Magnetic Effects of Electric Current – NCERT Exercises

• (a) False — a motor converts electrical energy into mechanical energy; the reverse applies to a generator.
• (b) True.
• (c) True — near the centre of a long solenoid (or a long coil of many turns) the field lines are parallel and straight along the axis.
• (d) False — green insulation marks the earth wire; the live wire is red (or brown).

1. A permanent bar magnet (or the Earth itself).
2. A current-carrying conductor (straight wire, loop or solenoid).
3. An electromagnet — a coil wound on a soft iron core carrying current.

The pattern of magnetic field lines outside a current-carrying solenoid is identical to that of a bar magnet — they emerge from one end, loop around and enter the other end. So a solenoid has a definite N end and S end and behaves like a bar magnet.

Identifying the poles. Bring the N pole of a known bar magnet close to one end of the solenoid. If that end repels the approaching N pole, the end is also N. If it attracts the N pole, that end is S. Repulsion is the sure test of polarity.

The electron beam moves from back to front, so conventional current (opposite to the electron flow) points from front to back. Apply Fleming's left-hand rule: point the middle finger from front to back (current) and the thumb toward the right side (force on the beam). The forefinger then points vertically downward.

Hence the magnetic field is directed downward in the chamber.

Principle. A rectangular current-carrying coil placed in a magnetic field experiences a couple, which rotates the coil.

Construction. A rectangular coil ABCD wound on a soft iron core is free to rotate about its axle between the poles of a strong permanent magnet. The two ends of the coil are attached to two halves (P, Q) of a split-ring commutator; two carbon brushes X, Y press against the split-rings and connect them to a battery via a key. (See Fig 12.9 in Part 2 for the full labelled diagram.)

Working. Current from the battery enters the coil via brush X, flows through AB, BC, CD, DA and returns through brush Y. In the uniform field, arm AB feels an upward force and arm CD feels an equal downward force — a couple that rotates the coil. After half a rotation the split-rings change contact with the brushes, so the current in the coil reverses and the couple continues to rotate the coil in the same sense.

Function of the split-ring. It reverses the direction of current in the coil at every half rotation, ensuring that the torque always acts in the same rotational sense so that the coil keeps spinning continuously.

• (i) Pushed in — magnetic flux through the coil increases. The galvanometer shows a momentary deflection in one direction, signalling an induced current.
• (ii) Withdrawn — flux through the coil decreases. The galvanometer now shows a momentary deflection in the opposite direction.
• (iii) Held stationary inside — flux is constant. No change in flux, so no induced current; the galvanometer shows no deflection.

• (i) Right-hand thumb rule. Grip the wire in the right hand with the thumb in the direction of the current; the curled fingers give the direction of the magnetic field lines.
• (ii) Fleming's left-hand rule. Stretch the thumb, forefinger and middle finger of the left hand perpendicular to one another. Forefinger = field, middle finger = current, thumb = force (direction of motion).
• (iii) Fleming's right-hand rule. Thumb = motion of the conductor, forefinger = field, middle finger = induced current.

Principle. Electromagnetic induction — when a coil is rotated in a magnetic field, the flux through it changes and an EMF is induced that drives a current through the external circuit.

Construction (AC generator). A rectangular coil ABCD is held between the poles of a strong magnet. The two ends of the coil are connected to two separate metal slip rings R₁ and R₂. Two carbon brushes press lightly against R₁ and R₂ and connect them to the external circuit (load). (See Fig 12.12 in Part 3.)

Working. As the coil is rotated, arm AB moves up while CD moves down (first half rotation). Using Fleming's right-hand rule, the induced current flows in the sense A → B → C → D and out through one brush, through the load and back in through the other brush. In the next half rotation, AB moves down and CD moves up — the induced current in the coil reverses, so the current in the external circuit also reverses. The output is therefore alternating current (AC). A DC generator uses a split-ring commutator instead of two slip rings so that the external current always flows in one direction.

Function of brushes. Brushes are graphite strips that maintain continuous sliding contact with the rotating slip rings (or split-rings). They transfer the induced current from the rotating coil to the stationary external circuit without tangling the wires.

The earth wire is a low-resistance metal wire (green insulation) connecting the metal body of an appliance to a thick copper or iron plate buried deep in the moist ground outside the house.

Function and necessity. If the insulation of the live wire fails and the bare live wire touches the metal body of the appliance, the body would be at 220 V and a person touching it would get a fatal shock. The earth wire provides a much easier path for this fault current to flow directly to the ground. A large current suddenly rushes to earth, which immediately blows the fuse / trips the MCB and isolates the circuit. Earthing therefore protects the user from electric shock and the appliance from damage.