What are magnetic lines of force? Give their important properties.

The curves which are used to represent the magnetic field, such that the number of lines represent the strength of the magnetic field at any point are called magnetic lines of force. The direction of magnetic force is given by the tangent of the curve at any point.

Properties of magnetic lines of force:

(i) These are closed curves which move from the N-pole to the S-pole and then return to the N-pole through the interior of the magnet. 

(ii) No two magnetic lines of force can intersect each other. 

(iii) Magnetic lines are continuous curves and they tend to contract longitudnally and expand laterally. This implies, attraction between unlike poles and repulsion between like poles.

(v) The relative closeness of the lines of force gives a measure of the strength of the magnetic field. It is found the magnetic strength is maximum at the poles.

 

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Draw a labelled diagram to explain the principle underlying the working of an electric generator.

AC. Generator: It is a device which converts mechanical energy into alternating form of electrical energy.
Principle: It works on the principle of electromagnetic induction. When a closed coil is rotated in a uniform magnetic field with its axis perpendicular to the magnetic field, the magnetic field lines passing through the coil change and an induced emf and hence a current is set-up in it.
Construction: It consists of the following main parts:
1. Field magnet: It is a strong horse shoe-type permanent magnet with concave poles.
2. Armature: ABCD is a rectangular armature coil. It consists of a large number of turns of insulated copper wire wound on a soft iron cylindrical core. It can be rotated about an axis perpendicular to the magnetic field of the field magnet.
3. Slip rings: These are two brass rings S1 and S2 rigidly connected to the two ends of the armature coil. As the coil rotates, slip rings also rotate about the same axis of rotation.
4. Brushes: These are two graphite rods B1 and B2 which are kept pressed against the slip rings S1 and S2. Through these brushes, the current induced in the armature coil is sent to the external circuit.

AC. Generator: It is a device which converts mechanical energy into a
Fig. A.C. generator

Working: As shown in Fig, suppose the armature coil ABCD is in the horizontal position. Now the coil is rotated clockwise. The coil cuts the magnetic lines of force. The arm AB moves upwards while the arm CD moves downwards. According to Fleming’s right hand rule, the induced current flows from A to B in arm AB and C to D in arm CD i.e., the induced current flows along ABCD. The induced current flows in the circuit through brush B2 to B1.
After half the rotation of the armature, the arm CD moves upwards and AB moves downwards. The induced current now flows in the reverse direction i.e., along DCBA. The current flows from B1 to B2. Thus the direction of current in the external circuit changes after every half rotation. Such a current which changes its direction after equal intervals of time is called alternating current. This device is called A.C. generator.

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With the help of a labelled diagram, explain the principle, construction and working of an electric motor. What is the function of a split ring in an electric motor?


Electric Motor:
We describe here a d.c. motor which operates on direct current obtained from a battery.
Principle: An electric motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force, the direction of force is given by Fleming’s left hand rule.
Construction: As shown in Fig.an electric motor consists of the following main parts:
1.    Field magnet: It is a strong horse shoe type magnet with concave poles.
2.    Armature: It is a rectangular coil ABCD having a large number of turns of thin insulated copper wire wound over a soft iron core. The armature is placed between the poles of the field magnet and it can be rotated about an axis perpendicular to the magnetic field lines.
 
Electric Motor:We describe here a d.c. motor which operates on direct
Fig. An electric d.c. motor
3.    Split ring commutator: It consists of a cylindrical metal ring split into two halves S1and S2. The two ends A and D of the armature coil are connected to the split rings S1 and S2 respectively. As the coil rotates, the split rings also rotate about the same axis of rotation. The function of the split ring commutator is to reverse the direction of current in the coil after every half rotation.
4.    Brushes: Two graphite or flexible metal rods maintain a sliding contact with split rings S1 and S2, alternately.
5. Battery: A battery of few cells is connected to the brushes. The current from the battery flows to the armature coil through the brushes and the split rings.
Working: Initially, suppose the plane of the coil is horizontal. The split ring S1 touches the brush B1 and split ring S2 touches the brush B2. The current in coil flows in the direction ABCD, as shown in Fig.(a)
Clearly, the currents in arms AB and CD are in opposite directions. On applying Fleming’s left hand rule for the direction of force on a current-carrying conductor in a magnetic field, we find that the force acting on arm AB pushes it downwards while the force acting on the arm CD pushes it upwards. Thus the armature coil alongwith the axle rotates anticlockwise. After half a rotation, as shown in Fig.(b), the split ring comes in contact with brush B2 and S2 in contact with brush B1. Therefore, the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called commutator. In electric motors, the split ring acts as a commutator. Thus the arm AB is now pushed up and the arm CD is pushed down. Therefore, the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.


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What is an electromagnet? On factors does the strength of an electromagnet depend?
Or
What is an electromagnet? Draw a circuit diagram to show how a soft iron piece can be transformed into an electromagnet.


Electromagnet: A soft iron core placed inside a solenoid behaves like a powerful magnet when a current is passed through the solenoid. This device is called an electromagnet. When the current is switched off, the iron core loses its magnetism and so it is no longer an electromagnet. Thus, electromagnets are temporary magnets.

Electromagnet: A soft iron core placed inside a solenoid behaves like
Fig. Electromagnet (a) Bar type  (b) Horse-shoe type

Factors on which the strength of an electromagnet depends:
(i) Number of turns in the coil: The larger the number of turns in the coil, greater is the strength of the electromagnet.
(ii) Strength of the current: The larger the current passed through the solenoid, more powerful is the electromagnet.
(iii) Nature of the core material: The core of the magnetic material like soft iron increases the strength of the electromagnet.

 

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Why does a current carrying conductor kept in a magnetic field experience force? On what factors does the directior of this force depend? Name and state the rule used for determination of direction of this force.

(i) A current carrying conductor has a magnetic field associated with it. The two magnetic fields, one due to the magnet and the other due to the current in the conductor, interact. This produces a force on the conductor.
(ii) The direction of the magnetic force depends on
         (a) the strength of electric current, and
         (b) the strength of the magnetic field.
(iii) Fleming’s left hand rule is used for determination of the direction of magnetic force on a current carrying conductor.
(iv) Fleming’s left hand rule: Stretch the forefinger, the central finger and the thumb of the left hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field, the central finger in the direction of current, then the thumb. points in the direction of force on the conductor.

(i) A current carrying conductor has a magnetic field associated with

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