♦ Take a straight thick copper wire and place it between the points X and Y in an electric circuit, as shown in Fig. 13.1.
♦ Place a small compass near to this copper wire. See the position of its needle.
♦ Pass the current through the circuit by inserting the key into the plug.
♦ Observe the change in the position of the compass needle.
♦ Fix a sheet of white paper on a drawing board using some adhesive material.
♦ Place a bar magnet in the centre of it.
♦ Sprinkle some iron filings uniformly around the bar magnet (Fig. 13.2). A salt-sprinkler may be used for this purpose.
♦ Now tap the board gently.
♦ What do you observe?
♦ Take a small compass and a bar magnet.
♦ Place the magnet on a sheet of white paper fixed on a drawing board, using some adhesive material.
♦ Mark the boundary of the magnet.
♦ Place the compass near the north pole of the magnet. How does it behave? The south pole of the needle points towards the north pole of the magnet. The north pole of the compass is directed away from the north pole of the magnet.
♦ Mark the position of two ends of the needle.
♦ Now move the needle to a new position such that its south pole occupies the position previously occupied by its north pole.
♦ In this way, proceed step by step till you reach the south pole of the magnet as shown in Fig. 13.3.
♦ Join the points marked on the paper by a smooth curve. This curve represents a field line.
♦ Repeat the above procedure and draw as many lines as you can. You will get a pattern shown in Fig. 13.4. These lines represent the magnetic field around the magnet. These are known as magnetic field lines.
♦ Observe the deflection in the compass needle as you move it along a field line. The deflection increases as the needle is moved towards the poles.
♦ Take a long straight copper wire, two or three cells of 1.5 V each, and a plug key. Connect all of them in series as shown in Fig. 13.5 (a).
♦ Place the straight wire parallel to and over a compass needle.
♦ Plug the key in the circuit.
♦ Observe the direction of deflection of the north pole of the needle. If the current flows from north to south, as shown in Fig. 13.5 (a), the north pole of the compass needle would move towards the east.
♦ Replace the cell connections in the circuit as shown in Fig. 13.5 (b). This would result in the change of the direction of current through the copper wire, that is, from south to north.
♦ Observe the change in the direction of deflection of the needle. You will see that now the needle moves in opposite direction, that is, towards the west [Fig. 13.5 (b)]. It means that the direction of magnetic field produced by the electric current is also reversed.
♦ Take a battery (12 V), a variable resistance (or a rheostat), an ammeter (0–5 A), a plug key, and a long straight thick copper wire.
♦ Insert the thick wire through the centre, normal to the plane of a rectangular cardboard. Take care that the cardboard is fixed and does not slide up or down.
♦ Connect the copper wire vertically between the points X and Y, as shown in Fig. 13.6 (a), in series with the battery, a plug and key.
♦ Sprinkle some iron filings uniformly on the cardboard. (You may use a salt sprinkler for this purpose.)
♦ Keep the variable of the rheostat at a fixed position and note the current through the ammeter.
♦ Close the key so that a current flows through the wire. Ensure that the copper wire placed between the points X and Y remains vertically straight.
♦ Gently tap the cardboard a few times. Observe the pattern of the iron filings. You would find that the iron filings align themselves showing a pattern of concentric circles around the copper wire (Fig. 13.6).
♦ What do these concentric circles represent? They represent the magnetic field lines.
♦ How can the direction of the magnetic field be found? Place a compass at a point (say P) over a circle. Observe the direction of the needle. The direction of the north pole of the compass needle would give the direction of the field lines produced by the electric current through the straight wire at point P. Show the direction by an arrow.
♦ Does the direction of magnetic field lines get reversed if the direction of current through the straight copper wire is reversed? Check it.
♦ Take a rectangular cardboard having two holes. Insert a circular coil having large number of turns through them, normal to the plane of the cardboard.
♦ Connect the ends of the coil in series with a battery, a key and a rheostat, as shown in Fig. 13.9.
♦ Sprinkle iron filings uniformly on the cardboard.
♦ Plug the key.
♦ Tap the cardboard gently a few times. Note the pattern of the iron filings that emerges on the cardboard.