SSLC, Physics Chapter 2.MAGNETIC EFFECT OF ELECTRIC CURRENT notes

How is a magnetic field created around a electromagnet?

The magnetic field around an electro magnet is created due to the flow of current through the coils in it.

When a magnetic needle is brought near a current carrying conductor, the magnetic needle deflects. Why?

Ans:

A magnetic field is developed around a current carrying conductor. The magnetic needle is deflected as a result of the mutual action of the magnetic field around the current carrying conductor and that around the magnetic needle.

Right hand thumb rule

This rule was put forward by James Clerk Maxwell. Hold a current carrying conductor with right hand in such a way that the thumb points in the direction of the current. The direction in which other figures encircle the conductor give the direction of the magnetic field.

Right hand screw rule

The direction of magnetic field around a current carrying conductor can also be found out by using right hand screw rule. If a right hand screw is rotated in such a way that its tip advances along the direction of the current in the conductor, then the direction of rotation of the screw gives the direction of magnetic field around the conductor.

What are the factors affecting the strength of an electromagnet?

Number of turns and intensity of current.

The strength of the magnetic field increases when the number of turns of the coil or current is increased.

What is a solenoid?

A solenoid is an insulated wire wound in the shape of a helix.

Electromagnet

If a soft iron core is inserted in a current carrying solenoid it will act as a magnet.

Polarity of an electromagnet

Difference between a bar magnet and a solenoid

Bar magnet

The magnetism is permanent.

Polarity is fixed.

Strength cannot be changed.

Solenoid

The magnetism is temporary.

Polarity depends on the direction of current.

Strength can be changed by changing the intensity of current.

Explain the working principle of electric motor with the help of an illustration.

Suspend a copper wire between the poles of a U shaped magnet, using thin conductors in such a way that the wire is perpendicular to the magnetic field and it is free to oscillate in the magnetic field. When current is passed through the conductor it will deflect or move. If the direction of the current or magnetic field is changed the direction of motion of the conductor will also change. When a current carrying conductor is brought in a magnetic field, a force is experienced. This is the working principle of electric motor.

FLEMING’S LEFT HAND RULE

Electric motor

N,S – Magnetic poles

XY – Axis of rotation of motor

ABCD – Armature

B1, B2 - Graphite brushes

R1, R2 - Split rings

Working of electric motor

When electricity is passed through the armature a force is experienced due to the mutual action of the magnetic field around the magnetic poles and the magnetic field around the armature. This force is converted into rotational motion.

Function of split ring commutator in the motor

To sustain the rotation of the armature, the current through the armature is kept on changing by the split ring commutator. This arrangement help to change the direction of current through the coil after every half rotation.

Moving coil loud speaker

Moving coil loud speaker is a device which works on the basis of motor principle.

Let us assess questions and answers

a. Towards east

b. Ampere’s swimming rule

c. Ampere’s swimming rule

Suppose a man swims in the direction of current flow in a conductor by looking towards a magnetic needle, the north pole deflects towards the direction of the left hand.

d. A freely suspended magnetic needle remains in the north south direction. When the current flows in the east west direction, the magnetic field produced will be in the north south direction. So magnetic needle does not deflect.

Ans:

If the direction of current flow in the end of the solenoid is in the clockwise direction, the polarity of the solenoid at that end is south. If the direction of current flow in the end of the solenoid is in the anticlockwise direction, the polarity of the solenoid at that end is north.

Methods to increase the strength of the magnetic field around a current carrying solenoid.

Increase the number of turns

Increase the intensity of current

Increase the area of cross section of the core

a. From B to A

b. The magnetic field across the conductor AB can be found out. The direction of the magnetic field around the conductor will be out of the plane(above the table). The rule which helps to find out this is right hand thumb rule.

Right hand thumb rule

c. The direction of magnetic field lines inside the coil will be from outside to inside the coil.

Ans:

Direction of current is from B to A

Ans:

b) It gradually increases towards the ends.

Ans: Yes this statement is correct. Current flows in the opposite direction of electrons. According to Fleming’s left hand rule, when the thumb, point finger and middle finger of the left hand are kept mutually perpendicular and if the point finger represents the direction of the magnetic field and the middle finger in the direction of current. Then the thumb represents the direction of force exprienced by the electrones and this will be into the plane of paper.

Ans: When the coil is kept in south north direction the magnetic field around the coil is free from the influence of the magnetic field due to the geomagnetism.

Ans:

To sustain the rotation of the armature, the current through the armature is kept on changing by the split ring commutator. This is made possible by the semi circular rings in the split ring commutator.

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The magnetic intensity will decrease. When the solenoid is stretched, the distance between the coils increases and so the number of magnetic lines decreases per unit area.

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Principle of motor

When a freely suspended current carrying conductor is kept in a magnetic field a force is experienced on the conductor. As a result the conductor moves. This is the principle of motor.

If the direction of current in the coil and direction of magnetic field are same, the conductor will not move.