Fleming’s Left-Hand Rule: Definition & Application

Fleming’s Left-Hand Rule: A current-carrying conductor when placed in a magnetic field will experience a force by this conductor. The direction of force acting on this conductor will depend upon the direction of the magnetic field and the current flowing through the conductor. Fleming’s Left-Hand Rule can be used to find the direction of this current flow.

Fleming’s left-hand rule tells us that if we stretch our thumb, middle finger and the index finger of our left hand in mutually perpendicular (make an angle of 90 degrees) directions to each other, we can see the relation between directions of force (F), current (I), and magnetic field (B). Thus, the conductor placed in the magnetic field experiences a magnetic force in a direction orthogonal both to that field and the direction of the current flow. Scroll down to understand this important concept in detail.

State Fleming’s Left Hand Rule

Fleming’s left hand thumb rule states that “if the thumb, forefinger, and middle finger are stretched to be perpendicular to each other such that the forefinger is stretched in the direction of the magnetic field, the middle finger is stretched in the direction of the current, then, the thumb represents the direction of the force“. John Ambrose Fleming in the late \(19^\rm{th}\) century gave Fleming’s left and the right-hand rule.

Visualizing Fleming’s Left Hand Thumb Rule

We know that whenever a current-carrying conductor is placed in a magnetic field, a force is experienced by this conductor in a direction that is perpendicular to the direction of the current and the direction of the magnetic field. With the help of Fleming’s left-hand rule, if we know the direction of any two quantities, we can easily determine the direction of the third quantity.

To picture this, consider a current-carrying conductor placed in-between a horseshoe magnet. This magnet produces some magnetic field in the direction indicated by the forefinger or Index finger. The current passing through the conductor will be in the direction of the middle finger. This conductor experiences a force that is indicated in the direction of the thumb. If the poles of this magnet were reversed, the applied force would also be in the reverse direction.

Fleming’s Left Hand Rule: Concept

When a current-carrying conductor comes under a magnetic field, there will naturally be a force acting on the conductor. Fleming’s Left-Hand Rule is a simple and accurate way to find the conductor’s direction of force/motion in an electric motor when the magnetic field direction and the current direction are known. The thumb, forefinger, and middle finger of the left hand are stretched in a perpendicular direction with respect to each other such that:
Thumb: The thumb indicates the direction of Thrust (or force) on the conductor.
Forefinger: The forefinger indicates the direction of the Magnetic Field.
Center(Middle) finger: The centre finger indicates the direction of the current.

Fleming’s Left Hand Rule: Applications

Electric motor: One of the most common and well-known applications of Fleming’s left-hand rule can be seen in the working of an electric motor. Let us try to understand how an electric motor works: We know,

1. When an electric current is passed through a conductor, it generates a cylindrical magnetic field around the conductor. If an external magnetic field is brought close to the current-carrying conductor, the magnetic field and the electromagnetic field interact.

2. This interaction between current and magnetic fields will produce a physical force.
3. To calculate the direction of this force, we use this rule. If the middle finger of the left hand gives the direction of the current, the forefinger represents the direction of an external magnetic field, then the thumb of our left hand will point in the direction of the force.
4. In a standard DC electric motor, the electromagnetic field interacts with the magnetic field generated by the permanent magnets, and due to this interaction, a physical force is generated.
5. Using this rule, we can determine the direction of this force and the direction of the motor’s motion.
6. It can be used to understand the motion in any of the multitude of different types of electric motors in a simplified way.

Fleming Left and Right Hand Rule Comparison

The comparison between Fleming’s Right Hand Rule and Left Hand Rule will help students differentiate between these concepts with utmost clarity. The difference between these concepts is stated as follows:

Fleming’s Left Hand Rule: Correct Approach of Application

Situation 1: Determine the direction of force acting on a proton.
We are given that a proton is moving towards the east, and a magnetic field is acting on it in the downward direction.
The direction of current will be along with east, i.e. in the direction of motion of proton. Applying Fleming’s left-hand rule, the force experienced by the proton will be along the north.
Situation 2: Determine the direction of force acting on an electron.
We are given that an electron is moving upwards, and an external magnetic field is acting on it along the east direction.
The direction of current will be downwards, i.e. opposite to the direction of motion of the electron. Applying Fleming’s left-hand rule, the force experienced by the electron will be along the south.
Situation 3: Determine the direction of the magnetic field when: An alpha- particle, when projected towards the west, is deflected towards the north by a magnetic field.
The direction of the current will be along the direction of the motion of the alpha particle. So, the direction of the current is towards the west. According to Fleming’s Left-Hand Rule, the middle finger shows the current direction, the forefinger shows the direction of the magnetic field, and the thumb shows the direction of motion. Applying this rule, the direction of the magnetic field would be upwards.

Summary

John Ambrose Fleming in the late \(19^\rm{th}\) century gave Fleming’s left and right-hand rule. Fleming’s Left-Hand Rule is a simple and accurate way to find the conductor’s direction of force/motion in an electric motor when the magnetic field direction and the current direction are known. The thumb, forefinger, and middle finger of the left hand are stretched in a perpendicular direction with respect to each other such that:
Thumb: The thumb indicates the direction of Thrust (or force) on the conductor.
Forefinger: The forefinger indicates the direction of the Magnetic Field.
Center(Middle) finger: The centre finger indicates the direction of the current.
The left-hand rule is used to indicate the direction of motion in an electric motor.

Frequently Asked Questions

The commonly asked questions about the topic are answered here:

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