CIE A Level Physics (9702) 2019-2021

Revision Notes

25.3.1 Fleming's Left-Hand Rule

Fleming's Left-Hand Rule

  • The direction of the force on a charge moving in a magnetic field is determined by the direction of the magnetic field and the current
  • Recall that the direction of the current is the direction of conventional current flow (positive to negative)
  • When the force, magnetic field and current are all mutually perpendicular to each other, the directions of each can be interpreted by Fleming’s left-hand rule:
    • On the left hand, with the thumb pointed upwards, first finger forwards and second finger to the right ie. all three are perpendicular to each other
    • The thumb points in the direction of motion of the rod (or the direction of the force) (F)
    • The first finger points in the direction of the external magnetic field (B)
    • The second finger points in the direction of conventional current flow (I)

Worked example: Using the left-hand rule

Fleming_s_Left_Hand_Rule_Worked_example_-_Using_the_Left_Hand_Rule_Question, downloadable AS & A Level Physics revision notes

Using Fleming’s left-hand rule:

B = into the page

F = vertically downwards

I = from right to left

Fleming_s_Left_Hand_Rule_Worked_example_-_Using_the_Left_Hand_Rule_Answer, downloadable AS & A Level Physics revision notes

Exam Tip

Don’t be afraid to use Fleming’s left-hand rule during an exam. Although, it is best to do it subtly in order not to give the answer away to other students!

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