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CNMS ICSE - 7 Std
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Answer key

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Chapter: 08. Magnetism

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Why does the electromagnet in an electric bell lose its magnetism after the hammer strikes the gong?
The displacement of the armature breaks the contact at the screw, which stops the current flow and causes the electromagnet to lose its magnetism.
State the basic law of magnetism.
The basic law of magnetism states that like poles of two magnets repel each other, while unlike poles attract each other.
What three factors can increase the magnitude of an induced current in a coil?
The magnitude of the induced current can be increased by increasing the number of turns in the coil, the strength of the magnet, or the relative speed between the magnet and the coil.
What are magnetic materials?
Magnetic materials are substances, such as iron, cobalt, steel, and nickel, that are attracted by a magnet.
What is the reading of a galvanometer when a magnet remains completely stationary inside a coil?
When the magnet is completely inside the coil but not moving, the galvanometer reading is zero because there is no relative motion to induce a current.
Distinguish between permanent and temporary magnets based on their magnetic retention.
Permanent magnets retain their magnetic properties without an external magnetic field, whereas temporary magnets act as magnets only in the presence of a strong magnetic field or current.
What is an artificial magnet?
An artificial magnet is a magnet in which magnetic properties have been imparted artificially, and they come in shapes such as bar, horseshoe, or cylindrical magnets.
Why are MRI scans potentially problematic for people with metallic implants?
MRI scans can cause problems for people with metallic implants because the machine generates extremely strong magnetic fields that can affect the metal.
Why does the hammer of an electric bell strike the gong when the push button is pressed?
When the button is pressed, current flows through the electromagnet, magnetizing it so that it attracts the iron armature, causing the attached hammer to hit the gong.
Explain the directive property of a magnet.
The directive property means that a freely suspended magnet always aligns itself in the geographic north-south direction.
What happens to the galvanometer needle when a magnet is moved away from a wire coil?
The galvanometer needle deflects in the opposite direction, indicating the presence of an induced current flowing in the reverse direction.
Why is repulsion considered the surest test of magnetism?
Repulsion is the surest test of magnetism because only a magnet can repel another magnet, whereas unmagnetized magnetic materials like iron are merely attracted to magnets.
What makes an MRI scan preferable over a CT scan for imaging organs and soft tissues?
An MRI scan is preferred over a CT scan because it produces clearer, more detailed images of organs and soft tissues without exposing the patient to radiation.
How does the direction of electric current determine the magnetic poles of an electromagnet?
The end of the electromagnet where the current flows clockwise acts as the south pole, while the end where current flows anticlockwise acts as the north pole.
Define electromagnetic induction.
Electromagnetic induction is the phenomenon of producing an electric current in a conductor due to a change in the magnetic lines of force associated with that conductor.
What happens to the poles of a bar magnet when it is broken into two parts?
When a bar magnet is broken into two parts, each part becomes a complete magnet with its own north and south poles.
State Faraday’s first law of electromagnetic induction.
Faraday’s first law of electromagnetic induction states that a current is induced in a coil when the number of magnetic lines of force associated with the coil changes.
Describe the construction of an electromagnet.
An electromagnet consists of a soft iron core on which an insulated copper wire is wound in a coil that carries an electric current.
List four different uses of electromagnets.
Electromagnets are used in electrical appliances like telephones and motors, in cranes to lift scrap iron, in bullet trains, and to remove steel splinters from patient wounds.
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