The magnetic field inside a current-carrying very long solenoid is
- (a)Uniform
- (b)Non-uniform
- (c)Zero
- (d)Highest at mid-point
Correct — A, Uniform. Inside a long current-carrying solenoid the magnetic field is essentially uniform — constant in both magnitude and direction, parallel to the axis, and given by B = mu0 x n x I (where n is the number of turns per unit length). The closely wound turns make the interior field lines straight, parallel and equally spaced, so a long solenoid is the magnetic analogue of the uniform field between the plates of a parallel-plate capacitor.
- (b)Non-uniform — The field is non-uniform only near the ENDS of the solenoid (edge effects). For a long solenoid the field throughout the interior is uniform, so 'non-uniform' is wrong for the region inside.
- (c)Zero — Zero is (approximately) the field OUTSIDE a long solenoid, not inside. Inside, the field is strong and uniform.
- (d)Highest at mid-point — The field does not peak at the centre; it is the same value throughout the interior of a long solenoid, tapering off only right at the ends.
A solenoid is a long, closely wound helical coil. When current flows, the fields of the individual turns add up; for a very long solenoid the interior field lines are straight and parallel, giving a uniform field B = mu0 x n x I directed along the axis and independent of position inside. Outside a long solenoid the field is nearly zero. This is why solenoids are used to build electromagnets and to provide uniform fields in the laboratory.
The keyword is 'very long'. When the length is much greater than the diameter, end (fringe) effects are negligible over most of the interior, so the field there is uniform. 'Zero' describes the field outside; 'non-uniform' and 'highest at mid-point' describe short coils or bar-magnet-like fields, not the interior of a long solenoid.
- Field inside a long solenoid: B = mu0 x n x I, uniform and directed along the axis (n = turns per unit length).
- The field outside a long solenoid is approximately zero.
- A current-carrying solenoid behaves like a bar magnet, developing a north and a south pole.
- Uniformity breaks down near the ends because of edge (fringe) effects.
Inside a LONG solenoid the field is uniform; it weakens at the ends and is nearly zero outside.
- Saying the field is zero inside — zero is the OUTSIDE field of a long solenoid.
- Ignoring the word 'long' — a short coil does NOT give a uniform interior field.
Solenoids are asked as 'field inside/outside' or 'B = ?' — remember uniform inside (mu0 x n x I) and nearly zero outside.
No directly related past PYQ was found.
- practice — not a real PYQ
The magnetic field inside a long solenoid carrying current I with n turns per unit length is
- (a)mu0 x n x I
- (b)mu0 x I / (2 x pi x r)
- (c)mu0 x I / (2 x r)
- (d)zero
Answer(a) mu0 x n x I — uniform along the axis inside the solenoid.
- practice — not a real PYQ
The magnetic field outside a very long current-carrying solenoid is approximately
- (a)uniform and strong
- (b)zero
- (c)infinite
- (d)equal to the field inside
Answer(b) zero — the field is confined almost entirely to the interior.