A wire carrying current can deflect a compass needle. That single observation, made by accident in 1820, tied electricity and magnetism together forever.
That link now powers electric motors, generators, and every home’s wiring — and it is a favourite topic across competitive exams.
MCQ Questions🏛️ Must Know
Magnetic Fields and Field Lines
- Oersted's Discovery In 1820, Hans Christian Oersted found that a compass needle deflects near a current-carrying wire, linking electricity and magnetism.
- Magnetic Field A magnetic field is the region around a magnet where its force can be detected. Field lines represent this field.
- Field Line Direction Field lines emerge from the north pole and merge at the south pole, outside the magnet. Inside, they run south to north.
- Field Strength Field lines crowd closer together where the field is stronger, and spread apart where it is weaker.
- No Crossing Two magnetic field lines never cross. A crossing point would mean the compass needle points in two directions at once.
🏛️ Must Know
Magnetic Field of a Current-Carrying Wire, Loop and Solenoid
- Straight Wire A current-carrying straight wire produces a magnetic field of concentric circles around it, growing larger with distance from the wire.
- Right-Hand Thumb Rule Point the right thumb along the current; the curled fingers give the direction of the field lines around the wire.
- Circular Loop A current-carrying circular loop produces a field that acts like straight lines at its centre, and adds up with each turn.
- Solenoid A solenoid is a cylindrical coil of many turns. Its field pattern outside resembles a bar magnet, with a north and south end.
- Electromagnet A solenoid's uniform internal field can magnetise a soft iron core placed inside it, creating an electromagnet.
📘 Good to Know
Force on a Conductor and the Electric Motor
- Force on a Conductor A current-carrying conductor placed in a magnetic field experiences a force, perpendicular to both the current and the field.
- Fleming's Left-Hand Rule Point the first finger along the field and the second along the current; the thumb then shows the force's direction.
- Motor Principle A rectangular coil carrying current in a magnetic field experiences forces that rotate it, converting electrical energy to mechanical energy.
- Split Ring Commutator The split ring reverses the coil's current every half rotation, so the force keeps pushing it the same way, giving continuous spin.
- Commercial Motors Commercial motors use an electromagnet, many wire turns, and a soft iron core (the armature) to boost their power.
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🌟 Great to Know
Electromagnetic Induction and the Electric Generator
- Faraday's Discovery In 1831, Michael Faraday found that a moving magnet, or a changing magnetic field, can induce a current in a coil.
- Fleming's Right-Hand Rule Point the first finger along the field and the thumb along the conductor's motion; the middle finger shows the induced current.
- Generator Principle An electric generator rotates a coil inside a magnetic field, using Faraday's law to convert mechanical energy into electrical energy.
- AC vs DC Slip rings give alternating current (AC), which reverses direction periodically; a split-ring commutator instead gives direct current (DC).
- AC in India Most power stations generate AC. In India, it reverses direction every 1/100 second, a frequency of 50 hertz.
🌟 Great to Know
Domestic Electric Circuits and Safety
- Live and Neutral Household mains have a live wire, red insulation, and a neutral wire, black insulation, with 220 V between them in India.
- Earth Wire The green-insulated earth wire connects an appliance's metal body to the ground, protecting users from shocks if current leaks.
- Separate Circuits Homes usually run a 15 A circuit for high-power appliances and a 5 A circuit for lights and fans.
- Parallel Wiring Appliances are wired in parallel, so each gets the same voltage and works independently of the others.
- Short-Circuiting If live and neutral wires touch directly, current surges suddenly; this is a short circuit, and a fuse breaks it before damage occurs.
📝 Exam Point of View
NDA GAT — Four Real Magnetism Questions
- Question NDA/NA 2023-II: to convert an AC generator into a DC generator, what does one need to use? The correct answer is a split-ring type commutator.
Why A split ring keeps one brush always in contact with the arm moving in the same direction, giving unidirectional, DC output instead of AC. - Question NDA/NA 2022-II: a DC generator works on the principle of what? The correct answer is Faraday's laws of electromagnetic induction.
Why Rotating a coil in a magnetic field changes the flux through it, inducing a current — exactly the effect Faraday discovered in 1831. - Question NDA/NA 2022-I: the magnetic field outside a current-carrying straight wire depends on distance how? The correct answer is inversely on the distance from it.
Why The concentric field-line circles around a straight wire grow larger, and weaker, the further out they are from the wire. - Question NDA 2021-II: given a straight conductor with anti-clockwise field lines, which rule and direction gives the current? The correct answer is the right-hand thumb rule, current in the upward direction.
Why Curling the right-hand fingers anti-clockwise (as seen from above) points the thumb upward, so that is the current's direction.
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D4452 — In India, the alternating current supplied to homes reverses its direction at a frequency of how many hertz?D4451 — What is the role of the split ring in a simple electric motor?D4450 — Inside a current-carrying solenoid, the magnetic field lines run as parallel straight lines. What does this indicate about the field?D4449 — In an electric motor, which rule gives the direction of the force acting on a current-carrying coil placed in a magnetic field?🎲 Take a General Science Quiz
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