Activity

• 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.(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.(a), the north pole of the compass needle would move towards the east.
  
• Replace the cell connections in the circuit as shown in Fig. (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.(b)]. It means that the direction of magnetic field produced by the electric current is also reversed.

Activity 

• Take a battery (12 V), a variable resistance (or a rheostat), an ammeter (0–5 A), a plug key, connecting wires, 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.  (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 
  
• 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.

Figure  (a) A design of concentric circles showing the field lines of a attractive field around a straight conducting wire. The bolts within the circles appear the heading of the field lines. (b) A near up of the design gotten.

What happens to the diversion of the compass needle set at a given point in case the current within the copper wire is changed? To see this, vary the current within the wire. We discover that the diversion within the needle too changes. In reality, in case the current is expanded, the diversion moreover increments. It shows that the greatness of the attractive field delivered at a given point increments as the current through the wire increments.

Example 

A current through a horizontal power line flows in east to west direction. What is the direction of magnetic field at a point directly below it and at a point directly above it?

Solution 

Ans. The current is in the east-west direction. Applying the right-hand thumb rule, we get that the magnetic field (at any point below or above the wire) turns clockwise in a plane perpendicular to the wire, when viewed from the east end, and anti-clockwise, when viewed from the west end.

Magnetic Field due to a Current through a Circular Loop

We have so distant watched the design of the attractive field lines created around a current-carrying straight wire. Assume this straight wire is bowed within the frame of a circular circle and a current is passed through it. How would the attractive field lines see like? We know that the attractive field created by a current-carrying straight wire depends contrarily on the separate from it. Essentially at each point of a current-carrying circular circle, the concentric circles speaking to the attractive field around it would gotten to be bigger and bigger as we move absent from the wire (Fig.3)

Figure-3 Magnetic field lines of the field created by a current-carrying circular loop

By the time we reach at the middle of the circular circle, the curves of these enormous circles would show up as straight lines. Each point on the wire carrying current would grant rise to the attractive field showing up as straight lines at the center of the circle. By applying the proper hand run the show, it is simple to check that each segment of the wire contributes to the attractive field lines within the same course inside the circle.

*This rule is also called Maxwell’s corkscrew rule. If we consider ourselves driving a corkscrew in the direction of the current, then the direction of the rotation of corkscrew is the direction of the magnetic field.

We know that the attractive field created by a current-carrying wire at a given point depends straightforwardly on the current passing through it. In this manner, in the event that there's a circular coil having n turns, the field created is n times as huge as that created by a single turn. This is often since the current in each circular turn has the same course, and the field due to each turn at that point fair includes up.