Fleming’s Left Hand Rule
When a current carrying conductor comes under a magnetic field, there will be force acting on the conductor and on the other hand, when a conductor is forcefully brought under a magnetic field, there will be an induced current in the conductor. This is the working principle of a motor and a generator respectively. So, in a motor and a generator there is a relation between magnetic field, current and force.
Whether you apply Left Hand Rule or Right Hand Rule depends on adopting a convention (or) understanding the implicit convention for one of the quantities. For instance,
- Conventional current flows in the direction of positive charges – i.e. an electron moving to the right is equivalent to conventional current flowing to the left
- Angular momentum points 90 degrees up from the plane where rotation is clockwise
- Magnetic lines spread outward from a North Pole.
These are just conventions that form a common basis to work from, and you don’t have to define it that way, it’s just customary.
Let’s place a conductor in a magnetic field in the direction of the index finger. When the thrust on the conductor goes up, conventional current flows from right to left. On the corollary, when you pass (conventional) current in the direction left to right without moving the conductor, the force on the conductor acts upward. This is because motors and generators have “opposite functions”: Motors use electricity and generators produce electricity.
Interpretation of Fleming’s Left Hand Rule
By assigning fingers to the directions of the three quantities,
- (conventional) current: middle finger
- magnetic field: index finger
- thrust (or) force: thumb
We get these two rules – Fleming’s left hand rule and Fleming’s Right Hand Rule, and we can observe that the left hand satisfies Motor, and the right hand – Generator.
But, why do these work? There is no intrinsic reason why a clock hands go clockwise, or that going clockwise creates a right handed vector into the wall. This is to preserve parity across a whole range of things, and so you reference it to a single event, which is readily available: that is the hand.
The Fleming’s left hand rule and Fleming’s right hand rule are a pair of visual mnemonics (mnemonics are learning techniques or memory aids, such as an abbreviation, rhyme or mental image that helps to remember something). In practice, these rules are never used except as a convenient trick to determine the direction of the resultant – either current or thrust. What gives the magnitude of force along this direction determined by these rules is the Lorentz’ Force.
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