Magnetic Effects of Electric Current

A MAGNETIC FIELD is the region around a magnet in which its force can be detected. It has both MAGNITUDE and DIRECTION, so it is a vector quantity. It is represented by MAGNETIC FIELD LINES.

FOUR PROPERTIES OF FIELD LINES (all four are asked): (1) OUTSIDE the magnet they run from the NORTH pole to the SOUTH pole; INSIDE the magnet they run from SOUTH to NORTH — so every field line is a CLOSED, continuous curve. (2) The direction of the field at a point is the direction in which the NORTH pole of a small compass placed there points. (3) Lines are CROWDED where the field is STRONG and spread out where it is weak. (4) TWO FIELD LINES NEVER INTERSECT.

WHY CAN TWO FIELD LINES NEVER CROSS? Because if they did, there would be TWO different directions of the magnetic field at the point of intersection — and the field can only have one direction at any point. (A guaranteed 1–2 mark question.)

A CURRENT-CARRYING STRAIGHT CONDUCTOR produces a magnetic field in the form of CONCENTRIC CIRCLES around it, in a plane perpendicular to the wire. The circles get larger and the field gets WEAKER as you move away.

RIGHT-HAND THUMB RULE (also called Maxwell's corkscrew rule): hold the current-carrying conductor in your RIGHT hand with the THUMB pointing along the direction of the CURRENT; then your CURLED FINGERS give the direction of the MAGNETIC FIELD LINES.

For a straight conductor the field is DIRECTLY proportional to the CURRENT (B ∝ I) and INVERSELY proportional to the DISTANCE from the wire (B ∝ 1/r).

FIELD DUE TO A CIRCULAR LOOP: at every point on the wire the field lines are concentric circles; as you approach the CENTRE the arcs grow larger, and at the centre they appear as STRAIGHT LINES all in the same direction. So the field at the centre is strong and nearly uniform.

THE FIELD AT THE CENTRE OF A LOOP INCREASES IF: (a) the CURRENT increases; (b) the RADIUS of the loop decreases; (c) the NUMBER OF TURNS n increases — with n turns the field becomes n TIMES as strong, because the field due to each turn adds in the SAME direction.

A SOLENOID is a long coil of many circular turns of insulated copper wire. Its field pattern is EXACTLY like that of a BAR MAGNET — one end acts as a NORTH pole and the other as a SOUTH pole. INSIDE the solenoid the field lines are parallel straight lines, so the field inside is UNIFORM (same strength and direction at every point).

ELECTROMAGNET: place a SOFT IRON core inside a solenoid; the iron becomes a strong magnet while the current flows. SOFT IRON is used, NOT steel, because soft iron LOSES its magnetism as soon as the current is switched off — which is exactly what an electromagnet needs.

FORCE ON A CURRENT-CARRYING CONDUCTOR: a conductor carrying current, placed in a magnetic field, experiences a FORCE. The force is the GREATEST when the direction of the current is at RIGHT ANGLES (90°) to the direction of the field, and it is ZERO when the current is parallel to the field.

FLEMING'S LEFT-HAND RULE (used for the MOTOR): stretch the THUMB, FOREFINGER and MIDDLE FINGER of your LEFT hand so that they are MUTUALLY PERPENDICULAR. FOREFINGER → direction of the magnetic FIELD. MIDDLE FINGER → direction of the CURRENT. THUMB → direction of the FORCE (the motion). Memory aid: Fore–Field, Middle–Current, Thumb–Motion.

[BEYOND THE CURRENT CBSE SYLLABUS — understand it, but it will NOT be asked in the board exam] ELECTRIC MOTOR — converts ELECTRICAL energy into MECHANICAL energy. A rectangular coil is placed between the poles of a magnet. The two sides of the coil carry current in OPPOSITE directions, so by Fleming's left-hand rule the forces on them act in opposite directions, and the coil ROTATES.

[BEYOND THE CURRENT CBSE SYLLABUS — understand it, but it will NOT be asked in the board exam] THE SPLIT RING COMMUTATOR: after every HALF rotation the split ring REVERSES the direction of the current in the coil. Without it the coil would stop after half a turn and rotate back. With it, the force keeps acting in the same rotational sense and the coil keeps spinning. So the FUNCTION of the split ring is to reverse the current, and it acts as a COMMUTATOR.

[BEYOND THE CURRENT CBSE SYLLABUS — understand it, but it will NOT be asked in the board exam] ELECTROMAGNETIC INDUCTION (Faraday): when the magnetic field through a coil CHANGES, a current is INDUCED in the coil — with no cell in that circuit. Move a magnet towards a coil and the galvanometer deflects one way; move it away and it deflects the other way; hold it still and there is NO deflection. What matters is the RELATIVE MOTION between the coil and the magnet.

[BEYOND THE CURRENT CBSE SYLLABUS — understand it, but it will NOT be asked in the board exam] FLEMING'S RIGHT-HAND RULE (used for the GENERATOR): stretch the THUMB, FOREFINGER and MIDDLE FINGER of your RIGHT hand mutually perpendicular. FOREFINGER → the magnetic FIELD. THUMB → the direction of MOTION of the conductor. MIDDLE FINGER → the direction of the INDUCED CURRENT.

LEFT HAND vs RIGHT HAND — do not mix these up. LEFT hand = MOTOR (current is supplied, and you find the FORCE). RIGHT hand = GENERATOR (motion is supplied, and you find the INDUCED CURRENT). (NOTE: Fleming's LEFT-hand rule is IN the syllabus; the RIGHT-hand rule and the generator are not.)

[BEYOND THE CURRENT CBSE SYLLABUS — understand it, but it will NOT be asked in the board exam] ELECTRIC GENERATOR — converts MECHANICAL energy into ELECTRICAL energy (the opposite of a motor). A coil is rotated between the poles of a magnet, and a current is induced in it.

[BEYOND THE CURRENT CBSE SYLLABUS — understand it, but it will NOT be asked in the board exam] AC vs DC GENERATOR — the ONLY difference is the rings. An AC generator uses TWO SEPARATE SLIP RINGS, so the current in the outer circuit REVERSES every half rotation → ALTERNATING CURRENT. A DC generator uses a SPLIT RING (a commutator), which cancels that reversal, so the current in the outer circuit always flows in the SAME direction → DIRECT CURRENT.

ADVANTAGE OF AC OVER DC (a named syllabus line and a standard 1-mark question): alternating current can be TRANSMITTED OVER LONG DISTANCES WITHOUT MUCH LOSS OF ENERGY. That is why electrical power is generated and distributed as AC.

AC IN INDIA: the alternating current changes direction after every 1/100 second, so its FREQUENCY is 50 Hz. (One complete cycle takes 1/50 s, and the current reverses twice in each cycle.) The big advantage of AC over DC is that it can be transmitted over long distances without much loss of energy.

DOMESTIC CIRCUITS — THREE WIRES. LIVE wire: RED or brown insulation, at 220 V. NEUTRAL wire: BLACK or blue insulation, at 0 V. EARTH wire: GREEN insulation, connected to a metal plate deep in the ground. The potential difference between live and neutral is 220 V.

EARTHING is a SAFETY measure. The earth wire is connected to the METAL BODY of appliances such as an electric iron, heater or refrigerator. If the live wire ever touches the metal casing, the current flows harmlessly to the EARTH instead of through the person who touches it, because the earth wire provides a low-resistance path.

THE FUSE is connected in the LIVE wire and protects the circuit: it is a wire of HIGH resistance and LOW melting point, which MELTS and breaks the circuit if the current exceeds a safe value.

TWO CAUSES OF EXCESSIVE CURRENT: (1) SHORT CIRCUIT — the LIVE and NEUTRAL wires come into direct contact, so the resistance becomes very small and the current very large; (2) OVERLOADING — too many appliances are connected to a single socket, so the total current drawn is too high. In both cases the fuse melts first and prevents damage or fire.

DOMESTIC APPLIANCES ARE CONNECTED IN PARALLEL, each with its own switch, so that every appliance gets the full 220 V, can be switched independently, and keeps working if another fails.

Field of a straight conductor

concentric circles; B ∝ I and B ∝ 1/r

Direction from the RIGHT-HAND THUMB rule (thumb = current, curled fingers = field).

Field at the centre of a loop

increases with I, increases with n turns, decreases with radius r

With n turns the field is n times as strong — each turn's field adds in the same direction.

Solenoid

field pattern = that of a BAR MAGNET; field INSIDE is UNIFORM

Add a SOFT IRON core and it becomes an ELECTROMAGNET.

Fleming's LEFT-hand rule

Forefinger = FIELD · Middle finger = CURRENT · Thumb = FORCE (motion)

LEFT hand = MOTOR. Current is given; you find the force.

Fleming's RIGHT-hand rule

Forefinger = FIELD · Thumb = MOTION · Middle finger = INDUCED CURRENT

RIGHT hand = GENERATOR. Motion is given; you find the induced current.

Motor vs generator

Motor: electrical → mechanical. Generator: mechanical → electrical

The motor uses a split ring; the AC generator uses two slip rings.

AC vs DC generator

AC = two SLIP RINGS. DC = one SPLIT RING (commutator)

That single difference is the whole answer to 'how does a DC generator differ from an AC one?'

AC frequency in India

reverses every 1/100 s → frequency = 50 Hz

One full cycle = 1/50 s; the current reverses twice per cycle.

Domestic wiring

Live = red/brown (220 V) · Neutral = black/blue (0 V) · Earth = green

The fuse goes in the LIVE wire. Appliances are wired in PARALLEL.

What is actually examinable here

IN: field lines · thumb rule · loop · solenoid · Fleming LEFT · AC/DC · domestic circuits

OUT (rationalised away): electric motor, electromagnetic induction, Fleming RIGHT, generator. They are covered for understanding and for Class 11-12, not for the board exam.