FLEMING’S LEFT AND RIGHT HAND RULE AND THEIR APPLICATIONS

FLEMINGS LEFT AND RIGHT HAND RULE AND ITS APPLICATION:

If a current carrying conductor placed in a magnetic field, it experiences a force due to the magnetic field.

On the other hand if a conductor moved in a magnetic field, an emf gets induced across the conductor (according to Faraday’s Law of Electromagnetic Induction).

There are two rules to determine the direction of motion (in electric motor) or the director of induced current (in electric generator). But these rules known as Fleming’s left hand rule (for motor) and Fleming’s right hand rule (for generator).

Fleming’s left hand rule:

Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a force. According to Flemings left hand rule if the thumb, forefinger and middle finger of left hand are stretched perpendicular to each other as shown the figure and if fore finger represents the direction of magnetic field, the middle finger represents the direction of current, and then the thumb represents the direction of force.

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  • Important point to remember:

This rule is applicable for electric motor.

Electric motor:

When a rectangular coil carrying current is placed in a magnetic field, a torque acts on the coil which rotates it continuously. When the coil rotates, the shaft attached to it also rotates and thus it is able to do mechanical work.

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ARMATURE:

A D.C. motor consists of a rectangular coil made of the insulated copper wire wound on a soft iron core. This coil is mounted on an exile and is placed between the cylindrical concave poles of a magnet.

Commutator:

A commutator is used to reverse the direction of current. Commutator is a copper ring split into two parts C1 and C2. The split rings are insulated from each other and mounted on the axle of the motor. The two ends of the coil are soldered to these rings. They rotate along with the coil. Commutator rings are connected to a battery. The wires from the battery are not connected to the rings but to the brushes which are in contact with the rings.

In short, a split ring wrapping around the axle, the commutator makes physical contact with brushes which connect to connect to opposite poles of a power source to deliver positive and negative charges to the commutator.

Brushes:

Two small strips of carbon known as brushes press slightly against the two split rings and the split rings rotate between the brushes. The carbon brushes are connected to D.C. source.

Working of a D.C. Motor:

When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn towards the right, causing rotation.

When the coil turns through 90°, the brushes lose contact with the commutator and the current stops flowing through the coil. However, the coil keeps turning because of its own momentum.

Now when the coil turns through 180°, the sides get interchanged. As a result, the commutator ring C1 is now in contact with brush B2 and commutator ring C2 is in contact with brush B1. Therefore, the current continues to flow in the same direction.

The efficiency of the D.C. motor increases by:

  1. By increasing the number of turns in the coil.
  2. By increasing the strength of the flowing current.
  3. By increasing the area of cross section of the coil.

Fleming’s right hand rule:

Whenever a conductor is moved in an electromagnetic field, an emf gets induced across the conductor (according to Faraday’s law of Electromagnetic induction).

According to Fleming’s right hand rule the thumb, fore finger and middle finger of right hand are stretched perpendicular to each other as shown in the figure and id thumb represents the direction of movement of conductor, forefinger represents direction of the magnetic field, then the middle finger represents direction of the induced current.

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Electric generator:

Electric generator is a machine that converts one form of energy into another, especially mechanical energy into electrical energy.

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Principle:

The generator is an application of electromagnetic induction. It works on the principle that when a wire is moved in a magnetic field, then the current is induced in the coil. A rectangular coil is made to rotate rapidly in the magnetic field between the poles of a horse shoe type magnet. When the coil rotates, it cuts the lines of the magnetic force due to which a current is produced in the generator coil. The current can be used to run the various electrical appliances.

Construction:

A simple electric generator (D.C. Generator) consists of a rectangular coil ABCD which can be rotated rapidly between the poles N and S of a strong horse shoe type magnet. The generator coil is made up of a large number of turns of insulated copper wire. The one end of the coil is connected to the copper haft ring R1 and the other end of the coil is connected to the other copper half ring R2. When the coil is rotated, the two half rings R1 and R2 touch the two carbon brushes B1 and B2 one by one. So the current produced in the rotating coil can be taped out through the commutator half rings and into the carbon brushes. From the carbon brushes B1 and B2, we can supply current into various electrical appliances.

Working:

Let us suppose that the generator coil ABCD is initially in the horizontally position. As the coil rotates in the anticlockwise direction between North and South poles of the magnet the side of AB of the coil moves down cutting the magnetic lines of force near the North pole of the magnet and side DC move up, cutting the lines of force near the South pole of the magnet.

Due to this the induced current is produced in the sides AB and DC of the coil. On applying Fleming’s right hand rule to the sides AB and DC of the coil we find that the current in the two sides of the coil are in the same direction and we get an effective induced current in the direction BADC. Due to this the brush B1 becomes the positive pole and brush B2 becomes the negative pole of the generator.

After half revolution, the sides AB and DC of the coil will interchange their position. The side AB will come on the right hand side and starts moving up whereas side CD will come on the left hand side and starts moving down.

But when sides of the coil interchange their positions then the two commutator rings R1 and R2 automatically change their contacts from one carbon brush to other. Due to this change the current keeps flowing in the same direction. Thus a DC generator supplies a current only in one direction.

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