Fleming's Left Hand Rule And Right Hand Rule

When a current-carrying conductor is placed under a magnetic field, a force acts on the conductor. The direction of this force can be identified using Fleming’s Left Hand Rule. Likewise, if a moving conductor is brought under a magnetic field, electric current will be induced in that conductor. The direction of the induced current can be found using Fleming’s Right Hand Rule. It is important to note that these rules do not determine the magnitude, instead show only the direction of the three parameters (magnetic field, current, force) when the direction of the other two parameters is known. Fleming’s Left-Hand Rule is mainly applicable to electric motors and Fleming’s Right-Hand Rule is mainly applicable to electric generators.

Interpreting Fleming’s Right-Hand Rule

 

Fleming's Right-Hand Rule

The thumb indicates the direction of the motion; the forefinger indicates the direction of the magnetic field and the middle finger represents the direction of the induced current.
 

According to Faraday’s law of electromagnetic induction, when a moving conductor is placed inside a magnetic field, a current will be induced in it. If the conductor is forcefully moved inside the magnetic field, there will be a relationship between the direction of applied force, magnetic field and the current.  This relation between these three directions is determined by Fleming’s right-hand rule.

As shown in the figure above, the right-hand rule requires you to
“Hold the right-hand forefinger, middle finger and the thumb at right angles to each other. If the forefinger represents the direction of the magnetic field, the thumb points in the direction of motion or applied force, then the middle finger points in the direction of the induced current.”

Interpreting Fleming’s Left-Hand Rule

 

Fleming's Left-Hand Rule

The forefinger indicates the direction of the magnetic field; the middle finger indicates the direction of the current and the thumb represents the direction of the motion.

When a current-carrying conductor is placed in an external magnetic field,  the conductor experiences a force perpendicular to both the field and to the direction of the current flow. A left hand can be held, as shown in the illustration, so as to represent three mutually orthogonal axes on the thumb, forefinger and middle finger.

The left-hand rule requires you to

“Hold out your left hand with the forefinger, second finger and thumb perpendicular to each other. If the forefinger represents the direction of the field and the second finger represents that of the current, then thumb gives the direction of the force.”

Difference Between Fleming’s Left-Hand and Right-Hand Rule

Left-Hand Rule

Right-Hand Rule

It was invented by John Ambrose Fleming

It was invented by John Ambrose Fleming

It is used for electric motors

It is used for electric generators

The purpose of the rule is to find the direction of motion in an electric motor

The purpose of the rule is to find the direction of induced current when a conductor moves in a magnetic field.
The thumb represents the direction of the thrust on the conductor

The thumb represents the direction of motion of the conductor

The index finger represents the direction of the Magnetic Field

The index finger represents the direction of the Magnetic Field

The middle finger represents the direction of the current

The middle finger represents the direction of the induced current

From this, we can observe that the left hand satisfies Motor, and the right hand – Generator. 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.

Fleming’s Left-Hand  Rule Examples

Q1. Determine the direction of the force acting on the proton, if the proton moves towards the east by entering a uniform magnetic field in the downward direction.

Answer: 
Fleming's Left-Hand Rule

Using Fleming’s left-hand rule, we can determine the direction of force acting on the proton.

It is given that the proton is moving towards the east and therefore, the motion of the current is towards the east. The direction of force is towards the north as the magnetic field is acting downwards. Therefore, we can say that the direction of the force acting is towards the north.

Q2. Find the direction of the magnetic field, if an electron is moving vertically upwards and gets deflected towards the south due to a uniform magnetic field.

Answer: 
Fleming's Left-Hand Rule

Using Fleming’s left-hand rule, we can determine the direction of the magnetic field acting on the electron.

We know that the charge on an electron is negative. When the electron moves upward, the direction of the current is in the opposite direction, that is the direction of the current is downwards. It is given that the force acting on the electron is in the south direction. Therefore, the direction of the magnetic field is towards the east.

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