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Which rule is used to find the direction of the magnetic field around a straight current-carrying conductor?
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Answer: Option B — Maxwell's corkscrew rule (right-hand thumb rule)
Detailed Solution
In 1820, the Danish scientist Hans Christian Ørsted discovered that a compass needle is deflected when placed near a wire carrying an electric current. This showed that an electric current produces a magnetic field around it. The magnetic field lines around a long, straight conductor form concentric circles, centred on the wire, in planes perpendicular to it. The field becomes weaker as the distance from the wire increases, and stronger when the current increases.
The direction of these field lines can be found using two equivalent rules:
Maxwell's corkscrew rule: imagine driving a corkscrew (or a right-handed screw) in the direction of the current. The direction in which the handle of the corkscrew turns gives the direction of the magnetic field lines.
Right-hand thumb rule: imagine holding the current-carrying wire in your right hand so that your thumb points in the direction of the current. Your fingers then wrap around the wire in the direction of the magnetic field.
Inside a solenoid (a long coil of wire), the field lines are nearly parallel straight lines, showing a uniform field, similar to that of a bar magnet. A solenoid with a soft iron core forms an electromagnet.
Other important rules:
Fleming's left-hand rule gives the direction of the force on a current-carrying conductor in a magnetic field. It is used for electric motors. The thumb, forefinger and middle finger, held at right angles, show the force (motion), the field and the current respectively.
Fleming's right-hand rule gives the direction of the induced current when a conductor moves in a magnetic field. It is used for electric generators (dynamos).
Lenz's law states that the direction of an induced current is such that it opposes the change that produced it.
Why not the other options?
- ✗Fleming's left-hand rule is for the force on a conductor (motors).
- ✗Fleming's right-hand rule is for induced current (generators).
- ✗Lenz's law gives the direction of induced current in terms of opposing change.
Quick tip
Remember: Field around a wire = right-hand thumb rule or Maxwell's corkscrew rule; motor = Fleming's left-hand rule; generator = Fleming's right-hand rule. Ørsted (1820) discovered the effect.
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Electricity & Magnetism — Preparation
This question tests your understanding of Electricity & Magnetism, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers electric charge, current, voltage and resistance, Ohm's law, series and parallel circuits, electric power and energy, the heating effect of current, magnets and magnetic fields, electromagnetic induction, and devices such as motors, generators, transformers and fuses. Questions commonly ask about units (the ampere, volt, ohm and kilowatt-hour), why household appliances are connected in parallel, what a fuse is made of and why, and which device converts mechanical energy into electrical energy (a generator). Remember that a transformer works only with alternating current. Practise the electricity and magnetism questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.