Chapter 12 · Question 3

What is the pattern of the magnetic field lines around a straight current-carrying conductor? Describe an activity to demonstrate this pattern, and state how the magnitude of the magnetic field depends on the current and the distance from the wire.

Answer Revealed

Direct Answer:

The magnetic field lines around a straight current-carrying conductor form concentric circles centred on the wire. To demonstrate: insert a thick copper wire vertically through the centre of a horizontal cardboard, sprinkle iron filings on the cardboard, and pass current through the wire. On gently tapping the cardboard, the iron filings align in concentric circles around the wire. The direction of the field lines is given by placing a compass on a circle — the north pole of the compass shows the direction. The magnitude of the magnetic field

B

at a given point is directly proportional to the current

I

(

B \propto I

) and inversely proportional to the distance

r

from the wire (

B \propto 1/r

). Reversing the current direction reverses the field direction.

Simple Explanation

If you pass current through a straight vertical wire, the iron filings around it form perfect concentric circles — like ripples from a stone dropped in water, but centred on the wire. That is the shape of the magnetic field: circular loops wrapping around the wire. Two things control how strong the field is: crank up the current and the field gets stronger; move your compass farther from the wire and the field gets weaker. If you reverse the battery connections, the circles flip direction — north pole points the opposite way.

Exam-Ready Structure

The magnetic field pattern around a straight current-carrying conductor is established through Activity 12.5 in the NCERT textbook. 1. Field pattern: The magnetic field lines around a straight current-carrying conductor consist of a series of concentric circles centred on the wire. These concentric circles become larger and larger as the distance from the wire increases. 2. Demonstration (Activity 12.5): (a) Take a battery (12 V), a variable resistance (rheostat), an ammeter (0–5 A), a plug key, connecting wires, and a long straight thick copper wire. (b) Insert the thick wire through the centre of a rectangular cardboard, perpendicular (normal) to its plane, and fix the cardboard so it does not slide. (c) Connect the copper wire vertically between points X and Y in series with the battery, plug key, and rheostat. (d) Sprinkle iron filings uniformly on the cardboard. (e) Close the key to allow current flow; ensure the wire remains vertically straight between X and Y. (f) Gently tap the cardboard a few times. Observe that the iron filings align in concentric circles around the copper wire (Fig. 12.6). 3. Determining direction: Place a compass at a point (say P) over a circle. The direction of the north pole of the compass needle gives the direction of the field lines at that point. 4. Dependence on current: If the current through the wire is increased, the deflection of a compass needle placed at a given point increases. This indicates that the magnitude of the magnetic field is directly proportional to the current:

B \propto I

. 5. Dependence on distance: If the compass is moved farther from the wire (say from point P to point Q), the needle's deflection decreases. The magnitude of the magnetic field is inversely proportional to the distance from the wire:

B \propto 1/r

. 6. Reversing current: Reversing the direction of the electric current reverses the direction of the magnetic field lines.

Key Points

Field lines around a straight current-carrying wire: concentric circles centred on the wire
Larger circles at greater distance — field lines spread out farther from the wire
Magnitude of magnetic field $B \propto I$ (directly proportional to current)
Magnitude of magnetic field $B \propto 1/r$ (inversely proportional to distance from wire)
Reversing the direction of current reverses the direction of the magnetic field

What is the pattern of the magnetic field lines around a straight current-carrying conductor? Describe an activity to demonstrate this pattern, and state how the magnitude of the magnetic field depends on the current and the distance from the wire.