MAGNETISM AND MATTER
Very Short Question Answers
Q1. A magnet dipole placed in a magnetic field experiences a net force what can you say about the nature of themagnetic field?
Ans. If a magnet dipole placed in a magnetic field experiences a net force the field may be non-uniform.
Q2. What happens to compass needles at the earth’s poles?
Ans. At the Earth’s poles, the compass needle can point along any direction, if the needle is only capable of moving in horizontal plane as $B_{H} =0.$
(or)
At the Earth’s poles, the compass needle can point straight down,if the needle is moving in a vertical plane as $B_{H} =0.$
Q3. What do you understand by the ‘magnetization’ of a sample?
Ans. Magnetization(M), of a sample is defined as its net magnetic moment per unit volume.
$$M = \frac {m_{net}}{V } $$
Unit∶$ A\,m^{-1}$
Q4. What is the magnetic moment associated with a solenoid?
Ans. Magnetic moment of a solenoid is $M=n\,i\,A=n\,i\,(\pi r^{2})$
n =number of turns, I = current, A = area of cross section.
Q5. What are units of magnetic moment, magnetic induction and magnetic field?
Ans. Magnetic moment $(M): Am^{2} $(or) $joule/tesla$
Magnetic induction$ (B):\: Tesla\, (or)\: wb/m^2$
Magnetic field strength $(H):Am^{-1}$
Q6. Magnetic lines form continuous closed loops. Why?
Ans. The magnetic lines of a magnet form continuous closed loops, because the magnetic poles north and south always exists together in pairs. (or) magnetic lines form continuous closed loops. Because magnetic monopoles do not exist.
Q7. Define magnetic declination
Ans. The angle between the geographic north and north shown by compass needle is called magnetic declination
(or) The angle between the geographical meridian and magnetic meridian of the earth is called magnetic declination.
Q8. Define magnetic inclination or angle of dip.
Ans. The angle between horizontal to earth surface and net magnetic field of the earth BE at that point is called “angle of Dip”.
(or) Angle of dip or magnetic inclination is the angle that the total magnetic field BE of the earth makes the surface of the earth.
Q9. Classify the following materials with regard to magnetism: manganese, cobalt, nickel, bismuth, oxygen, copper
Ans.| Diamagnetic | Para magnetic | Ferro magnetic |
| Bismuth, copper | Manganese, oxygen | Nickel, cobalt |
Q10. Define Curie temperature.
Ans. The temperature of transition from ferromagnetic to paramagnetic is called the Curie temperature.
Q11. State Gauss Law for Magnetism.
Ans. The Net magnetic flux through any closed surface is Zero.
$$ \phi_{B}=\oint B.ds =0$$
Q12. Which needle to be used at poles
Ans. Dip needle
Short Question Answers
Q1. Derive an expression for the axial field of a solenoid of radius 'r' containing n turns per unit length and carrying current i.
Q2. The force between two magnet poles separated by a distance 'd' in air is F. At what distance between them does the force become doubled?
Q3. Compare the properties of para, dia and ferromagnetic substances.
Q4. Explain the elements of the Earth's magnetic field and draw a sketch showing the relationship between the vertical component, horizontal component and angle of dip.
Q5. Define retentivity and coercivity. Draw the hysteresis curve for soft iron and steel. What do you infer from these curves?
Q6. If B is the magnetic field produced at the centre of a circular coil of one turn of length L carrying current I then what is the magnetic field at the centre of the same coll which is made into 10 turns?
Q7. If the number of turns of a solenoid is doubled, keeping the other factors constant, how does the magnetic field at the axis of the solenoid change?
Long Question Answers
Q1. Derive an expression for the magnetic field at a point on the axis of a current carrying circular loop.
Q2. Prove that a bar magnet and a solenoid produce similar fields.
Q3. A small magnetic needle is set into oscillations in a magnetic field B. Obtain an expression for the time period of oscillation.
Q4. A bar magnet, held horizontally, is set into angular oscillations in the Earth's magnetic field. It has time periods T, and T, at two places. where the angles of dip are theta 1 , and theta 2, respectively. Deduce an expression for the ratio of the resultant magnetic fields at the two places:
Q5. Define magnetic susceptibility of a material. Name two elements one having positive susceptibility and other having negative susceptibility.
Q6. Obtain Gauss' Law for magnetism and explain it.
Q7. What do you understand by hysterisis? How does this property influence the choice of materials used in different appliances where electromagnets are used?
Problems Question Answers
Q1. What is the torque acting on a plane coil of n turns carrying a current and having an area A when placed in a constant magnetic field B?
Q2. A coil of 20 turns has an area of 800mm and carries a current of 0.5A. If it is placed in a magnetic field of intensity 0.3T with its plane parallel to the field, what is the torque that it experiences?
Q3. In the Bohr atom model the electrons move around the nucleus in circular orbits.Obtain an expression of the magnetic moment(u) of the electron in a Hydrogen atom in terms of its angular momentum L.
Q4. A solenoid of length 22.5 cm has a total of 900 turns and carries a current of 0.8A. What is the magnetising field H near the centre and far away from the ends of the solenoid?
Q5. A bar magnet of length 0.1m and with a magnetic moment of 5Am is placed in a uniform magnetic field of intensity 0.4T, with its axis making an angle of 60 with the field. What is the torque on the magnet?
Q6. If the Earth's magnetic field at the equator is about 4 x 10 T. what is its approximate magnetic dipole moment?
Q7. The horizontal component of the earth's magnetic field at a certain place is 2.6 x 10°T and the angle of dip is 60°. What is the magnetic field of the earth at this location?
Q8. A solenoid, of insulated wire, is wound on a core with relative permeability 400. If the number of turns per metre is 1000 and the solenoid carries a current of 2A, calculate H. B and the magnetisation M.