# CIE A Level Physics复习笔记20.1.3 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) Fleming’s left hand rule

#### Worked Example

State the direction of the current flowing in the wire in the diagram below. Using Fleming’s left-hand rule:

B = into the page

F = vertically downwards

I = from right to left #### 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! 