1. Magnetic field and field lines
A compass needle is a small bar magnet; its north-seeking end is the north pole. Like poles repel, unlike poles attract. The magnetic field is the region around a magnet in which its force can be detected; it has magnitude and direction. The direction at a point is the direction in which the north pole of a compass needle points there.
Field lines (traced with iron filings or step by step with a compass):
- outside a magnet they run from N to S; inside, from S to N — they are closed curves;
- the tangent at a point gives the field direction;
- they are closer together where the field is stronger (near the poles);
- no two field lines cross — a compass at the crossing would point two ways at once.
| Property of magnetic field lines | What it tells us |
|---|---|
| They come out of the N pole and go into the S pole outside the magnet; inside, they run from S to N | Field lines are closed curves |
| The tangent at any point gives the field direction | A compass needle sets along the tangent |
| Lines are crowded where the field is strong | The field is strongest near the poles |
| No two lines ever cross | At a crossing, a compass would point two ways at once — impossible |
Key terms
- Magnetic field
The region around a magnet in which its magnetic force can be detected.
- Magnetic field line
The path along which a free north pole would tend to move; its tangent gives the field direction.
2. Magnetic field of a current: straight wire, loop and solenoid
Oersted (1820): a compass needle near a wire is deflected when current flows; reversing the current reverses the deflection. A current produces a magnetic field.
Straight wire: field lines are concentric circles centred on the wire, in planes perpendicular to it. The field is stronger for a larger current and weaker further from the wire. Right-hand thumb rule: grip the wire with the right hand, thumb along the current; the curled fingers give the direction of the field lines.
Circular loop: near the wire the lines are circles; at the centre they are nearly straight, perpendicular to the plane of the loop, because every part of the loop contributes in the same direction. The field at the centre increases with the current and with the number of turns (the fields of the turns add up).
Solenoid: a coil of many closely wound turns in the shape of a cylinder. Its field is like that of a bar magnet, and inside it is uniform (parallel straight lines). With the right hand, fingers curled along the current, the thumb points to the N pole; equivalently, current anticlockwise as seen from an end makes that end N, clockwise makes it S.
Electromagnet: a solenoid with a soft iron core. It is a magnet only while current flows; it is made stronger by more turns, more current and a suitable core; its polarity reverses with the current. Soft iron loses its magnetism quickly when switched off; steel keeps it (permanent magnets).
Key terms
- Right-hand thumb rule
Thumb along the current, curled fingers give the direction of the magnetic field.
- Solenoid
A long coil of many circular turns of insulated wire wound closely in the shape of a cylinder.
- Electromagnet
A solenoid with a soft iron core, magnetised only while current flows.
3. Force on a current-carrying conductor
A conductor carrying current in a magnetic field experiences a force (suggested by Ampère: the magnet and the conductor exert equal and opposite forces on each other). The force is perpendicular to both the current and the field.
- Reversing the current or the field reverses the force; reversing both leaves it unchanged.
- The force is largest when the current is perpendicular to the field and zero when it is parallel.
- It increases with the current and with the strength of the field.
Fleming’s left-hand rule: stretch the thumb, first finger and second finger of the left hand mutually perpendicular. First finger → field, second finger → current, thumb → force (motion). For moving electrons, the current is opposite to their motion.
Uses: electric motors (fans, mixers, pumps), loudspeakers, moving-coil meters.
Key terms
- Fleming’s left-hand rule
First finger field, second finger current, thumb motion, on the left hand held mutually perpendicular.
4. Direct current and alternating current
Direct current (DC) flows in one direction only (cells, batteries, solar panels). Alternating current (AC) reverses direction periodically (power stations, mains).
In India the mains supply is AC at 220 V and 50 Hz: 50 cycles per second, period s ms; the current reverses 100 times each second, i.e. every s.
Advantage of AC: it can be transmitted over long distances without much loss of energy (its voltage is easily stepped up and down).
- AC: reverses every 10 ms (50 Hz)
- DC: steady, one direction
Formulas
- Period of Indian mains AC
Key terms
- Direct current
Current that flows in one direction only.
- Alternating current
Current that reverses its direction periodically; 50 Hz in India.
5. Domestic electric circuits and safety
Mains power reaches a house through live (red insulation, high potential) and neutral (black, about zero potential) wires; the potential difference between them is 220 V. It passes through the main fuse and the electricity meter to a distribution box, which feeds separate circuits with their own fuses: a 15 A power circuit for high-power appliances (geyser, heater, air conditioner) and a 5 A lighting circuit (bulbs, fans). Appliances are connected in parallel, each with a switch in the live wire.
Earth wire (green) — joined to a metal plate deep in the ground and to the metal bodies of appliances. If the live wire touches the body, the earth wire gives a low-resistance path, the body stays at earth potential, and the user does not get a severe shock; the large current also blows the fuse.
Short-circuiting — the live and neutral wires come into direct contact (damaged insulation, a fault in an appliance); the current rises abruptly. Overloading — too much current is drawn: too many appliances on one socket, or an accidental rise in supply voltage. Protection: the fuse (low melting point, in series in the live wire) melts; a miniature circuit breaker (MCB) switches off and can be reset. The fuse protects wiring and appliances; earthing protects people.
| Wire | Colour | Role |
|---|---|---|
| Live | Red | High potential; current to the appliance |
| Neutral | Black | Return path; about zero potential |
| Earth | Green | Safety path to the ground |
Key terms
- Earthing
Connecting the metal body of an appliance to the ground through a low-resistance wire for safety.
- Short circuit
Direct contact between live and neutral wires, causing a sudden large current.
- Overloading
Drawing more current from a circuit than it is designed to carry.
6. Motor, electromagnetic induction and generator (formative assessment only)
Taught for projects and internal (formative) assessment; not assessed in the 2026-27 board theory paper.
Electric motor: converts electrical energy into mechanical energy. A coil in a magnetic field experiences forces in opposite directions on its two sides and rotates; split rings (commutator) reverse the current in the coil every half rotation so that it keeps turning the same way.
Electromagnetic induction: a current is induced in a coil when the magnetic field linked with it changes — for example while a magnet is moved into or out of the coil; no current flows while the magnet is still.
Electric generator: uses electromagnetic induction to convert mechanical energy into electrical energy; power-station generators produce AC.
Key terms
- Electromagnetic induction
Production of a current in a coil by a changing magnetic field.
Exam tips
Common mistakes
- Field-line diagrams without arrows, with crossing lines, or with arrows from S to N outside the magnet.
- Using the left hand for the field of a current (use the right-hand thumb rule) or the right hand for the force (use Fleming’s left-hand rule).
- Forgetting that the left-hand rule uses conventional current — reverse the direction for electrons.
- Saying the force is largest when the wire is along the field (it is zero then).
- Writing 50 reversals per second for 50 Hz AC (it is 100).
- Calling the neutral wire "the earth wire", or putting switches and fuses in the neutral wire.
- Saying a fuse protects people from shocks (that is the earth wire; the fuse protects the circuit).
How marks are usually awarded
- Field-line diagrams: correct shape (1) + arrows and labels (1).
- Rule questions: statement of the rule (1) + correct application with direction (1).
- Domestic-circuit answers: key phrases — "low-resistance path", "earth potential", "live and neutral in direct contact", "low melting point, in series".
- Case-based questions: two 1-mark parts from the passage and a 2-mark part with an internal choice.
- Motor, induction and generator: formative only in 2026-27.