The rule that determines the direction of a magnetic field produced around a straight conductor carrying current is :
- A.Right-hand thumb rule
- B.Fleming's left-hand rule
- C.Fleming's right-hand rule
- D.Hund's rule
Correct answer
Explanation
The correct answer is A, Right-hand thumb rule. If you hold a current-carrying straight wire in your right hand with the thumb pointing along the current, your curled fingers show the direction of the magnetic field lines around it.
Hans Christian Oersted found in 1820 that a current in a wire deflects a nearby compass needle, which showed that an electric current produces a magnetic field. Around a straight conductor the field lines are concentric circles centred on the wire, and the field becomes weaker farther from the wire. The rule is also called Maxwell's corkscrew rule.
B is wrong: Fleming's left-hand rule gives the direction of the force on a current-carrying conductor placed in a magnetic field, the principle of the electric motor.
C is wrong: Fleming's right-hand rule gives the direction of induced current when a conductor moves in a magnetic field, the principle of the generator.
D is wrong: Hund's rule is from chemistry; it describes how electrons fill orbitals of equal energy.
Exam tip: field around a wire = right-hand thumb rule; motor = Fleming's left hand; generator (dynamo) = Fleming's right hand.
Practice Questions
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1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously
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3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
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- B.Only two
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- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
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