Magnetic Effects of Electric Current — NCERT Solutions, AI Tutor & Practice

Understand: A compass needle is a small magnet that aligns itself with any magnetic field present.

Analyze: When an electric current flows through a wire, it produces a magnetic field in the region around it (Oersted's discovery). This magnetic field exerts a force on the magnetic needle.

Conclude: Since the current-carrying wire behaves like a source of magnetic field, the compass needle experiences a torque and gets deflected from its usual north–south position. This shows that an electric current produces a magnetic effect.

Properties of magnetic field lines:

1. Magnetic field lines emerge from the north pole and merge at the south pole outside the magnet; inside the magnet they run from south to north (forming closed loops).
2. They are closed continuous curves.
3. The direction of the field at any point is given by the tangent to the field line at that point.
4. Field lines are closer together where the field is stronger and farther apart where it is weaker.
5. No two field lines ever intersect each other — if they did, there would be two directions of the field at one point, which is impossible.

Right-hand thumb rule (Maxwell's corkscrew rule):

Imagine holding a straight current-carrying conductor in your right hand such that the thumb points in the direction of the current. Then the direction in which the fingers curl around the conductor gives the direction of the magnetic field lines.

Use: It helps determine the direction of the magnetic field produced around a straight wire carrying current, so we can predict the deflection of a compass placed nearby.

The magnetic field at the centre of a circular loop depends on:

1. Current ($I$): The field is directly proportional to the current. More current → stronger field.
2. Radius of the loop ($r$): The field is inversely proportional to the radius. A smaller loop → stronger field at the centre.
3. Number of turns ($n$): If the loop has $n$ turns, the field becomes $n$ times stronger, because the field due to each turn adds up in the same direction.

Every point on the wire contributes a field, and at the centre all these contributions point the same way, so they combine to give a strong field represented by nearly straight parallel lines through the centre.

Solenoid: A solenoid is a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder.

Field pattern:

• Inside the solenoid the field lines are parallel straight lines, showing the field is uniform and strong.
• Outside, the field pattern is exactly like that of a bar magnet: one end acts as the north pole and the other as the south pole.

Comparison: A current-carrying solenoid behaves like a bar magnet. The magnetic field of both has similar external field lines emerging from N and entering S. A strong magnetic field inside the solenoid can be used to magnetise a piece of magnetic material placed inside it, forming an electromagnet.

Fleming's left-hand rule: Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points along the magnetic field and the middle finger points along the direction of current, then the thumb points in the direction of force (motion) on the conductor.

Factors affecting the force:

1. The magnitude of current ($I$) through the conductor — larger current, larger force.
2. The strength of the magnetic field ($B$) — stronger field, larger force.
3. The length of conductor in the field.
4. The force is maximum when the conductor is perpendicular to the field and zero when it is parallel to the field.

Principle: An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a force (Fleming's left-hand rule). This converts electrical energy into mechanical energy.

Construction: It consists of a rectangular coil ABCD placed between the poles of a magnet, a split ring (commutator), brushes, and a battery.

Working:

1. Current enters the coil, flowing in arm AB one way and arm CD the opposite way.
2. By Fleming's left-hand rule, arm AB experiences a downward force and arm CD an upward force, forming a couple that rotates the coil.
3. After half a rotation, the split ring reverses the direction of current in the coil. This keeps the force acting in the same rotational sense.
4. Thus the coil continues to rotate continuously in one direction.

The split ring (commutator) is the device that reverses the current direction to maintain continuous rotation.

Electromagnetic induction: The phenomenon of producing an induced current in a coil due to a change in the magnetic field (or magnetic flux) linked with it is called electromagnetic induction. It was discovered by Michael Faraday.

Moving a magnet towards or away from a coil, or changing the current in a nearby coil, produces an induced current — even without any battery in the coil circuit.

Direction of induced current — Fleming's right-hand rule: Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular. If the forefinger points along the magnetic field and the thumb points along the direction of motion of the conductor, then the middle finger gives the direction of the induced current.

AC Generator:

• Uses two slip rings (full rings).
• The direction of the induced current in the outer circuit reverses periodically.
• Produces alternating current.

DC Generator:

• Uses a split ring (commutator).
• The split ring changes the connections at the moment the current would reverse, so the output current flows in one direction only.
• Produces direct current.

Key point: The only major difference in construction is the ring arrangement — slip rings give AC, a split-ring commutator gives DC.

Note: In India, AC changes direction every $1/100$ second, i.e. it reverses after each half cycle, with a frequency of 50 Hz.

Three wires in domestic supply:

1. Live wire — usually with red insulation, at a potential of about 220 V.
2. Neutral wire — usually with black insulation, at zero potential.
3. Earth wire — usually with green insulation, connected to the earth.

Function of the earth wire: It is connected to the metallic body of appliances. If the live wire touches the metal casing, the earth wire provides a low-resistance path for the current to flow directly to the ground, protecting the user from a fatal electric shock.

Function of the fuse: A fuse is a safety device made of a wire with a low melting point. When the current exceeds a safe limit (due to overloading or short-circuiting), the fuse wire heats up and melts, breaking the circuit and preventing damage to appliances and fires.

Short circuit: occurs when live and neutral wires touch directly, causing a very large current.

Given: Direction of motion of positive charge (current) = West. Direction of force (deflection) = North.

Apply Fleming's left-hand rule: (positive charge motion = direction of conventional current)

• Middle finger → current direction = West
• Thumb → force direction = North
• Forefinger → magnetic field direction = ?

Aligning the left hand so the middle finger points west and the thumb points north, the forefinger points downwards (vertically downward).

Answer: The magnetic field is directed vertically downward.

Rule (clock-face rule):

• If the current in a loop appears to flow anticlockwise when viewed from a face, that face is a North pole.
• If the current appears to flow clockwise, that face is a South pole.

First loop (anticlockwise current): The face towards the viewer is a North pole.

Second loop (clockwise current): The face towards the viewer is a South pole (so its opposite/far face is the North pole).

This can be verified using the right-hand thumb rule applied to the circular loop — curl the fingers along the current and the thumb points towards the north pole.