The SI unit of magnetic field strength (magnetic flux density) is the Tesla (T).
Note: 1 Tesla = 1 Weber per square metre (Wb/m²).

Outside a bar magnet, magnetic field lines emerge from the North pole and enter the South pole, forming closed loops.

The magnetic field lines around a straight current-carrying conductor are concentric circles centred on the wire.

At any given point in space, there can be only one direction of the magnetic field. If field lines intersected, it would imply two directions at a single point — which is physically impossible. Hence, they never cross.

The magnetic field inside a solenoid is given by $B = \mu_0 n I$, where $n$ is the number of turns per unit length. If the total number of turns $N$ is doubled (keeping length constant), $n$ doubles, so $B$ also doubles.

A Fuse (or MCB — Miniature Circuit Breaker) is used to protect household circuits from overloading.

The frequency of AC supply in India is 50 Hz (i.e., the current changes direction 50 times per second).

Fleming's Left-Hand Rule: Stretch the thumb, index finger, and middle finger of the left hand mutually perpendicular to each other. If the index finger points in the direction of the magnetic field (B), the middle finger in the direction of the current (I), then the thumb points in the direction of the force (F) on the conductor.

In India (as per IS/IEC standards), the live wire is coloured Brown. (Older wiring used Red for live.)

An electromagnet is a temporary magnet made by winding an insulated copper wire around a soft iron core and passing electric current through it. It behaves as a magnet only when current flows through the coil.

Diagram description: Closed loops emerging from the North pole, curving through the air, entering the South pole, and passing through the magnet internally from S to N.

Two properties:

1. Magnetic field lines emerge from the North pole and merge into the South pole outside the magnet; inside the magnet they travel from S to N — thus forming closed loops.
2. The closer the field lines, the stronger the magnetic field. Lines are most densely packed near the poles.

The force on the conductor acts in the East direction.

• AC (Alternating Current): The direction of current reverses periodically. In India, frequency = 50 Hz, voltage = 220 V. Example: Main electricity supply to homes.
• DC (Direct Current): The direction of current remains constant (one-way flow). Example: Current from a cell or battery (torch cells, car batteries).

Key difference: AC can be transmitted over long distances efficiently via transformers; DC cannot be stepped up/down easily.

1. The strength of an electromagnet can be controlled by varying the current or number of turns; a permanent magnet has fixed strength.
2. An electromagnet can be switched on and off as needed; a permanent magnet cannot be demagnetised easily for quick release of loads.

A solenoid is a long coil of insulated copper wire wound in the form of a helix (many close turns). When current flows:

• The magnetic field inside is strong, uniform, and parallel to the axis — similar to the field inside a bar magnet.
• One end acts as the North pole (field lines emerge) and the other as the South pole (field lines enter), determined by the direction of current flow (right-hand rule).
• It attracts magnetic materials and aligns with external fields, exactly like a bar magnet.

A three-pin plug includes the earth wire connection (the top, thicker pin). This wire provides a low-resistance path to the ground for any leakage current, protecting the user from electric shock. In a two-pin plug, no earth connection is present, so if the metal casing of an appliance becomes live (due to insulation failure), anyone touching it may receive a fatal shock. Hence, three-pin plugs are essential for all heavy metallic appliances (iron, geysers, refrigerators).

Short circuit: When the live and neutral wires come in direct contact with each other (without resistance), an extremely large current flows through the circuit. This can cause fire and damage to appliances.

Two causes:

1. The insulation on wires wears out or is damaged, allowing the bare live and neutral wires to touch.
2. Connecting too many appliances to a single socket (via adapters) causing overheating and insulation melting.

1. Increase the current flowing through the solenoid ($B \propto I$).
2. Increase the number of turns per unit length ($B \propto n$).
3. (Bonus) Insert a soft iron core inside the solenoid to make it an electromagnet, which greatly amplifies the field.

Activity:

1. Take a small aluminium rod AB and place it horizontally across two conducting rails inside a horseshoe magnet such that the rod lies perpendicular to the magnetic field.
2. Connect the rails to a battery through a switch and rheostat.
3. When the switch is closed, current flows through the rod AB.
4. Observation: The rod experiences a force and is displaced in a specific direction.
5. On reversing either the current direction or the magnetic field direction, the rod moves in the opposite direction.

Rule Used: Fleming's Left-Hand Rule — If the index finger points in the direction of B, middle finger in the direction of I, the thumb gives the direction of force F on the conductor.
This principle is used in the working of a galvanometer.

Working: When current flows through a circular loop, each small segment of the wire produces concentric circular field lines around it. At the centre of the loop, all these fields add up in the same direction (perpendicular to the plane of the loop), creating a strong, concentrated magnetic field.

Direction: Determined by the right-hand rule — curl the fingers of the right hand in the direction of current flow; the thumb points in the direction of the magnetic field at the centre.

Two factors affecting field strength at the centre:

1. Current (I): The field is directly proportional to the current $B \propto I$.
2. Radius (r): The field is inversely proportional to the radius $B \propto \dfrac{1}{r}$. Smaller loops produce stronger fields at their centres.

Comparison:

• The magnetic field pattern of a current-carrying solenoid is identical to that of a bar magnet — with closed field lines, a strong uniform field inside, and similar pole behavior at the ends.
• One end of the solenoid behaves as the North pole and the other as the South pole.

Right-Hand Thumb Rule for solenoid: Wrap the right hand around the solenoid so that the fingers curl in the direction of the current in the turns. The thumb then points toward the North pole of the solenoid.

Why equivalent? Each turn of the solenoid acts as a tiny circular magnet. When stacked, their individual fields add up to create a net dipole field — exactly like a bar magnet.

Domestic Circuit:

• Electricity is supplied to homes at 220 V, 50 Hz AC through three wires: Live (Brown/Red), Neutral (Blue/Black), and Earth (Green/Yellow).
• The live wire carries current at high potential (220 V); the neutral wire is at zero potential and completes the circuit; the earth wire is a safety wire connected to a metal rod buried deep in the ground.
• All appliances are connected in parallel so each gets the full 220 V and can be switched on/off independently.
• A main switch and fuses/MCBs are present in the distribution board for protection.

Why colour-coded? To allow electricians to identify wires quickly and safely, preventing accidental connection of live wires to neutral or earth — which could cause short circuits or electrocution.

Schematic (text form): Live ? Switch ? Bulb ? Neutral (with Earth wire connected to the metal body of the appliance).

Overloading: When the total current drawn from the mains exceeds the safe limit of the wiring, the circuit is said to be overloaded. This causes excessive heating of wires and can lead to fire.

Two causes:

1. Connecting too many high-power appliances to a single circuit simultaneously (e.g., AC, geyser, iron, and washing machine on the same circuit).
2. Using wires with insufficient current-carrying capacity (undersized wires) for the load connected.

How a fuse prevents damage: A fuse is a thin wire made of a low-melting-point alloy (e.g., tin-lead alloy). It is connected in series with the live wire. When excess current flows, the fuse wire heats up and melts, breaking the circuit before the wiring or appliances overheat. This protects both the wiring and the connected appliances.

Right-Hand Thumb Rule (Maxwell's Corkscrew Rule): Hold the current-carrying conductor in the right hand with the thumb pointing in the direction of current. The fingers curl in the direction of the magnetic field lines around the conductor.

(a) Conductor with current flowing West to East:
Thumb points East. Above the conductor, fingers come out of the page (toward the observer). Magnetic field above the conductor points out of the page (North direction if horizontal).

(b) Circular loop with anticlockwise current (viewed from front):
Using the right-hand rule for a loop: curl fingers in the direction of current (anticlockwise). Thumb points toward the observer (out of the page). So the magnetic field at the centre is directed toward the viewer (the front face is a North pole).

(a) Electric Bell (3 marks):
Construction: An electric bell consists of:

• An electromagnet (soft iron core wound with copper coil)
• A springy metal strip (armature) with a soft iron piece at one end and a hammer (striker) at the other
• A gong (bell)
• A contact screw (acts as the switch)

Working:

1. When the push button is pressed, the circuit is complete and current flows through the electromagnet.
2. The electromagnet attracts the iron piece on the armature strip, pulling it toward itself.
3. The hammer strikes the gong, producing sound.
4. Simultaneously, the armature strip breaks contact with the contact screw, interrupting the circuit.
5. The electromagnet loses magnetism, the spring pulls the armature back to original position, contact is restored, and the cycle repeats rapidly — producing a continuous ringing sound.

(b) Numerical (2 marks):
(Or using $P = I^2 R = (2.4)^2 \times 5 = 5.76 \times 5 = 28.8\ \text{W}$)

(a) Solenoid field (3 marks):
Pattern: Inside the solenoid, the magnetic field lines are parallel, uniform, and along the axis. Outside, the pattern resembles a bar magnet with closed loops from N to S through the air.

Comparison with bar magnet: Identical external field pattern; one end is N-pole, other is S-pole.

Two factors affecting field strength:

1. Number of turns per unit length (n): $B \propto n$ — more turns = stronger field.
2. Current (I): $B \propto I$ — more current = stronger field.

(b) Solenoid expression & soft iron core (2 marks):
Effect of soft iron core: Replacing the air core with a soft iron core dramatically increases the magnetic field (by a factor of the relative permeability $\mu_r$ of iron, which can be several hundred to thousands). The iron core becomes magnetised and amplifies the total field, making it a powerful electromagnet.

1. Why separate circuits: Separate circuits allow independent control of each section, prevent overloading of a single circuit, and allow one section to be repaired without affecting the other. Heavy appliances require higher-rated cables and fuses.
2. Earth wire function: The earth wire provides a safe path for leakage current to flow into the ground, preventing the metal body of an appliance from becoming live. All heavy metallic appliances (refrigerator, washing machine, geyser, AC) must be earthed because their metal bodies could become live if internal insulation fails — causing fatal shock if touched.
3. MCB advantage over wire fuse: An MCB automatically trips (disconnects) when excess current flows and can be reset manually without replacement. A wire fuse must be replaced every time it blows.
4. Calculation:
   $I = \dfrac{P}{V} = \dfrac{3300}{220} = 15\ \text{A}$
   Minimum fuse rating should be 15 A or slightly above (e.g., a 16 A fuse). A fuse rated below 15 A would blow under normal operating conditions.

1. Why soft iron: Soft iron has high magnetic permeability (gets strongly magnetised) and low retentivity (loses magnetism immediately when current is switched off). This is essential for quick release of loads in a scrap yard. Steel has high retentivity and remains magnetised even after current is off.
2. Two ways to increase lifting power:
   • Increase the current through the coil.
   • Increase the number of turns in the coil.
3. Other application: Electromagnets are used in electric bells, magnetic cranes, MRI machines, loudspeakers, relays, and circuit breakers. (Any one correct answer.)
4. Disadvantage of steel core: Steel has high retentivity — once magnetised, it does not lose magnetism easily when the current is switched off. This means the electromagnet would not release the scrap metal when required, making the sorting operation impossible.

1. Rule used: Fleming's Left-Hand Rule is used to determine the direction of force on a current-carrying conductor in a magnetic field.
2. Three quantities determining magnitude of force:
   • The current (I) in the conductor.
   • The length (L) of the conductor in the field.
   • The magnetic field strength (B).
   ($F = BIL\sin\theta$, where $\theta$ is the angle between the conductor and the field.)
3. Direction of force: Current in rod AB flows from A to B (East to West); Magnetic field acts vertically upward.
   Using Fleming's Left-Hand Rule: Index finger = upward (B), Middle finger = East to West (I).
   Thumb points toward the South — the rod experiences a force in the South direction.
4. Why galvanometer uses this and not EMI: A galvanometer measures steady (or slowly varying) currents by the direct force on a current-carrying coil placed in a fixed magnet. Electromagnetic induction (EMI) requires a changing magnetic flux to generate an EMF — it is not suitable for detecting steady DC current since a static field produces no induced EMF. Hence, the motor effect (force on current) is the correct principle for a galvanometer.