Magnetic Effects of Electric Current
Field lines, right-hand thumb rule, solenoids, motors, induction and domestic circuits.
Based on NCERT Class 10 Science, Chapter 12
12.1 Magnetic field and field lines
- A magnet has a field around it; a compass needle is a tiny magnet that aligns with this field.
- Field lines run from the north pole to the south pole outside the magnet and from south to north inside it.
- Field lines never intersect — at any point the field can have only one direction. Crowded lines mean a stronger field.
Q1.List three properties of magnetic field lines.
2 marks12.2 Magnetic field due to a current-carrying conductor
- Oersted found that a current-carrying wire deflects a nearby compass needle: current produces a magnetic field.
- For a straight wire the field lines are concentric circles; the field strength increases with current and decreases with distance from the wire.
- Right-hand thumb rule: point the right thumb along the current, and the curl of the fingers gives the direction of the field lines.
- In a circular loop the field near the wire is circular, and at the centre the lines are nearly straight; with n turns the field becomes n times stronger.
B ∝ I, B ∝ 1/r (straight wire)Q1.State the right-hand thumb rule and how the field around a straight current-carrying wire depends on current and distance.
3 marks12.2.4 Solenoid and electromagnet
- A solenoid is a coil of many circular turns of insulated wire. Its field pattern is like that of a bar magnet, with one end a north pole and the other a south.
- Inside the solenoid the field is uniform and parallel.
- Placing a soft iron core inside a solenoid makes an electromagnet, whose strength can be switched on and off.
Watch it happen — Solenoid Magnetic Field
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Live result
The field inside a long solenoid is uniform and behaves like a bar magnet outside.
Q1.What is a solenoid? How can it be used to make an electromagnet?
3 marks12.3 Force on a current-carrying conductor & the electric motor
- A current-carrying conductor placed in a magnetic field experiences a force; the force is maximum when the current is perpendicular to the field.
- Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular — forefinger = field, middle finger = current, thumb = force (motion).
- An electric motor uses this force on a rectangular coil; a split-ring commutator reverses the current every half rotation so the coil keeps spinning in the same direction.
Watch it happen — Force on a Current-Carrying Wire
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Live result
Force is maximum when the wire is perpendicular to the field and zero when parallel.
Q1.State the right-hand thumb rule and how the field around a straight current-carrying wire depends on current and distance.
3 marksElectromagnetic induction and the generator
- Faraday showed that a changing magnetic field around a coil induces a current in it — electromagnetic induction.
- Fleming's right-hand rule gives the direction of the induced current: forefinger = field, thumb = motion, middle finger = induced current.
- An AC generator rotates a coil in a magnetic field; slip rings give alternating current, while a split ring gives DC. In India AC is supplied at 220 V and 50 Hz, so the current reverses direction 100 times per second.
Watch it happen — Electromagnetic Induction
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Live result
EMF = −N·ΔΦ/Δt. The minus sign (Lenz's law) shows the induced current opposes the change.
Q1.State Fleming's left-hand and right-hand rules and name one device based on each.
5 marks12.4 Domestic electric circuits
- Power reaches a house through a live wire (red/brown insulation), a neutral wire (black/blue) and an earth wire (green), with 220 V between live and neutral.
- Separate circuits are used: a 15 A circuit for heavy appliances such as geysers and coolers and a 5 A circuit for lights and fans.
- The earth wire connects the metal body of an appliance to the earth so any leakage current flows away safely.
- Overloading or a short circuit (live and neutral touching) causes a large current; the fuse or MCB melts/trips and protects the circuit.
Watch it happen — Domestic Circuit Safety
Controls
Live result
The fuse is placed in the live wire; earthing protects from shocks by giving leakage current a low-resistance path.
Q1.What is short-circuiting and overloading? What is the role of an earth wire?
2 marks