The rule that determines the direction of a magnetic field produced around a straight conductor carrying current is:

Correct — A, Right-hand thumb rule. Grip the wire with your right hand so that the extended thumb points along the direction of the conventional current; the curl of your fingers then gives the direction in which the magnetic field lines encircle the wire. The field around a straight current-carrying conductor consists of concentric circles in the plane perpendicular to the wire, so a rule of this kind is exactly what is needed — a rule that returns a sense of rotation rather than a single direction. It is also called Maxwell's corkscrew rule, because driving a right-handed corkscrew in the direction of the current turns it the way the field points. The rule works in reverse as well, which is how it is usually examined: if you are told the field lines run anti-clockwise as seen from one end, the same right hand gives you the direction of the current.

• (b)Fleming's left-hand rule — A different question altogether — it gives the direction of the FORCE on a current-carrying conductor placed in a magnetic field, with the forefinger for field, the middle finger for current and the thumb for the resulting motion. It is the motor rule, and it presupposes a field rather than locating one.
• (c)Fleming's right-hand rule — The generator rule. It gives the direction of the INDUCED current when a conductor is moved through a magnetic field — again with the field already given. It does not tell you how a current creates a field.
• (d)Hund's rule — Not an electromagnetism rule at all. Hund's rule belongs to atomic structure and governs how electrons occupy orbitals of equal energy, singly and with parallel spins before any pairing.

A current-carrying conductor sets up a magnetic field around itself — the discovery Oersted made when a compass needle deflected beside a wire. For a long straight conductor the field lines are concentric circles centred on the wire, growing weaker with distance and stronger with current. For a circular loop the lines are circles near the wire that straighten towards the centre of the loop, and for a solenoid the field inside is uniform and resembles that of a bar magnet. Each geometry has its own field pattern, but the same right-hand rule fixes the sense of all of them.

Three of the four options are genuine electromagnetism rules, so the item is testing whether you can separate them by what each takes as given and what it returns. Sort them by that: the right-hand thumb rule takes a current and returns a field; Fleming's left-hand rule takes a current and a field and returns a force; Fleming's right-hand rule takes a motion and a field and returns a current. Left hand for the motor, right hand for the generator is the standard mnemonic for the last two. Hund's rule is the odd one out, planted to catch a candidate matching on the word 'rule' rather than on the physics.

• The magnetic field around a long straight current-carrying conductor consists of concentric circles in the plane perpendicular to the conductor.
• The right-hand thumb rule, also called Maxwell's corkscrew rule, gives the sense of that field: thumb along the current, curled fingers along the field lines.
• Fleming's left-hand rule gives the direction of the force on a current-carrying conductor in a magnetic field — the motor rule.
• Fleming's right-hand rule gives the direction of the current induced in a conductor moving in a magnetic field — the generator rule.
• Hund's rule belongs to atomic structure, not electromagnetism: electrons occupy orbitals of equal energy singly before pairing.

• Swapping Fleming's two rules — left hand for the motor, right hand for the generator.
• Matching on the word 'rule' and picking Hund's rule, which belongs to a different branch of physics.
• Confusing the right-hand thumb rule with Fleming's right-hand rule; the first locates a field from a current, the second locates an induced current from a motion.

As a rule-to-situation matching item, as a 'which rule does not give the direction of the magnetic field' item, or as a diagram question giving field lines and asking for the direction of the current.

• practice — not a real PYQ

  The direction of the force experienced by a current-carrying conductor placed at right angles to a magnetic field is given by which one of the following?

  • (a)Right-hand thumb rule
  • (b)Fleming's left-hand rule
  • (c)Fleming's right-hand rule
  • (d)Lenz's law

  Answer(b) Fleming's left-hand rule — the motor rule, with forefinger for field, middle finger for current and thumb for the force.

• practice — not a real PYQ

  The magnetic field lines around a long straight current-carrying conductor take which one of the following forms?

  • (a)Straight lines parallel to the conductor
  • (b)Concentric circles centred on the conductor
  • (c)Radial lines pointing away from the conductor
  • (d)Ellipses with the conductor at one focus

  Answer(b) Concentric circles centred on the conductor — in the plane perpendicular to it, with the sense given by the right-hand thumb rule.