3.7.5.1
Fleming's left-hand rule finds the force direction
Magnetic Fields & Forces — AQA A-Level Physics
- Force, field, and currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). on a conductor are all mutually perpendicular.
- Fleming's left-hand rule lets you work out which direction the force acts when you know the other two.
Key Definition
Fleming's left-hand rule — Point the thumb, first finger, and second finger of the left hand at right angles to each other. Thumb = force/motion (F), first finger = magnetic field (B), second finger = conventional current (I).
Dots and crosses: 3D on paper
- A $dot = field coming out$ of the page. Think of an arrow flying towards you — you see the tip.
- A $cross = field going$ into the page. An arrow flying away — you see the flights.
- The left-hand rule is for motors. Fleming's right-hand rule is for generators. Don't mix them.
Fleming's left-hand rule
Left hand with thumb pointing upward (Force), first finger pointing forward (B-field), second finger pointing right (CurrentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).). Labelled F, B, I.
Worked Example
A wire sits in a magnetic field directed into the page. The force on the wire acts vertically downwards. State the direction of the currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). in the wire.
Show Solution
1
Identify what you know
- Field $B$: into the page (crosses)
- Force $F$: vertically downwards
- Find: current direction
2
Set up Fleming's left-hand rule
Point your first finger into the page (field direction).
Point your thumb downwards (force direction).
Your second finger now shows the current direction.
3
Read off the current direction
With the first finger into the page and the thumb pointing down, the second finger points from right to left.
Answer
The current flows from right to left.
Examiner Tips and Tricks
- Physically use Fleming's left-hand rule in the exam.
- It works.