Ch. 8 Magnetism
Q. What is the magnetic effect of electric current?
A. Magnetic effect of electric current means that an electric current flowing through a conductor produces a magnetic field around it.
Q. What is a magnet?
A. A magnet is an object which attracts iron, steel, nickel, and cobalt.
A. Magnetic effect of electric current means that an electric current flowing through a conductor produces a magnetic field around it.
Q. What is a magnet?
A. A magnet is an object which attracts iron, steel, nickel, and cobalt.
Q. What are the north and south poles of a magnet?
A. The end of a freely suspended (/pivoted) magnet which points towards the north direction is called the north pole of the magnet. The end of a freely pivoted magnet which points towards the south direction is called the south pole of a magnet.
A. The end of a freely suspended (/pivoted) magnet which points towards the north direction is called the north pole of the magnet. The end of a freely pivoted magnet which points towards the south direction is called the south pole of a magnet.
Q. Write a few uses of magnets.
A. i) Magnets are used in electronic goods like radios, televisions, speakers, refrigerators, audio and video cassette tapes, hard discs, computers, toys etc.
ii) Magnets are also used in making electric generators and electric motors.
iii) Magnets are used in the working of MRI (Magnetic Resonance Image) technique which is used to scan inner body parts.
A. i) Magnets are used in electronic goods like radios, televisions, speakers, refrigerators, audio and video cassette tapes, hard discs, computers, toys etc.
ii) Magnets are also used in making electric generators and electric motors.
iii) Magnets are used in the working of MRI (Magnetic Resonance Image) technique which is used to scan inner body parts.
Q. What is a magnetic field?
A. The space surrounding a magnet in which magnetic force is exerted is called a magnetic field.
A. The space surrounding a magnet in which magnetic force is exerted is called a magnetic field.
Q. What are magnetic field lines?
A. The path traced by a north magnetic pole free to move under the influence of a magnetic field is called a magnetic field line.
A. The path traced by a north magnetic pole free to move under the influence of a magnetic field is called a magnetic field line.
Q. Write the properties of magnetic field lines.
A. i) Magnetic field lines originate from the north pole and end at the south pole of a magnet.
ii) The field lines come close to each other near the poles but they are widely separated at other places i.e. magnetic field gets weaker as we go away from the pole.
iii) The magnetic field lines never intersect each other.
A. i) Magnetic field lines originate from the north pole and end at the south pole of a magnet.
ii) The field lines come close to each other near the poles but they are widely separated at other places i.e. magnetic field gets weaker as we go away from the pole.
iii) The magnetic field lines never intersect each other.
Q. Why does a compass needle get deflected when brought near a bar magnet?
A. When a compass is placed in a magnetic field, then the bar magnet exerts a magnetic force on the compass and its freely pivoted needle is deflected from its original north-south position.
A. When a compass is placed in a magnetic field, then the bar magnet exerts a magnetic force on the compass and its freely pivoted needle is deflected from its original north-south position.
Q. What does the degree of closeness of magnetic field lines signify?
A. The closeness of field lines signifies that the magnetic field is more i.e. the magnetic force is more. This happens near the poles of the magnet. The field lines are closest at the poles i.e. the magnetic force exerted at the poles is maximum.
Q. State the rule which is used to determine the direction of induced current produced in a conductor moving in a magnetic field.
A. The rule used is Flemming's right-hand rule. It states that if the forefinger, middle finger and the thumb of the right hand are kept mutually perpendicular to each other, then the forefinger represents the direction of the magnetic field, the thumb represents the direction of motion (i.e. the direction of exerted force), and the middle finger represents the direction of the induced current.
Q. What is electric motor? On what principle does it work?
A. Electric motor is a device which converts electrical energy into mechanical energy.
Principle: When a current carrying conductor is placed in a magnetic field, it experiences a force.
Q. Define:
i) Solenoid
ii) Electromagnetic Induction
A. i) Solenoid is an insulated copper wire wound around a cylindrical core, such that its length is greater than its diameter.
ii) Electromagnetic induction is the phenomenon of inducing a current in a coil by changing magnetic field around it.
Q. What is AC Generator? On what principle does it work?
A. AC Generator is a device which converts mechanical energy into electrical energy.
Principle: It works on the principle of electromagnetic induction i.e. by inducing a current in a coil by changing magnetic field around it.
Q. Differentiate between Electromagnet and Permanent Magnet.
A.
Q. Explain the working of MCB.
A. MCBs (Miniature Circuit Breakers) work on the magnetic effect of electric current. It contains an electromagnet which when, the current exceeds the rated value, become strong enough to separate a pair of contacts (i.e. by putting off the switch), and breaks the circuit.
Q. Why does the acceleration of a bar magnet decreases while falling through a solenoid?
A. The acceleration of a bar magnet decreases while falling through a solenoid because it experiences magnet force (magnetic attraction) when it falls through the magnetic field of a solenoid.
Q. Can two magnetic lines of force ever intersect? Justify your answer.
A. No. Two magnetic field lines can never intersect. This is because the resultant force at a point on the north pole of a magnet can only be in one direction. If two magnetic field lines intersect, then this resultant force on a north pole placed at the point of intersection would be in two directions, (i.e. the compass needle would point towards two directions) which is not possible.
Q. The frequency of alternating current in India is 50Hz. What does it mean?
A. This means that the coil of an AC (alternating current) generator rotates at the rate of 50 revolutions per second. As the current reverses its direction 2 times in a revolution, therefore the AC supply in India reverses its direction 100 times in one second. Or the AC supply reverses its direction every 1/100 second.
Q. Why does the bulk of iron filings stick to the ends of a bar magnet and not at its center?
A. The bulk of iron filings stick to the ends of a bar magnet because the magnetic field (/force) is strongest at the poles i.e. the ends of the bar magnet. The filings do not stick at the center because here the magnetic field (/force) is weakest.
A. The closeness of field lines signifies that the magnetic field is more i.e. the magnetic force is more. This happens near the poles of the magnet. The field lines are closest at the poles i.e. the magnetic force exerted at the poles is maximum.
Q. State the rule which is used to determine the direction of induced current produced in a conductor moving in a magnetic field.
A. The rule used is Flemming's right-hand rule. It states that if the forefinger, middle finger and the thumb of the right hand are kept mutually perpendicular to each other, then the forefinger represents the direction of the magnetic field, the thumb represents the direction of motion (i.e. the direction of exerted force), and the middle finger represents the direction of the induced current.
Q. What is electric motor? On what principle does it work?
A. Electric motor is a device which converts electrical energy into mechanical energy.
Principle: When a current carrying conductor is placed in a magnetic field, it experiences a force.
Q. Define:
i) Solenoid
ii) Electromagnetic Induction
A. i) Solenoid is an insulated copper wire wound around a cylindrical core, such that its length is greater than its diameter.
ii) Electromagnetic induction is the phenomenon of inducing a current in a coil by changing magnetic field around it.
Q. What is AC Generator? On what principle does it work?
A. AC Generator is a device which converts mechanical energy into electrical energy.
Principle: It works on the principle of electromagnetic induction i.e. by inducing a current in a coil by changing magnetic field around it.
Q. Differentiate between Electromagnet and Permanent Magnet.
A.
| Electromagnet | Permanent Magnet |
| It is a temporary magnet | It is a permanent magnet |
| Strength of magnet can be changed | Strength cannot be changed |
| Poles can be interchanged | Poles cannot be interchanged |
Q. Explain the working of MCB.
A. MCBs (Miniature Circuit Breakers) work on the magnetic effect of electric current. It contains an electromagnet which when, the current exceeds the rated value, become strong enough to separate a pair of contacts (i.e. by putting off the switch), and breaks the circuit.
Q. Why does the acceleration of a bar magnet decreases while falling through a solenoid?
A. The acceleration of a bar magnet decreases while falling through a solenoid because it experiences magnet force (magnetic attraction) when it falls through the magnetic field of a solenoid.
Q. Can two magnetic lines of force ever intersect? Justify your answer.
A. No. Two magnetic field lines can never intersect. This is because the resultant force at a point on the north pole of a magnet can only be in one direction. If two magnetic field lines intersect, then this resultant force on a north pole placed at the point of intersection would be in two directions, (i.e. the compass needle would point towards two directions) which is not possible.
Q. The frequency of alternating current in India is 50Hz. What does it mean?
A. This means that the coil of an AC (alternating current) generator rotates at the rate of 50 revolutions per second. As the current reverses its direction 2 times in a revolution, therefore the AC supply in India reverses its direction 100 times in one second. Or the AC supply reverses its direction every 1/100 second.
Q. Why does the bulk of iron filings stick to the ends of a bar magnet and not at its center?
A. The bulk of iron filings stick to the ends of a bar magnet because the magnetic field (/force) is strongest at the poles i.e. the ends of the bar magnet. The filings do not stick at the center because here the magnetic field (/force) is weakest.
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