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)
Fleming's left hand rule
The thumb represents the motion (force), the first finger the field and the second finger the current
Worked example
State the direction of the current flowing in the wire in the diagram below.
Answer:
- Using Fleming’s left-hand rule:
- B = into the page
- F = vertically downwards
- I = from right to left
Worked example
A 15 cm length of wire is placed vertically and at right angles to a magnetic field. When a current of 3.0 A flows in the wire vertically upwards, a force of 0.04 N acts on it to the left.
Determine the direction of the field.
Answer:
- Using Fleming’s left-hand rule :
- F = to the left
- I = vertically upwards
- therefore, B = into the page
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!
