Question

Magnetic field inside a long straight solenoid carrying current :

• A. is zero
• B. decreases as we move towards its end
• C. increases as we move towards its end
• D. is the same at all points

Hint:

For a {long} solenoid, the magnetic field deep inside is remarkably {uniform} (same strength and direction everywhere inside, pointing along the axis). The field lines are straight, parallel, and evenly spaced inside. The field only weakens and spreads out near the very ends. Option (4) describes the ideal behavior well inside.

Answer 1

The Correct Option is D

Concept: A long straight solenoid carrying current produces a magnetic field. The characteristics of this field, particularly inside the solenoid, are important. Step 1: Visualizing the magnetic field of a solenoid When current flows through the coil of a solenoid, it generates a magnetic field. The field lines inside a long solenoid are nearly parallel to its axis, straight, and evenly spaced, especially far from the ends. Outside the solenoid, the field is much weaker and spreads out, resembling the field of a bar magnet. Step 2: Properties of the magnetic field inside a long solenoid For an {ideal} long solenoid (infinitely long, or very long compared to its diameter), the magnetic field inside is:
Uniform: The strength (magnitude) of the magnetic field is the same at all points well within the interior of the solenoid, away from the ends. The direction of the field is also uniform, parallel to the axis of the solenoid.
Directed along the axis: The magnetic field lines run parallel to the axis of the solenoid.
Calculable: The magnitude of the field is given by \(B = \mu_0 n I\), where \(\mu_0\) is the permeability of free space, \(n\) is the number of turns per unit length, and \(I\) is the current. Step 3: Considering the "long straight solenoid" in the question The phrase "long straight solenoid" implies we are considering a situation close to the ideal case, particularly for points well inside.
(1) is zero: This is incorrect. A current-carrying solenoid produces a magnetic field.
(2) decreases as we move towards its end: The field does become weaker and starts to fringe (spread out) near the ends. At the very end of a long solenoid, the field strength is approximately half of what it is deep inside.
(3) increases as we move towards its end: This is incorrect.
(4) is the same at all points: This statement is true for the region {well inside} a long solenoid, away from the effects of the ends. Given the options, this is the best description of the field characteristics within the main body of a long solenoid. If the question implies "deep inside" or "far from the ends", then this is accurate. Step 4: Choosing the best fit option While the field strength does change near the ends, the defining characteristic of the magnetic field {inside} a long solenoid (implying the main internal region) is its uniformity. Therefore, option (4) is the most appropriate answer in the context of typical idealizations taught at an introductory level.