Magnetic Effects of Electric Current Test - 25

The soft iron core helps in concentrating the magnetic lines of forces through the solenoid, so that the magnetic field is almost uniform at the end face of the core. In the absence of a core, the magnetic lines of forces start to diverge by curving sharply immediately outside the coils. The smaller the space between the coils and the core, the stronger is the magnetism induced in the core. Therefore, the best electromagnets have coils wound tightly around the core.

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The magnetic fields produced by the solenoid are similar to the magnetic field produced by a magnet. As shown in the figure, polarity of a magnetic pole in a solenoid is south pole if current is flowing clockwise and north pole if current flows anti-clockwise. Polarity can also be found using right-hand thumb rule.
Thus, the polarity of the solenoid XY at X is south pole and at Y is north pole.

When a bar magnet is suspended freely it will come to rest along the direction of the earth's magnetic field. The magnetic field lines that are produced by the solenoid are similar to that of the bar magnet and behaves like a bar magnet.

The polarity of the magnetic field (that is, which end is magnetic-north or magnetic-south) can easily be found from the Right hand thumb rule. If we hold the solenoid in our right hand, so that the fingers curl around it in the direction of the conventional current flow, then the thumb will point to the north pole of the magnet. Also, a solenoid when suspended freely, aligns itself in the north-south direction, thus behaving like a bar magnet. One end of the solenoid acts like a north pole and the other end the south pole. 
When magnetic compass with the north end is brought closer to the 1st end of the solenoid, the needle deflects towards the west, that is, away from the solenoid because the magnetic compass that is pointing in the direction of the earth's magnetic field, i.e. north- south direction repels with the north pole of current carrying solenoid. Like poles repel each other. Thus, the needle of the compass will rest in the direction of magnetic field corresponding to the solenoid at that point.

A solenoid behaves like a bar magnet and when suspended freely, aligns itself in the north-south direction by the directive property of the magnets. 

The closed curves in a magnetic field is called the magnetic lines of force.

According to Fleming's left Hand Rule rule, extend the thumb, forefinger, and the middle finger of the left hand in such a way that all the three are mutually perpendicular to each another. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then, the thumb points in the direction of the force exerted on the conductor.So, if the current is reversed in the conductor placed in a magnetic field the direction of force is also reversed.

The force on a charged particle moving in a magnetic field is at its maximum when the particle is moving at right angles to the direction of the magnetic field and zero when the particle is moving parallel to the field. The direction of the drift velocity of electrons in a conductor is along the length of the conductor. Thus, the magnitude of the force on a current-carrying conductor in a magnetic field varies with the angle between the direction of the length of the conductor and the direction of the magnetic field. 
Hence, the magnitude of force is zero when the conductor lies parallel to the field.