Test: Magnetism - Class 9 MCQ

When the current flowing through an electromagnet is turned off, the magnetic field disappears, as electromagnets only produce a magnetic field while current is present, due to the nature of the materials used.

Induced magnetism occurs when a magnetic material, such as soft iron, develops magnetic properties due to its proximity to a magnet. This effect is temporary and disappears once the external magnetic field is removed.

Iron nails become temporarily magnetized when placed in a magnetic field due to induction. They will magnetize and attract each other but will lose this magnetism when the external magnetic field is removed.

Electromagnets are widely used in various applications, including DC motors, cranes for lifting heavy materials, medical devices like MRI machines, and many other devices that require controlled magnetic fields.

Neutral points are locations where the net magnetic field is zero, meaning the magnetic forces acting on a compass needle cancel each other out, allowing the needle to point in any direction.

A battery is essential for constructing an electromagnet as it provides the electrical current that flows through the wire coil, creating the magnetic field around the core material.

An electric bell operates on the principle of electromagnetic attraction and release. When the circuit is closed, the electromagnet attracts the armature, causing the hammer to strike the bell, and when the circuit opens, the armature returns to its original position.

At the magnetic equator, Earth's magnetic field lines are horizontal. They tilt upwards towards the poles, becoming vertical at the magnetic poles, illustrating the three-dimensional nature of Earth's magnetic field.

Lodestone is an iron oxide ore (Fe3O4) recognized for its natural magnetic properties. It was historically significant as one of the first known magnets, used by ancient civilizations for navigation.

The primary distinction between an electromagnet and a permanent magnet is that an electromagnet can be turned on and off by controlling the electric current, while a permanent magnet always has a constant magnetic field.

Electromagnets are commonly made from soft iron because it becomes magnetized when current flows through the coil wrapped around it, and it loses this magnetism quickly when the current is turned off.

Magnetic field lines always run from the north pole to the south pole outside of a magnet, creating closed loops. They do not cross because that would imply multiple magnetic directions at a single point.

The strength of a magnetic field weakens as the distance from the magnet increases. This is because the magnetic force diminishes over distance, affecting the interaction with nearby objects.

Magnetic field lines around a bar magnet are curved, illustrating the direction and strength of the magnetic field. The lines are denser near the poles, indicating a stronger magnetic force.

A galvanometer is used to measure electric current. It operates based on the deflection of a needle in response to the magnetic field generated by the current flow.

The strength of an electromagnet can be increased by increasing the number of turns in the solenoid or increasing the current flowing through it. More turns create a stronger magnetic field.

Unlike poles (north and south) attract each other, while like poles (north-north or south-south) repel each other. This fundamental property of magnets is crucial for understanding magnetic interactions.

The Earth's magnetic field causes a compass needle to align with magnetic north, which is near the geographic north pole. This alignment is essential for navigation.

Retentivity refers to a material's ability to maintain its magnetic properties after the external magnetic field is removed. Materials with high retentivity, like steel, are often used to create permanent magnets.

A permanent magnet is characterized by its stable and long-lasting magnetic field, often made from materials like steel, which retain their magnetism over time due to high retentivity.