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Fleming's Left Hand Rule

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Fleming's Left Hand Rule

Did you know that a current-carrying wire in a magnetic field will experience force? This force is called the electromotive force, or emf. This property forms the basis for all modern electric motors and generators. So we know that there is a force that is induced on the current-carrying conductor, but how do we know its direction? If only there was an easy method that could be applied in almost all conditions. Wait! Fleming's left-hand rule does just that. John Ambrose Fleming in the late 19th century discovered a method using which the direction of this electromotive force can be determined. Fleming devised a simple method of finding the direction of force, current, and magnetic field, and to make things better, all you need is your left hand!

Definition of Fleming's left-hand rule

Fleming's left-hand rule The image shows the orientation of the left hand for applying the Fleming's left-hand rule StudySmarterFleming's left-hand rule shows the direction of the thrust on a conductor carrying a current in a magnetic field, Wikimedia Commons.

The rule states that if we stretch the thumb, middle finger and index finger of the left hand in such a way that they make an angle of, then the thumb will point in the direction of the force, the middle finger will point in the direction of the currentand the index finger represents the direction of the magnetic field.

An easy way to remember this is by using the acronym FBI, where the letters stand for the properties defined above. The letters represent each of the fingers starting from the thumb and ending at the middle finger. Now let us work on a few simple example problems where we can apply Fleming's left-hand rule.

Examples of Fleming's left-hand rule

Imagine a wire across the computer screen, where the current flows from the left to the right of the screen and the external magnetic field is flowing from the top to the bottom of the screen. What will the direction of the force on the conductor be?

Fleming's left-hand rule Examples of Fleming's left-hand rule StudySmarterGiven the direction of the current and magnetic field, the force or thrust will point towards the screen, StudySmarter Originals.

Using Fleming's left-hand rule, position the index finger and the middle finger in the direction of the magnetic field and the current, respectively. The thumb will be pointing towards the screen, so the force is directed into the screen.

Imagine a wire placed on the ground, carrying a current that is flowing from south to north. It experiences a force to the east. Does the magnetic field point towards the ground or the sky?

Fleming's left-hand rule The image shows an example of Fleming's left-hand rule StudySmarterThe direction of the external magnetic field can be found using Fleming's left-hand rule. by aligning the thumb and index finger in the direction of the current and force, StudySmarter Originals

Placing the middle finger in the direction of the current and the thumb towards the east, we see that the magnetic field is pointing towards the sky.

After these introductory examples, let us look at a use case in which Fleming's left-hand rule can be applied.

Use case of Fleming's left-hand rule

Fleming's left-hand rule can be applied in the circuit of an electric motor, where it can be used to determine the force acting on the current-carrying wire that is present inside the electric motor.

Fleming's left-hand rule Fleming's left-hand rule on an electric motor StudySmarterThe image shows how an electric motor works due to the interaction of the current-carrying coil and the magnetic field, StudySmarter Originals.

Applying Fleming's left-hand rule

The figure above shows the working of an electric motor. We see that the current enters the loop from the right arm. Point the middle finger in the direction of the current in the right arm of the coil. The index finger points to the south pole as it represents the direction of the magnetic field (north to south). Pointing the middle finger in the direction of the current will give you the direction of the force on both arms of the coil as shown in the picture. In conclusion, the coil experiences two forces equal in magnitude but opposite in direction, which are responsible for driving the electric motor.

A couple is a pair of equal and opposite forces acting on either end of a body. These forces together can cause an object to rotate around its center. The forces due to the motor effect exert a couple on the current-carrying wire inside the electric motor.

Difference between the left-hand and right-hand rule

Fleming's Left-Hand Rule A drawing of a right hand illustrating Fleming's right hand rule StudySmarterIn the right-hand rule, the thumb shows the direction of motion of the conductor, Wikimedia Commons.

Fleming also has a right-hand rule, which is used in the context of electromagnetic induction.

Electromagnetic induction is the phenomenon due to which a current is induced in a conductor when the magnetic field around it fluctuates.

Here, the thumb indicates the motion of the conductor instead of the force on the conductor. These two directions are each other's opposite, so it's important to remember this major difference with the function of the thumb in Fleming's left-hand rule! The index and middle finger both have the same function as in the left-hand rule. We see that this right-hand rule tells us the same as Fleming's left-hand rule. We can also see this by positioning the index fingers of our hands together, and then placing the middle fingers together: our thumbs point in opposite directions, displaying the difference in their function between the two hand rules.

Importance of Fleming's left-hand rule

Fleming's left-hand rule is important because it provides a simple and accurate way of finding the direction of the force experienced by a current-carrying conductor in a magnetic field, given that you know the direction of both the current and the magnetic field. As we saw in the use case of an electric motor, using Fleming's left-hand rule, we can easily identify the direction in which the force acts on the arms of an electric motor. Considering that the workings of the all-important electromotor are well-understood in part due to Fleming's left-hand rule, we can conclude that Fleming's left-hand rule is, although indirectly, important in everyday life as well.

Fleming's Left-Hand Rule - Key takeaways

  • Fleming's left-hand rule states that the thumb will point in the direction of the force, the index finger will point in the direction of the magnetic field, and the middle finger will point in the direction of the current. A mnemonic is FBI.
  • Fleming's right-hand rule states that the thumb points in the direction of the motion of the conductor, the index finger points in the direction of the magnetic field, and the middle finger points in the direction of the induced current.
  • Beware of the difference in the function of the thumb between the two hand rules of Fleming.
  • Fleming's left-hand rule can be used to identify the direction of the forces in an electric motor.
  • Fleming's right-hand rule can be used to identify the direction of the induced current in an electric generator.

Frequently Asked Questions about Fleming's Left Hand Rule

Fleming's left-hand rule is a rule that uses the left hand to find the direction of the force on a current-carrying conductor in a magnetic field. Stretch the thumb, index finger, and middle finger such that they're perpendicular to each other. The thumb points in the direction of the force (F), the middle finger in the direction of the current (I), and the index finger in the direction of the magnetic field (B).

The rule states that if we stretch the thumb, middle finger, and index finger of the left hand in such a way that they make an angle of 90 degrees with each other, then the thumb will point in the direction of the force (F), the middle finger will point in the direction of the current (I), and the index finger will point in the direction of the magnetic field (B).

The right-hand rule is sometimes applied in the context of electromagnetic induction to find the direction of the induced current (I) through a conductor, whereas the left-hand rule is applied in the context of the motor effect to find the direction of the force (F) through a conductor. The right-hand rule is applied more in the case of electric generators, while the left-hand rule is applied in the case of electric motors.

Fleming's left-hand rule is important because it provides an accurate way of finding the direction of the force experienced by a current-carrying conductor in a magnetic field.

An application of Fleming's left-hand rule is seen in the use case of an electromotor.

Final Fleming's Left Hand Rule Quiz

Question

What does the index finger represent in Fleming's left-hand rule?

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Answer

Magnetic field.

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Question

The thumb represents the direction of the force in Fleming's left-hand rule.

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Answer

 True.

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Question

Fleming's left-hand rule can be used to find out the direction of the force on the coils in electric motors.

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Answer

True.

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Question

Name the important application in which Fleming's left-hand rule plays a major role.

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Answer

The electric motor.

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Question

In Fleming's right-hand rule, the thumb represents the direction of the ...

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Answer

motion of the conductor.

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Question

Which rule is best used to predict the direction of a force on a wire in an electric motor?

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Answer

Fleming's left-hand rule.

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Question

The direction of the magnetic field is always from ...

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Answer

the north pole to the south pole.

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Question

Is there an electromotive force on a wire if the current through it is parallel to the surrounding magnetic field lines?

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Answer

No.

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Question

Find the direction of the force if the flow of current is from geographic south to geographic north and the magnetic field is from geographic east to geographic west.

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Answer

Up.

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Question

What is Fleming's left-hand rule used for?

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Answer

The Fleming's left-hand rule can be used to find the direction of the force on a current-carrying wire in a magnetic field.

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Question

Which rule is best used to predict the induced current in a conductor?

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Answer

Fleming's right-hand rule.

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Question

What is electromagnetic induction?

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Answer

Electromagnetic induction is the phenomenon due to which a current is induced in a conductor when the magnetic field around it fluctuates.

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