Class 10 – Science Extra Questions – Ch. 12 – Magnetic Effects of Electric Current

Ans: D

Explanation: Magnets attract ferromagnetic materials. Of the options provided, iron and steel are ferromagnetic. Plastics are non-magnetic. While aluminium is a metal, it is not ferromagnetic and therefore not strongly attracted to a magnet. Not all metals are magnetic; for instance, copper is not.
Correct Option: D

Ans: A

Explanation: A generator is a device that converts mechanical energy into electrical energy. A galvanometer is an instrument for detecting and measuring small electric currents. An ammeter is an instrument for measuring electric current in amperes. A motor is a machine that converts electrical energy into mechanical energy. Therefore, the apparatus that generates electricity is a generator.
Correct Option: A

Ans: C

Explanation: A plotting compass needle is a small magnet itself. The north pole of the compass needle is attracted to the south pole of a magnet, and the south pole of the compass needle is attracted to the north pole of a magnet. Therefore, when held close to the south pole of a bar magnet, the north pole of the compass needle will point towards the south pole of the bar magnet.
Correct Option: C

Ans: A

Explanation: A temporary electromagnet requires a core material that can be easily magnetized when current flows through the solenoid and quickly demagnetized when the current is removed. Soft iron fits this description perfectly. It has high magnetic permeability, meaning it readily allows magnetic field lines to pass through it, enhancing the magnetic field of the solenoid. Crucially, it has low retentivity, meaning it loses most of its induced magnetism as soon as the external magnetic field (from the solenoid’s current) is removed. Steel, on the other hand, has high retentivity, making it suitable for permanent magnets, not temporary ones. Copper and aluminum are not ferromagnetic materials and therefore do not significantly enhance the magnetic field of the solenoid in the way that soft iron does.
Correct Option: A

Ans: A

Explanation: Electromagnets rely on the ability of their core material to become strongly magnetized when a current flows through the coil, and importantly, to lose most of that magnetism when the current is switched off. Soft iron is a ferromagnetic material with high magnetic permeability and low retentivity. This means it can be easily magnetized and demagnetized, making it ideal for applications like electromagnets where the magnetic field needs to be switched on and off. Brass and aluminum are non-magnetic, so they would not enhance the magnetic field. Steel, while ferromagnetic, has higher retentivity than soft iron, meaning it retains magnetism after the current is removed, which is undesirable for a typical electromagnet.
Correct Option: A

Ans: B

Explanation: A d.c. generator operates on the principle of electromagnetic induction, where a changing magnetic field induces an electromotive force (voltage) in a conductor. This is the fundamental principle behind how generators convert mechanical energy into electrical energy. Electrochemical induction is related to chemical reactions producing electricity, the magnetic effect of current describes how a current creates a magnetic field, and the heating effect of current describes the heat produced by current flow. None of these are the primary operating principle of a d.c. generator.
Correct Option: B

Ans: B

Explanation: A circuit breaker is an automatic electrical switch designed to protect an electrical circuit from damage caused by overcurrent or short circuit. Its primary function is to interrupt the flow of electricity when the current exceeds a safe level, thus preventing overheating of wires and potential fires. A voltage regulator maintains a constant voltage. A wattmeter measures power consumption. A ground fault interrupter (GFI) interrupts current when it detects an imbalance between the outgoing and returning electrical currents, usually due to current flowing to ground, preventing electric shock.
Correct Option: B

Ans: C

Explanation: Faraday’s Law of Induction states that the magnitude of the induced emf in any circuit is equal to the rate of change of the magnetic flux through the circuit. The magnetic flux is proportional to the strength of the magnet and the area of the coil, and the rate of change of flux is influenced by the speed of movement and the number of turns. Resistivity of the coil does not directly affect the induced emf, though it would affect the induced current if a closed circuit is formed.
Correct Option: C

Ans: C

Explanation: The magnetic force on a charged particle moving in a magnetic field is given by the Lorentz force formula: F = q(v x B), where q is the charge, v is the velocity, and B is the magnetic field. The direction of the cross product (v x B) is perpendicular to both v and B. Since the particle is positively charged (q > 0), the direction of the force F is the same as the direction of (v x B). Therefore, the magnetic force is perpendicular to both the velocity and the magnetic field.
Correct Option: C

Ans: C

Explanation: Option C is incorrect because for a current to be induced, there must be a closed circuit for the charges to flow. A changing magnetic field will induce an EMF, but without a closed circuit, no current will flow. Options A, B, and D accurately describe fundamental principles of electromagnetic induction.
Correct Option: C