Samacheer Kalvi 9th Science Magnetism and Electromagnetism Exercise Solutions

Stretch the thumb, forefinger, and middle finger of the left hand so that they are mutually perpendicular to each other. Then the forefinger points in the direction of the magnetic field (B), the middle finger indicates the direction of current (I), and the thumb shows the direction of the force or motion (F) acting on the conductor. This rule is used to determine the direction of force in an electric motor, which is why it is also called the Motor rule.

Magnetic flux density (B) is defined as the number of magnetic field lines crossing a unit area kept normal (perpendicular) to the direction of the field lines. Its formula is B = Φ/A, where Φ is the magnetic flux in Weber and A is the area in square metres. The SI unit of magnetic flux density is Weber per square metre (Wb/m²), which is also called Tesla (T). A stronger magnetic field has a greater flux density.

An electric motor consists of five main parts. The field magnet produces a strong, uniform magnetic field between its poles. The armature is a rectangular coil of insulated copper wire wound on a soft iron core, mounted on an axle for free rotation. The split ring commutator consists of two half-rings that reverse the current direction every half rotation to maintain unidirectional rotation. The brushes are carbon or metal strips that maintain sliding contact with the commutator to supply current. The battery provides the direct current needed to drive the motor.

AC has several practical advantages over DC for power distribution. First, AC generation is more economical than DC generation. Second, AC can be easily converted to DC using rectifiers when needed. Third, AC voltage can be stepped up or stepped down efficiently using transformers, which is impossible with DC since transformers require changing magnetic flux. Fourth, AC can be transmitted over long distances with less power loss because stepping up voltage reduces current, thereby reducing I²R losses in transmission lines.

A step-up transformer increases voltage by having more turns on the secondary coil, making it ideal for power transmission over long distances. A step-down transformer decreases voltage by having fewer turns on the secondary coil, making it suitable for running household appliances and electronic devices safely. Both types operate only on AC and work on the principle of mutual induction.

A portable radio can operate on batteries or mains power. The transformer inside is a step-down transformer that reduces 230 V AC mains to a low AC voltage matching the radio's operating voltage. This low AC is then rectified to DC using a rectifier circuit so the radio can run on mains supply. Without the transformer, the high mains voltage would damage the radio's electronic components. The transformer thus enables safe, efficient power conversion.

First law: Whenever the magnetic flux linked with a closed circuit changes, an EMF is induced in the circuit. The induced EMF lasts only as long as the change in flux continues. Second law: The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux linkage through the circuit. Mathematically, e = -N × (dΦ/dt), where N is the number of turns, dΦ is the change in flux, and dt is the time interval. The negative sign represents Lenz's law, indicating the induced EMF opposes the change that causes it.

Principle: A DC motor works on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. The magnitude of this force is given by F = BIL, where B is the magnetic flux density, I is the current, and L is the length of the conductor. The direction of the force is determined by Fleming's Left Hand Rule.

Construction:

• Field magnet: A permanent magnet or electromagnet that produces a strong, uniform magnetic field between its poles. The armature rotates within this field.
• Armature: A rectangular coil of insulated copper wire wound on a soft iron core, mounted on an axle so it can rotate freely between the poles of the field magnet.
• Split ring commutator: Two half-rings of conducting material attached to the two ends of the armature coil. It reverses the direction of current through the coil every half rotation, ensuring continuous rotation in one direction.
• Brushes: Two carbon or metal strips that press against the commutator halves, providing electrical contact between the external battery circuit and the rotating coil.
• Battery: Provides the direct current needed to drive the motor.

A transformer is a device that changes (steps up or steps down) AC voltage using the principle of mutual induction. It consists of a laminated soft iron core with two coils — the primary coil (connected to input) and the secondary coil (connected to output). When alternating current flows through the primary coil, it creates a changing magnetic flux in the core, which induces an EMF in the secondary coil.

Step-up transformer:

• The secondary coil has more turns than the primary coil (Ns > Np).
• Output voltage is greater than input voltage (Vs > Vp).
• Output current is less than input current (Is < Ip), since power is conserved in an ideal transformer.
• Used at power generating stations to step up voltage for long-distance transmission, reducing I²R power losses.
• Example: A 11 kV generation output stepped up to 220 kV for transmission.

Step-down transformer:

• The secondary coil has fewer turns than the primary coil (Ns < Np).
• Output voltage is less than input voltage (Vs < Vp).
• Output current is greater than input current (Is > Ip).
• Used in electronic appliances, mobile chargers, doorbells, and adapters to reduce mains voltage to safe operating levels.
• Example: A 230 V mains supply stepped down to 12 V for a doorbell.

Key relations: The turns ratio formula Vp/Vs = Np/Ns relates voltage to number of turns. For an ideal transformer, power is conserved: Vp × Ip = Vs × Is. No transformer is 100% efficient — some energy is lost as heat in the coils (copper loss) and in the core (eddy current loss and hysteresis loss).

Principle: An AC generator works on Faraday's law of electromagnetic induction — when a coil rotates in a magnetic field, the changing magnetic flux through the coil induces an alternating EMF. The direction of the induced current is given by Fleming's Right Hand Rule.

Construction:

• Armature: A rectangular coil ABCD of insulated copper wire that rotates between the poles of a strong magnet. The coil is wound on a soft iron core to increase magnetic flux.
• Slip rings: Two continuous, separate metallic rings (S₁ and S₂) connected to the two ends of the armature coil. They rotate with the coil and provide continuous electrical contact.
• Brushes: Two stationary carbon strips (B₁ and B₂) that press against the slip rings to transfer current from the rotating coil to the external circuit.