A current through a horizontal power line flows in east to west direction. What will be the direction of magnetic field at a point directly below it when viewed from east end?
- (a)Clockwise in a plane perpendicular to the wire
- (b)Anti-clockwise in a plane perpendicular to the wire
- (c)Clockwise in a plane of parallel to the wire
- (d)Anti-clockwise in a plane of parallel to the wire
Correct — A, Clockwise in a plane perpendicular to the wire. The magnetic field around a straight current-carrying wire forms concentric circles that lie in planes perpendicular to the wire. Viewed from the east end, the observer looks west, so the east-to-west current flows away from them (into the page). By the right-hand rule — thumb along the current, fingers curling in the direction of the field — the field circles clockwise as seen by this observer.
- (b)Anti-clockwise in a plane perpendicular to the wire — The plane is right, but the sense is wrong: with the current flowing away from the east-end observer, the right-hand rule gives a clockwise, not anti-clockwise, circulation.
- (c)Clockwise in a plane of parallel to the wire — The circulation sense is right, but the field lines of a straight wire lie in planes perpendicular to it, not parallel to it.
- (d)Anti-clockwise in a plane of parallel to the wire — Both parts are wrong — the plane should be perpendicular to the wire and the sense should be clockwise for this viewpoint.
A steady current in a long straight wire sets up a magnetic field whose lines are concentric circles centred on the wire, lying in planes at right angles to the wire. The direction of circulation is fixed by the right-hand rule (or Maxwell's corkscrew rule): point the right thumb along the current and the curling fingers show the field's sense.
Two things must be settled — the plane of the circles (always perpendicular to the wire) and the sense (clockwise or anti-clockwise), which depends on the observer's viewpoint. Fixing the viewpoint 'from the east end' makes the current point into the page, giving a clockwise field.
- Field lines of a straight current-carrying wire are concentric circles in planes perpendicular to the wire.
- Field strength B = μ₀I / (2πr) falls off with distance r from the wire.
- The right-hand rule: thumb along the current, curled fingers give the direction of the magnetic field.
- When the current points into the page (away from the viewer), the field circulates clockwise as seen by that viewer.
- Placing the field lines in a plane parallel to the wire — they are always perpendicular to it.
- Getting the clockwise/anti-clockwise sense wrong by ignoring the stated viewpoint.
Asked by giving a current direction and a viewpoint and asking the field's sense, or by asking the shape/plane of the field lines around a wire.
No directly related past PYQ was found.
- practice — not a real PYQ
The magnetic field lines produced around a long straight current-carrying wire are
- (a)Straight lines parallel to the wire
- (b)Concentric circles in planes perpendicular to the wire
- (c)Ellipses in the plane of the wire
- (d)Radial lines pointing away from the wire
Answer(b) Concentric circles in planes perpendicular to the wire — the standard pattern given by the right-hand rule.
- practice — not a real PYQ
The rule used to find the direction of the magnetic field around a straight current-carrying conductor is
- (a)Fleming's left-hand rule
- (b)Right-hand thumb (corkscrew) rule
- (c)Fleming's right-hand rule
- (d)Lenz's law
Answer(b) Right-hand thumb (corkscrew) rule — thumb along the current, curled fingers give the field.