Samacheer Class 9 Science - Magnetism and Electromagnetism

• (a) AC only
• (b) DC only
• (c) Both AC and DC

Answer:
(a) AC only

• (a) field magnet
• (b) split rings
• (c) slip rings
• (d) brushes

Answer:
(d) brushes

Slip rings rotate with the coil, while brushes collect current.

True. A generator converts mechanical energy into electrical energy. When a coil rotates in a magnetic field, the changing magnetic flux through the coil induces an electromotive force (EMF) according to Faraday's law of electromagnetic induction. This induced EMF drives electrons through an external circuit, producing electric current. The mechanical energy used to rotate the coil is thus transformed into electrical energy that can be transmitted and used in various applications.

False. The correct statement is that magnetic field lines never intersect each other. The statement that magnetic field lines repel each other is not accurate. Magnetic field lines are imaginary lines used to represent the direction and strength of a magnetic field. The key property is that they never cross or intersect because at any given point in space, the magnetic field can have only one direction. If two field lines were to intersect at a point, it would mean the magnetic field has two different directions at that same point, which is physically impossible. Therefore, the non-intersection of magnetic field lines is a fundamental characteristic that ensures the uniqueness of the magnetic field direction at every point in space.

False. The correct statement is that Fleming's Left Hand Rule is called the Motor Rule, not the Dynamo Rule. Fleming's Left Hand Rule is used to determine the direction of force on a current-carrying conductor placed in a magnetic field, which is the principle used in electric motors. Fleming's Right Hand Rule, on the other hand, is associated with generators and is sometimes called the Dynamo Rule. In Fleming's Right Hand Rule, the thumb, forefinger, and middle finger of the right hand are used to determine the direction of induced current in a conductor moving through a magnetic field.

False. The correct statement is that the speed of an electric motor increases by increasing the area of the coil, not by decreasing it. The speed of a motor depends on the torque produced, which is given by the formula τ = NBIA, where N is the number of turns, B is the magnetic field strength, I is the current, and A is the area of the coil. Since speed is proportional to the torque and torque is directly proportional to the area of the coil, a larger coil area results in greater torque and thus higher speed of rotation.

False. The correct statement is that a transformer can only work with alternating current (AC), not direct current (DC). A transformer operates on the principle of electromagnetic induction, which requires a changing magnetic flux through the primary coil to induce a voltage in the secondary coil. Alternating current produces a continuously changing magnetic field that induces the required voltage. Direct current, however, produces a constant magnetic field with no change, so no voltage is induced in the secondary coil. Therefore, transformers are ineffective with direct current and cannot step up or step down DC voltage.

True. In a step-down transformer, the primary coil has more turns than the secondary coil. A step-down transformer reduces the voltage from the primary coil to a lower voltage in the secondary coil. The voltage transformation ratio is determined by the turns ratio, expressed as V_s/V_p = N_s/N_p, where V represents voltage and N represents the number of turns. Since a step-down transformer produces a lower secondary voltage, the secondary coil must have fewer turns than the primary coil.

Answer:

Stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular to each other.

• Forefinger → magnetic field
• Middle finger → current
• Thumb → force or motion

This rule gives the direction of motion in an electric motor.

Answer:

The number of magnetic field lines passing normally through unit area is called magnetic flux density.

Where:

• (B) = magnetic flux density
• (\Phi) = magnetic flux
• (A) = area

SI Unit:

or

Answer:

• Field magnet
• Armature coil
• Split ring commutator
• Carbon brushes
• Battery

Answer:

• AC can be transmitted over long distances with less power loss.
• AC voltage can be stepped up or stepped down easily using transformers.
• Generation cost of AC is lower.
• AC can easily be converted into DC.

Where:

• (N_s) = turns in secondary coil
• (N_p) = turns in primary coil

A portable radio uses a step-down transformer because household AC voltage is typically much higher than the voltage required by the radio's internal circuits. The step-down transformer reduces the high household AC voltage to a lower, safer voltage suitable for the radio. After the voltage is stepped down by the transformer, the reduced AC voltage is then rectified to convert it into direct current, which is necessary to power the radio's electronic components such as transistors and integrated circuits. This combination of stepping down the voltage and then rectifying it ensures that the radio receives the correct voltage and current type needed for proper operation.

First Law

Whenever magnetic flux linked with a coil changes, an emf is induced.

Second Law

The induced emf is equal to the negative rate of change of magnetic flux. The negative sign shows Lenz's law: the induced emf opposes the change in flux.