Physics [3 Mark Question Answer]

The conductor is kept in N-S direction and its ends are connected to a battery, rheostat and a tapping key. When the key is open i.e., no current flows in the conductor, the needle points in N-S direction. As the key is pressed and a current passes in the conductor, the needle deflects. This shows that a magnetic field Exists around a current carrying conductor.

The important properties are:
(i) The magnetic lines of force near the wire are nearly circular, especially at the points where
the current enters or leaves the coil.
(ii) At the centre of the coil, the lines of force are almost parallel to each other, showing that the magnetic field at the centre or the coil may be considered uniform.

In case of a step-up transformer, the e.m.f. across the secondary coil is greater than across the primary coil. So the current in the secondary coil is less than in the primary coil. Therefore to reduce heat loss in the windings, the primary coil is made of thicker wire than the secondary coil.

The core of a transformer is made of soft iron because it has high permeability so it provides complete linkage of magnetic flux of the primary coil to the secondary coil. Secondly, the hysteresis loss of energy in soft iron is less.

(i) The strength of current in the coil,
(ii) The number of turns in the coil,
(iii) The strength of the magnetic field.

Electromagnet	Bar Magnet
1. An electromagnet exhibits a much stronger magnetic field.	A bar magnet is not able to exhibit a strong magnetic field.
2. Its polarity can be reversed by reversing the direction of current flowing in the turns of the coil (or solenoid).	Its polarity is fixed.
3. It is a temporary magnet, it behaves as a magnet only when the current flows through the solenoid.	It is a permanent magnet and cannot be readily demagnetized.

The magnitude of force acting on a current carrying conductor placed in a magnetic field depends on the following factors:
(i) The force F is directly proportional to the strength of the current flowing in the conductor,
i.e., F ∝ i.
(ii) The force F is directly proportional to the strength of the magnetic field, i.e., F ∝ B; and
(iii) The force F is directly proportional to the length of the conductor, i.e., F ∝ l.

Experimentally it is found that the magnitude of the force acting on a current-carrying conductor kept in a magnetic field in a direction perpendicular to it, depends on the following factors:
(i) The F is directly proportional to the length of the current-carrying conductor, i.e., F ∝ l.
(ii) F is directly proportional to the strength of the magnetic field, i.e., F ∝ B.
(iii) F is directly proportional to the strength of the current flowing in the conductor, i.e., F ∝ i.
Combining the above factors we find that:
or F ∝ i Bl
F = KiBl

Fleming's right hand rule: Stretch the forefinger, middle finger and the thumb of your right hand mutually perpendicular to each other. If the forefinger indicates the direction of magnetic field and the thumb will indicates the direction of motion of conductor, then the middle finger indicates the direction of induced current.

Faraday's formulated two laws of electromagnetic induction:

(i) Whenever there is a change in the magnetic flux linked with a coil, an e.m.f. is induced. The induced e.m.f. lasts so long as there is a change in the magnetic flux linked with the coil.

(ii) The magnitude of the e.m.f. induced is directly proportional to the rate of change of the magnetic flux linked with the coil. If the rate of change of magnetic flux remains uniform, a steady e.m.f. is induced.

(i) Heat losses in the windings of transformer. They are minimized by taking a thicker wire in the primary coil than in the secondary coil in a step-up transformer, whereas a thicker wire in secondary coil than in primary coil in a step-down transformer.
(ii) Hysteresis losses in the core of transformer. They are minimized by using soft iron core.
(iii) Eddy current losses in the core of transformer. They are minimized by taking the core laminated instead of taking it as one solid piece.

The main properties are:
(i) The magnetic lines of force are in the form of concentric circles around the conductor but as their distance from the conductor increases, the curvature of lines of force decreases.
(ii) The direction of the magnetic lines of force reverses if the direction of the flow of current in the conductor reverses.
(iii) With the increase of current in the conductor, the number of lines of force or the intensity of the magnetic field also increases.

According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of the current, then the thumb will point in the direction of motion of the conductor or the force acting on the conductor.

Maxwell’s Cork Screw Rule:

Imagine a right handed cork screw placed parallel to a current carrying conductor such that when it is rotated, it advances in the direction of the current. Then the direction in which the thumb rotates gives the direction of the magnetic lines of forces.

The magnetic lines of force due to current in the straight conductor XY are shown in figure given alongside. The arrows on the magnetic lines of force shows the direction of magnetic field.

The magnitude of the magnetic field at a point depends on:
(i) The strength of the current in the conductor, and
(ii) The distance of a point from the conductor.

The magnetic field of a solenoid is very similar to that of a bar magnet. the given figure shows, respectively, the lines of force of the magnetic field of a current-carrying solenoid and a bar magnet.

Figure shows the magnetic lines of force due to current-carrying coil.

The magnitude of the magnetic field at the center of coil depends on:
(i) the strength of the current in the coil, and
(ii) the radius of coil.

Step-up transformer is used to change low voltage alternating e.m.f. to high voltage alternating e.m.f. Its essential parts are shown in the given diagram:

As shown in the diagram the only change is that both the slip-rings of an A.C. generator are replaced by a split-ring (X-Y) (a ring split in two halves) which acts as a commutator or current-reverser.
A. C. Generator

D. C. Generator

The other parts in both are the same. By this device the current in the external circuit becomes unidirectional.

When the coil of a D. C motor turns through 180°, the split ring reverses the direction of the current in the coil from CBAD to DABC.


In this way, the coil continues to rotate continuously in the same direction and positive and negative polarities are maintained.

The electromagnet is a temporary magnet which can be magnetized or demagnetized in a very short time as per our requirements. In the simplest form, an electromagnet consists simply of a soft iron core around which a large number of turns of insulated wire are wound. Two of the simple forms of an electro magnet are shown in the figure alongside.

If a conducting wire (coated with an insulating paint or varnish) is closely wound on a (long) cylindrical core, the resulting coil is referred to as solenoid. It is usual to keep the diameter of the solenoid small compared to its length. This is done to bring about a greater degree of uniformity (and simplicity) in the axial magnetic field of the solenoid.

As the switch is pressed, the magnet will get repelled. The reason is that at the face of solenoid near the north pole of a magnet, the current flows in an anti-clockwise direction, so the face becomes north pole. The like poles repel, so the magnet is repelled.

In Ampere’s swimming rule’, imagine a man swimming along the wire in the direction of the current with the face of the man towards the needle, such that current enters his feet and leaves his head, then the direction of the magnetic field produced is such the magnetic held, as shown in the diagram, towards his left hand.