AQA A Level Physics

Revision Notes

7.8.2 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 in the direction of conventional current flow (positive to negative)
  • When the force, the magnetic field and the 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 the 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)

Flemings left hand rule, downloadable AS & A Level Physics revision notes

Fleming’s left hand rule

  • Since this is represented in 3D space, sometimes the current, force or magnetic field could be directed into or out of the page, not just left, right, up and down
  • The direction of the magnetic field into or out of the page in 3D is represented by the following symbols:
    • Dots (sometimes with a circle around them) represent the magnetic field directed out of the plane of the page
    • Crosses represent the magnetic field directed into the plane of the page

 

Direction of B field, downloadable AS & A Level Physics revision notes

The magnetic field into or out of the page is represented by circles with dots or crosses

  • The way to remember this is by imagining an arrow used in archery or darts:
    • If the arrow is approaching head-on, such as out of a page, only the very tip of the arrow can be seen (a dot)
    • When the arrow is receding away, such as into a page, only the cross of the feathers at the back can be seen (a cross)

Worked Example

State the direction of the current flowing in the wire in the diagram below.

Worked example - LH rule question image, 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

Worked example - LH rule solution image, 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!

Direction of the Magnetic Force

  • The direction of the magnetic force on positive and negative charged particles depends on
    • The direction of flow of the charged particles
    • The direction of the magnetic field
  • This can be found using Fleming’s left-hand rule by remembering that the second finger represents the current flow or the flow of positive charge
    • This means that for negative charges, such as electrons, their flow will be in the opposite direction to which the second finger points
    • Therefore, if a particle carries a negative charge, the second finger should be pointed in the opposite direction to its motion

  • For example, when a positively charged particle enters a magnetic field into the page from left to right:
    • Using Fleming’s left-hand rule, the first finger points into the page and the second finger (current) points to the right
    • This means the force is upwards
    • The particle is then pulled in the direction of this force (upwards). This means the direction of the current also changes direction slightly (slanting upwards)
    • This means the force will also change direction since it still needs to keep perpendicular to the current and the field
  • Therefore, the moving charges will follow a circular trajectory

Exam Tip

Remember not to get this mixed up with Fleming’s right-hand rule. That is used for a dynamo, where a current is induced in the conductor (not following through it initially). Fleming’s left-hand rule is sometimes referred to as the ‘Fleming’s left-hand rule for motors’.

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top