ICSE Class 10 Physics Chapter 10 Electro Magnetism

Chapter 10: Electro-magnetism

Syllabus

Magnetic effect of current (principles only, laws not required); Electromagnetic induction (elementary); transformer.

Scope of syllabus: Oersted's experiment on the magnetic effect of electric current; magnetic field (B) and field lines due to current in a straight wire (qualitative only), right hand rule; magnetic field due to a current in a loop; electromagnets, their uses; comparison with a permanent magnet; Fleming's left hand rule, the dc electric motor - simple sketch of main parts (coil, magnet, split ring commutator and brushes); brief description and type of energy transfer (working not required); simple introduction to electromagnetic induction, frequency of a.c. in household supplies, ac generator - simple sketch of main parts, brief description and type of energy transfer (working not required); advantage of a.c. over d.c.; transformer - its types, characteristics of primary and secondary coils in each type (simple labelled diagram and its uses).

Section A: Magnetic Effect Of Electric Current

10.1 Oersted's Experiment On The Magnetic Effect Of Electric Current

Hans Oersted, in 1820, in his experiments observed that when an electric current is passed through a conducting wire, a magnetic field is produced around it. The presence of magnetic field at any point around the current-carrying wire can be detected with the help of a compass needle.

In absence of any magnetic field, the needle of compass can rest in any direction. But in presence of a magnetic field, the needle of compass rests only along the direction of the magnetic field. In the earth's magnetic field alone, the needle rests only along the north-south direction.

Experiment: In Fig. 10.1, AB is a wire lying in the north-south direction and connected to a battery through a rheostat and a tapping key. A compass needle is placed just below the wire.

Observations: (1) When the key is open i.e., no current passes through the wire, the needle points in the N-S direction (i.e., along the earth's magnetic field) with the north pole of needle pointing towards the north direction. In this position, the needle is parallel to the wire as shown in Fig. 10.1 (a).

(2) When the key is pressed, a current passes in the wire in the direction from A to B (i.e., from south to north) and the north pole (N) of the needle deflects towards the west [Fig. 10.1 (b)]. If current in the wire is increased, the deflection of the needle also increases.

(3) When the direction of current in the wire is reversed by reversing the connections at the terminals of the battery, the north pole (N) of the needle deflects towards the east [Fig. 10.1 (c)].

(4) If the compass needle is placed just above the wire, the north pole (N) deflects towards east when the direction of current in wire is from A to B [Fig. 10.1 (d)], but the needle deflects towards west as shown in Fig. 10.1 (e) if the direction of current in wire is from B to A.

Explanation: The above observations of the experiment suggest that a current carrying wire produces a magnetic field around it and the magnetic needle of compass experiences a torque in this magnetic field, so it deflects to align itself in the direction of magnetic field at that point. On increasing current in the wire, the deflection of the compass needle increases which implies that the strength of magnetic field around the wire increases. On reversing the direction of current in the wire, the direction of deflection of the compass needle of compass reverses because the direction of magnetic field reverses.

Inference: A current (or moving charge) produces a magnetic field around it. This is called the magnetic effect of current. The strength of magnetic field depends on the magnitude of current and its direction depends on the direction of current.

10.2 Magnetic Field And Field Lines Due To Current In A Straight Wire

When a current is passed through a conducting wire, a magnetic field is produced around it. At a point, the direction of magnetic field will be along the tangent drawn on the magnetic field line at that point. The magnetic field lines can be drawn by means of iron filings as follows.

Experiment: Take a sheet of smooth cardboard with a hole at its centre. Place it horizontal and pass a thick copper wire XY vertically through the hole. Connect an ammeter A, battery B, rheostat Rh and a key K between the ends X and Y of the wire as shown in Fig. 10.2. Sprinkle some iron filings on the cardboard and pass an electric current through the wire by inserting the plug in the key K. Gently tap the cardboard.

It is observed that the iron filings get arranged along the concentric circles around the wire as shown by the dotted lines in Fig. 10.2.

If a small compass needle is placed anywhere on the board near the wire, the direction in which the north pole of needle points, gives the direction of magnetic field at that point which is marked by an arrow on the field line.

From the magnetic field lines patterns, we note that

(1) The magnetic field lines form the concentric circles around the wire, with their plane perpendicular to the straight wire and with their centres lying on the wire.

(2) When the direction of current in the wire is reversed, the pattern of iron filings does not change, but the direction of deflection of the compass needle gets reversed. The north pole of the compass needle now points in a direction opposite to the previous direction showing that the direction of magnetic field has reversed.

(3) On increasing current in the wire, the magnetic field lines become denser and the iron filings get arranged in circles up to a larger distance from the wire, showing that the magnetic field strength has increased and it is effective up to a larger distance.

Note: The magnetic field at any point is the combined effect of the magnetic field due to current in the wire and the magnetic field of the earth. At points near the wire, the magnetic field due to current in the wire predominates due to which iron filings arrange themselves in circles, while at points far away from the wire, the magnetic field due to earth becomes more pronounced as compared to the magnetic field due to current in the wire, as a result filings tend to arrange themselves in straight lines. The point where the two fields are equal and opposite, is called a neutral point. At the neutral point, the resultant magnetic field is zero and the compass needle at this point rests in any direction.

10.3 Rule To Find The Direction Of Magnetic Field

Experimentally the direction of magnetic field at a point is determined with the help of compass needle. But theoretically the direction of magnetic field (or magnetic field lines) produced due to flow of current in a conductor can be determined by various rules. One such rule is the right hand thumb rule.

Right hand thumb rule

If we hold the current carrying conductor in our right hand such that the thumb points in the direction of flow of current, then the fingers encircle the wire in the direction of the magnetic field lines (Fig. 10.3).

In Fig. 10.3, the direction of current in the conductor XY is upwards (i.e., X to Y). The magnetic field lines are anticlockwise around the conductor.

10.4 Magnetic Field Due To Current In A Loop (Or Circular Coil)

The magnetic field lines due to current in a loop (or circular coil) can be obtained by the following experiment.

Experiment: Take a piece of thick wire A bent in the form of a loop (or coil). It is passed at two points P and Q through a horizontal cardboard C such that the points P and Q lie in a plane through the diameter of the coil. Then sprinkle some iron filings on the cardboard. Connect a battery through a rheostat and a key between the ends X and Y of the loop as shown in Fig. 10.4.

When current is passed through the coil by closing the key and the cardboard is gently tapped, we find that the iron filings get arranged in a definite pattern representing the magnetic field lines of the current carrying loop (Fig. 10.4).

To find the direction of magnetic field lines, a compass needle is used. The direction of magnetic field at a point is along the needle in direction in which its north pole points. An arrow is marked on each magnetic field line to indicate the direction of magnetic field.

From the pattern of magnetic field lines, it is noted that

(1) In the vicinity of wire at P and Q, the magnetic field lines are nearly circular.

(2) Within the space enclosed by the wire (i.e., between P and Q), the magnetic field lines are in the same direction.

(3) Near the centre of loop, the magnetic field lines are nearly parallel to each other, so the magnetic field may be assumed to be nearly uniform in a small space near the centre.

(4) At the centre, the magnetic field lines are along the axis of loop and normal to its plane.

(5) The magnetic field lines become denser (i.e., the magnetic field strength is increased) if (i) the strength of current in loop is increased, and (ii) the number of turns in the loop are increased.

(6) The magnetic field lines pass through the loop in same direction. They appear to come out from the face of loop on other side, so that face behaves as a north pole, while the magnetic field lines appear to enter in at the front face of loop, so it behaves as a south pole. Thus the loop acts like a dipole and it has a magnetic field similar to that of a magnetised disc of radius same as that of the loop.

The polarity at the two faces of loop depends on the direction of current in the loop. On reversing the direction of current in loop, the polarity at the faces of loop gets reversed. The polarity at the two faces of loop is determined by the clock rule.

Clock rule (clockwise current-south pole and anticlockwise current-north pole)

Looking at the face of loop, if the current around that face is in anticlockwise direction, the face has the north polarity, while if the current at that face is in clockwise direction, the face has the south polarity.

This can be tested by using a compass needle. In Fig. 10.5 (a), current at the face of loop is anticlockwise, so it behaves as north pole, while in Fig. 10.5 (b), current at the face of loop is clockwise so it behaves as south pole.

10.5 Magnetic Field Due To A Current Carrying Cylindrical Coil (Or Solenoid)

If a conducting wire is wound in form of a cylindrical coil whose diameter is less in comparison to its length, the coil is called a solenoid. It looks like a helical spring. To obtain the magnetic field lines due to a current carrying solenoid, the following experiment is performed.

Experiment: Take a cardboard having two slits PQ and P'Q' parallel to each other, at a small separation and parallel to its length. Wind a uniform spiral of an insulated thick copper wire through the two slits such that the axis of spiral is in the plane of the cardboard. Connect a battery through a rheostat and a key between the ends X and Y of the solenoid. Sprinkle some iron filings on the cardboard and pass current through the solenoid by closing the key. Gently tap the cardboard.

It is found that the iron filings on the cardboard get arranged in a definite pattern as shown by the dotted lines in Fig. 10.6, representing the pattern of magnetic field lines due to the current carrying solenoid. The direction of magnetic field at a point is determined by using a compass needle and arrows are marked on these lines in the direction in which the north pole of the compass needle points.

From the pattern of magnetic field lines, it is found that

(1) The magnetic field lines inside the solenoid are nearly straight and parallel to the axis of solenoid i.e., the magnetic field is uniform inside the solenoid.

(2) The magnetic field lines become denser (i.e., a strong magnetic field is obtained) on increasing current in the solenoid.

(3) The magnetic field is increased, if the number of turns in the solenoid of given length is increased.

(4) The magnetic field is also increased, if a soft iron rod (core) is placed along the axis of solenoid. The soft iron increases the strength of magnetic field of the solenoid since soft iron has a high magnetic permeability.

(5) In Fig. 10.6, the end P at which the direction of current is anticlockwise behaves as a north pole (N), while the end Q at which the direction of current is clockwise behaves as a south pole (S). On reversing the direction of current in the solenoid, the polarities at the ends of solenoid are reversed because the direction of magnetic field has reversed.

Similarities between a current carrying solenoid and a bar magnet

(1) The magnetic field lines of a current carrying solenoid are similar to the magnetic field lines of a bar magnet. Thus a current carrying solenoid behaves just like a bar magnet.

(2) A current carrying solenoid when suspended freely sets itself in the north-south direction exactly in the same manner as a bar magnet does.

(3) A current carrying solenoid also acquires the attractive property of a magnet. If iron filings are brought near the current carrying solenoid, it attracts them.

Dissimilarities between a current carrying solenoid and a bar magnet

(1) The strength of magnetic field due to a solenoid can be changed by changing the current in it, while the strength of magnetic field due to a bar magnet cannot be changed.

(2) The direction of magnetic field due to a solenoid can be reversed by reversing the direction of current in it, but the direction of magnetic field due to a bar magnet cannot be reversed.

10.6 Electromagnet

An electromagnet is a temporary strong magnet made by passing current in a coil wound around a piece of soft iron. It is an artificial magnet.

An electromagnet can be made in any shape, but usually the following two shapes of electromagnet are in use:

(1) I-shape (or bar) magnet, and

(2) U-shape (or horse-shoe) magnet.

Teacher's Note

Electromagnets are found in everyday devices like doorbells, electric motors, and MRI machines, showing how fundamental principles of electromagnetism have practical applications in modern technology.

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