Permanent Magnets
Questions for Short Answer
1. Can we have a single north pole? A single south pole?
ANSWER: No, we can not have a single north pole or single south pole. Magnetic poles always occur in pairs. Even if a magnet is broken into pieces, each and every piece will have a north and a south pole.
2. Do two distinct poles actually exist at two nearby points in a magnetic dipole?
ANSWER: No, two distinct poles can not actually exist at two nearby points. In a magnetic dipole, two distinct poles are located near its end.
3. An iron needle is attracted to the ends of a bar magnet but not to the middle region of the magnet. Is the material making up the ends of a bar magnet different from that of the middle region?
ANSWER: No, the material making up the ends and middle region of a bar magnet is not different. When an iron needle is brought near a magnet, each pole at the ends of the magnet induces an opposite pole in the needle. When the needle is brought near the end of the magnet, the pole at this end induces the opposite pole in the needle. The pole at the far end has less influence. Hence the needle is attracted towards the nearer end. But when the needle is brought near the middle of the magnet, the poles at each end have equal and opposite influences on the needle. Thus the forces cancel each other and the needle is not attracted.
4. Compare the direction of the magnetic field inside a solenoid with that of the field there if the solenoid is replaced by its equivalent combination of a north pole and south pole.
ANSWER: The end of solenoid where current is anticlockwise when seen from outside, acts as the magnetic north pole of a magnet and the other end as the south pole. So, the magnetic field inside the solenoid is from the apparent south pole to the apparent north pole.
When the solenoid is replaced by equivalent north and south poles, the magnetic fields remain the same.
5. Sketch the magnetic field lines for a current-carrying circular loop near its center. Replace the loop with an equivalent magnetic dipole and sketch the magnetic field lines near the center of the dipole. Identify the difference.
ANSWER: In a current-carrying circular loop the magnetic lines of forces are directed perpendicular to the loop and coming out towards the viewer that sees the anticlockwise current. So, if we consider one side of the loop as the north pole and the other side as the south pole, then the direction of the fields near the center is from south to north. 
Diagram for Q-5
Now replace the loop with a magnetic dipole. As we see in the diagram, the magnetic lines of force emerge from the N-pole, and in a smooth curve, it enters the S-pole. So near the center, the direction of the field is from the north to the south, which is just opposite to the case of the loop.
6. The force on a north pole, F =mB, is parallel to field B. Does it contradict our earlier knowledge that a magnetic field can exert forces only perpendicular to itself?
ANSWER: The earlier knowledge that a magnetic field can exert forces only perpendicular to itself is related to a moving electrical charge. Here the force on a north pole F =mB, is directed on a magnetic charge m. It is just like an electric charge in an electric field. So, though our earlier knowledge seems to be contradictory in this case the entity in the magnetic field is different.
7. Two bar magnets are placed close to each other with their opposite poles facing each other. In absence of other forces, the magnets are pulled towards each other and their kinetic energy increases. Does it contradict our earlier knowledge that magnetic forces cannot do any work and hence cannot increase the kinetic energy of a system?
ANSWER: The earlier knowledge that magnetic forces cannot do any work was related to the magnetic force on a moving charged particle, where the magnetic force is always perpendicular to the direction of movement. The force here only can change the direction of movement and cannot increase the speed. Thus the kinetic energy cannot be increased.
The given case is not the case of moving charged particles. Here the magnetic force is on another magnet. The movement of the magnet is in the direction of the force, hence the work is done and the kinetic energy is increased.
So yes it contradicts, but the condition is different.
8. Magnetic scalar potential is defined as
U(r₂) -U(r₁) =-∫r2r₁(B.dl).
Apply this equation to a closed curve enclosing a long straight wire. The RHS of the above equation is then -µₒi by Ampere's law. We see that U(r₂) ≠U(r₁) even when r₂ =r₁. Can we have a magnetic scaler potential in this case?
ANSWER: We apply Ampere's law for the magnetic field due to current-carrying wires. The magnetic scalar potential is calculated at a point at distance r due to a pole of pole strength m and is equal to µₒm/4πr. So in this case we can not have a magnetic scalar potential.
9. Can the earth's magnetic field be vertical at a place? What will happen to a freely suspended magnet at such a place? What is the value of dip here?
ANSWER: Yes, the earth's magnetic field is vertical above the magnetic north and south poles of the earth.
A freely suspended magnet will hang vertically at these places. Above the north magnetic pole of the earth, the north pole of the magnet will be down towards the north magnetic pole of the earth. And just reverse at the south magnetic pole of the earth.
The value of dip above the north pole will be 90° and above the south pole, it will be -90°.
10. Can the dip at a place be zero? 90°?
ANSWER: Yes, the dip at the magnetic equator is zero. Here a freely suspended magnet will rest horizontally.
The dip at the north magnetic pole of the earth is 90°. Here the freely suspended magnet will rest vertically with its north pole pointing towards the magnetic north pole of the earth.
11. The reduction factor K of a tangent galvanometer is written on the instrument. The manual says that the current is obtained by multiplying this factor to tanθ. The procedure works well at Bhubneswar. Will the procedure work if the instrument is taken to Nepal? If there is some error, can it be corrected by correcting the manual or the instrument will have to be taken back to the factory?
ANSWER: The reduction factor K of the tangent galvanometer is fixed only for a place because it is proportional to the horizontal component of the earth's magnetic field which varies from place to place. So this procedure that works well at Bhubneswar will not work well at Nepal if the same reduction factor K is used.
The error can be corrected by correcting the manual with the actual reduction facor at Nepal. Actual rection factor can be obtained by passing a known current through the instrument and measuring the angle.
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Links to the Chapters
Links to the Chapters
CHAPTER- 35- Magnetic Field due to a Current
CHAPTER- 34- Magnetic Field
CHAPTER- 33- Thermal and Chemical Effects of Electric Current
CHAPTER- 35- Magnetic Field due to a Current
CHAPTER- 34- Magnetic Field
CHAPTER- 33- Thermal and Chemical Effects of Electric Current
CHAPTER- 32- Electric Current in ConductorsCHAPTER- 31- CapacitorsCHAPTER- 30- Gauss's Law
CHAPTER- 29- Electric Field and Potential
CHAPTER- 28- Heat Transfer
OBJECTIVE -I
CHAPTER- 26-Laws of Thermodynamics
CHAPTER- 25-CALORIMETRY
Questions for Short Answer
OBJECTIVE-I
OBJECTIVE-II
EXERCISES - Q-11 to Q-18
CHAPTER- 24-Kinetic Theory of Gases
CHAPTER- 23 - Heat and Temperature
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 32- Electric Current in Conductors
CHAPTER- 31- Capacitors
CHAPTER- 30- Gauss's Law
CHAPTER- 29- Electric Field and Potential
CHAPTER- 28- Heat Transfer
CHAPTER- 26-Laws of Thermodynamics
CHAPTER- 25-CALORIMETRY
Questions for Short Answer
OBJECTIVE-I
OBJECTIVE-II
CHAPTER- 24-Kinetic Theory of Gases
CHAPTER- 23 - Heat and Temperature
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 8 - Work and Energy
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CHAPTER- 7 - Circular Motion
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CHAPTER- 6 - Friction
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CHAPTER- 6 - Friction
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CHAPTER- 5 - Newton's Laws of Motion
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Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)
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CHAPTER- 4 - The Forces
The Forces-
"Questions for short Answers"
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CHAPTER- 3 - Kinematics - Rest and Motion
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CHAPTER- 2 - "Physics and Mathematics"
CHAPTER- 2 - "Physics and Mathematics"
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