the plane of the armature rectangle is perpendicular to the lines of force (ie the armature rectangle is vertical, in the linked diagram) then that is correct. Given that definition, the armature shown in the diagram is close to the 90 degree position.
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