Chapter 12: Magnetic Effects of Electric Current
1. Magnets and Magnetic Field
A magnet attracts materials like iron, cobalt and nickel. A freely suspended bar magnet always comes to rest in the north-south direction. The end pointing towards geographic north is the north pole (N); the other end is the south pole (S). Like poles repel; unlike poles attract.
Magnetic field: The region surrounding a magnet in which the force of the magnet can be detected. It is a vector quantity — it has both direction and magnitude. SI unit: tesla (T).
2. Magnetic Field Lines
Magnetic field lines are imaginary lines used to represent a magnetic field. If iron filings are sprinkled around a bar magnet, they arrange in a pattern of curved lines from one pole to the other.
Properties of magnetic field lines:
- Outside the magnet, field lines emerge from the north pole and merge at the south pole; inside the magnet, they go from south pole to north pole. Thus field lines are closed curves.
- The direction of the field at a point is given by the tangent to the field line at that point (direction in which a north pole would move).
- Field is stronger where lines are crowded (e.g., near the poles) and weaker where they are far apart.
- No two field lines ever cross — if they did, the compass needle at the point of intersection would point in two directions at once, which is impossible.
3. Magnetic Field due to a Current-Carrying Conductor
Oersted's experiment: Hans Christian Oersted (1820) found that a compass needle placed near a current-carrying wire gets deflected, showing that electricity and magnetism are linked — a current-carrying conductor produces a magnetic field around it.
(a) Straight conductor
The magnetic field lines around a straight current-carrying conductor are concentric circles centred on the wire, lying in planes perpendicular to it. The magnitude of the field:
Right-hand thumb rule: Imagine holding the current-carrying straight conductor in your right hand so that the thumb points in the direction of current. Then the curled fingers show the direction of the magnetic field lines around the conductor. (Also called Maxwell's corkscrew rule.)
(b) Circular loop
Every point of a circular loop behaves like a small straight wire, so concentric circles of field surround every section. Near the wire the circles are small; towards the centre of the loop the arcs appear as straight lines — the field at the centre is uniform and perpendicular to the plane of the loop.
- B at the centre ∝ current I
- B at the centre ∝ 1/r (radius of the loop)
- For a coil of n turns, the field is n times that of a single turn (fields of all turns add up as current in each turn has the same direction).
(c) Solenoid
A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. The field pattern of a current-carrying solenoid is similar to that of a bar magnet — one end behaves as N-pole, the other as S-pole. Inside the solenoid the field lines are parallel straight lines, meaning the field is uniform (same at all points inside).
4. Electromagnet
An electromagnet is a solenoid with a soft iron core placed inside it. The strong uniform field inside the solenoid magnetises the core, producing a powerful temporary magnet whose magnetism can be switched on/off with the current. Soft iron is used because it gains and loses magnetism quickly. Uses: electric bells, cranes to lift scrap iron, loudspeakers, MRI machines, relays.
5. Force on a Current-Carrying Conductor in a Magnetic Field
Just as a current exerts force on a magnet (Oersted), by Newton's third law a magnet exerts an equal and opposite force on a current-carrying conductor. In the classroom activity, an aluminium rod suspended between the poles of a horseshoe magnet gets displaced when current flows; the displacement reverses if either the direction of current or the direction of the field is reversed.
The force is largest when the current is perpendicular to the magnetic field, and zero when parallel.
Fleming's Left-Hand Rule (for force on conductor / motor): Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular. If the Forefinger points along the magnetic Field and the Middle finger along the Current, then the Thumb gives the direction of Force (motion) on the conductor.
6. Electric Motor
Principle: A current-carrying rectangular coil placed in a magnetic field experiences forces (Fleming's left-hand rule) on its two arms in opposite directions, producing a turning effect — it converts electrical energy into mechanical energy.
Main parts:
| Part | Function |
|---|---|
| Armature coil (ABCD) | Rectangular coil of insulated copper wire that rotates between the magnet poles |
| Strong field magnet (N, S) | Provides the magnetic field in which the coil rotates |
| Split-ring commutator (P, Q) | Two halves of a metal ring; reverses the direction of current in the coil every half rotation so that the coil keeps rotating in the same direction |
| Brushes (X, Y) | Carbon contacts pressing on the split rings; connect the rotating coil to the battery |
Working (in words): Current enters the coil; arm AB (near N-pole) experiences a downward force and arm CD an upward force (by Fleming's left-hand rule), so the coil rotates anticlockwise. After half a rotation, the split rings interchange contact with the brushes, reversing the current in the coil; the forces on the arms also reverse, so the rotation continues in the same direction. Commercial motors use (i) an electromagnet instead of a permanent magnet, (ii) a coil of many turns, and (iii) a soft iron core on which the coil is wound (coil + core = armature) to increase power.
7. Electromagnetic Induction
Discovered by Michael Faraday (1831). When the magnetic field around a conductor changes (or a conductor moves relative to a magnetic field), a current is induced in the conductor. This is electromagnetic induction; the current is called induced current and the potential difference, induced potential difference.
- Activity 1 (coil and magnet): Pushing a bar magnet into a coil deflects the galvanometer one way; withdrawing it deflects the opposite way; keeping the magnet still gives no deflection. Moving the coil instead of the magnet works equally well — relative motion is what matters.
- Activity 2 (two coils): When current in a primary coil is switched on or off, a momentary current is induced in a nearby secondary coil, because the magnetic field through it changes. Steady current induces nothing.
Fleming's Right-Hand Rule (for induced current / generator): Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular. If the forefinger points along the field and the thumb along the direction of motion of the conductor, then the middle finger shows the direction of the induced current.
A galvanometer is an instrument that detects current in a circuit; its pointer deflects to either side of zero depending on the direction of current.
8. Electric Generator
Principle: Electromagnetic induction — when a coil is rotated in a magnetic field, an induced current is produced in it (direction by Fleming's right-hand rule). It converts mechanical energy into electrical energy.
Construction (AC generator): a rectangular coil ABCD rotated between the poles of a magnet; the two ends of the coil are connected to two full slip rings, on which two brushes press and lead current to the external circuit.
Working: As the axle rotates the coil, arm AB moves up while CD moves down, inducing current along ABCD. After half a rotation the arms exchange positions, so the induced current reverses direction every half rotation. Such periodically reversing current is alternating current (AC). In India AC reverses every 1/100 s — frequency 50 Hz.
DC generator: Replace the two slip rings with a split-ring commutator; then one brush always stays in contact with the arm moving up and the other with the arm moving down, so the current in the external circuit flows in one direction only (DC).
| Alternating current (AC) | Direct current (DC) |
|---|---|
| Reverses direction periodically (50 Hz in India) | Flows in one direction only |
| Can be transmitted over long distances with little energy loss | More energy is lost in long-distance transmission |
| Source: AC generator, mains supply | Source: cell, battery, DC generator |
9. Domestic Electric Circuits
Power reaches homes through a mains supply of 220 V, 50 Hz via three wires:
- Live wire (red insulation) — at 220 V; carries current to appliances.
- Neutral wire (black insulation) — return path; at nearly zero potential. Potential difference between live and neutral = 220 V.
- Earth wire (green insulation) — connected to a metal plate buried deep in the earth; a safety measure for appliances with metallic bodies (iron, refrigerator, toaster). It provides a low-resistance path so that any leakage current flows to the earth and keeps the appliance body at earth potential, saving the user from severe electric shock.
In a house, separate circuits are used: a 15 A circuit for high-power appliances (geyser, air cooler) and a 5 A circuit for bulbs and fans. All appliances are connected in parallel so each gets the full 220 V, works independently, and can have its own switch.
Overloading: Current exceeding the rated safe value of a circuit. Causes: (i) connecting too many appliances to a single socket, (ii) accidental rise in supply voltage, (iii) short-circuiting. It can overheat wires and cause fire.
Short-circuiting: When the live wire directly touches the neutral wire (e.g., insulation damage), the resistance of the circuit becomes very small, so the current rises abruptly and the wires overheat — this may cause fire/sparks.
Electric fuse: The most important safety device — a wire of a metal/alloy of low melting point connected in series with the live wire. When current exceeds the safe limit (overload/short circuit), the fuse wire heats up and melts, breaking the circuit and protecting the appliances and wiring.