KENDRIYA VIDYALYA SANGATHAN, AHMEDABAD REGION
PHYSICS WORKSHOP (II) JULY 2016
QUESTION BANK-PHYSICS
CHAPTER 5 MAGNETISM
SECTION A (1 MARK QUESTIONS)
1. Write the name of a physical quantity having unit J/T. State, whether it is a scalar or a vector
2. A short bar magnet placed with its axis making an angle Ө with uniform magnetic field experience a torque. What is the magnetic moment of the magnet?
3. If the horizontal and vertical component of earth’s magnetic field are equal at a place. Find the angle of dip.
4. Which is the material used in making the core of a moving coil galvanometer?
5. A magnetic needle free to rotate in vertical plane orients itself vertically at a certain place on the earth. What are the values of (i) Horizontal component of the earth’s magnetic field (ii) angle of dip at this place?
SECTION B (2 MARK QUESTIONS)
1. An electron beam passes through a region of crossed electric and magnetic field of strength E and B respectively. For what values of the electron speed the beam will remain undeflected?
2. Write any three characteristics of Ferro magnetic substance should possesses if it is to be used to make a permanent magnet.. Give an example of such a material.
3. If χ stands for the magnetic susceptibility of a given material. Identify the class of materials for which (i) -1≥ χ<0 (ii) 0< χ<a (A small positive number).
Write the range of relative magnetic permeability of these materials.
4. The susceptibility of a magnetic material is 2.6 X10-5.Identify the type of the magnetic material and states its two properties.
5. State Curie’s law in magnetism. Name the material which does not obey this law.
SECTION C (3 MARK QUESTIONS)
1. Define magnetic dipole moment. Derive an expression for the magnetic moment of a dipole placed in uniform magnetic field.
2.The horizontal component of earth’s magnetic field at a given place is 0.4x10-4 Wb/m2 and andle of dip is 30°.Calculate the value of (i)Vertical component (ii)Total intensity of earth’s magnetic field.
3. Define intensity of magnetisation and Magnetic susceptibility. Write their expression also.
4. Define magnetic elements of the earth. Define them.
5.A bar magnet of magnetic moment 1.5 JT-1 lies aligned with the direction of a uniform magnetic field of 0.22 T
(a) What is the amount of work required by an external torque to turn the magnet so as to align its magnetic moment? (i)Normal to the field direction (ii) Opposite to the field direction.
(b) What is the torque on the magnet in case (i) and (ii)?
SECTION D (5 MARK QUESTIONS)
1. Define paramagnetic, diamagnetic and ferromagnetic substances. Write three properties of each.
2. An electron is revolving around the nucleus in a circular orbit of radius r with speed v. Derive an expression for its orbital angular momentum. Hence define Bohr’s magneton.
VALUE BASED QUESTION
1. Mrs Lata found that her son could not hear properly. The specialist prescribed hearing aid for her son. Hearing aids consist of electromagnets in the loudspeakers used in the device..
(i) What two values does Ms Nita exhibit towards her son and students? Mention any two .
(caring attitude, sensitive towards society, concern for others
(ii) What is an electromagnet? In what way its hysteresis curve is different from that used for permanent magnets?
Ans. Electromagnet- temporary magnet. Hysteresis curve has small are, small coercivity, small retentivity.
2. Ms Sumathy wife of Mr Varadan complained about the non availability of gas cylinders and explained to him to look out for alternate method for cooking Mr Varadan bought an induction stove to over come the fuel problem. The next day Ms Sumathy used her copper bottom cooker and kept it on the induction stove. But even after using it for half an hour she found that the cooker was not hot and food not cooked. AS she was not aware of the method to use the induction stove, she asked her elder daughter Dhanya, studying first year engineering about it. She told her that some vessels cannot be used on this stove. She took the instruction manual and explained to her mother, that the stove works on magnetic induction and copper being a diamagnetic material will not respond to it.
(I).What values did Mr Varadan and Dhanya exhibit towards Ms Sumathy?
(II).Give few example of diamagnetic material and explain how their susceptibility varies with temperature?
3. Rohit’s mother put many clothes for washing in the washing machine. She could not start the washing machine as an indicator was showing that the lid was not closed. On seeing his mother struggling, Rohit tried his best to close the lid but soon he realised that the closing mechanism was not a manual system but a magnetic system. He went to the shop and purchased a magnetic closer. He replaced the older closer by the new one. The machine started working. The mother was happy that her son helped her to save three hundred rupees.
(a) What are the values being displaced by Rohit?
(b)A magnetised needle in a uniform magnetic field experiences a torque but no net force. An iron nail near a bar magnet, however experiences a force of attraction in addition to a torque. Why?
HOTS
1. From molecular point of view discuss the temperature dependence susceptibility for diamagnetism, paramagnetism and ferromagnetism.
2. The following figure shows the variation of intensity of magnetisation versus applied the magnetic field intensity H, for two magnetic materials A and B.
(a) Identify the materials A and B.
(b) For the material A, plot the variation of intensity of magnetisation verses temperature.
B
I A
H
3. A proton has spin and magnetic moment just like an electron. Why then its effect is neglected in the magnetism of the material?
SOLUTION OF QUESTION BANK
CHAPTER 5 MAGNETISM
SECTION A (1 MARK QUESTIONS)
1. Magnetic dipole moment, vector
2. M=Ƭ/ (B Siin Ө)
3. Tan δ BV/BH =1, δ =450
4. Soft iron core, because it has high initial value of magnetic permeability and low hysteresis loss.
5. δ =00 & δ =900
SECTION B (2 MARK QUESTIONS)
1. An electron beam passes through undeflected if electric and magnetic force on electron is equal and opposite
Where q E =q v B so v =E/B
2. Characteristics of permanent magnet
(i) High permeability
(ii) High retentivity
(iii) High coercivity
Example:-- steel, alnico
3. (i) Diamagnetic material (ii) paramagnetic material
µr =1+ χ
(i)range of relative magnetic permeability for diamagnetic is 0≤µr <1
(ii)range of relative magnetic permeability 1<µr <1+a
4. Paramagnetic material.
(i) They have tendencies to move from weak magnetic field to the stronger magnetic field.
(ii) When placed in external magnetic field, the field lines get concentrated inside the material and the field inside is enhanced.
5. The magnetic susceptibility of the paramagnetic material is inversely proportional to the absolute temperature. Diamagnetic substance does not obey this law.
SECTION C (3 MARK QUESTIONS)
1.
1. Dipole moment M®: - Product of the pole strength of one of the pole & effective length of a magnet is called dipole moment M vector. It is a vector quantity directed from South Pole to North Pole.
5.5 Torque on a Bar magnet in a magnetic field: - Consider a magnet ns placed in a magnetic field of intensity H vector (uniform) making angle q with H vector. Then it will experience two equal but opposite forces F = m H parallel to the field. This two equal & opposite force will setup a couple.
Torque of the couple t = Force ´ perpendicular distance - - - - - (1)
Putting value in eq1
So
Hence Direction of the torque is perpendicular to both M & B & can be given by the Fleming's right hand rule.
2.(i)Given H=0.4x10-4Wb/m2 ,Ө=30°
tan Ө= V/H
Vertical component=H Tan Ө= H=0.4x10-4xtan30= 0.23x10-4Wb/m2
(ii) Total intensityof earth’s magnetic field
Be=(H2+V2)1/2= ((0.4x10-4)2+(0.4x10-4/(3)1/2)2)1/2
Be=0.46x10-4Wb/m2
3. Intensity of magnetisation: - When a magnetic substance placed in a magnetic field it becomes magnetized and acquires a magnetic moment pet unit volume of the substance is known as intensity of magnetization I.
Amp/m.
Pole strength developed per unit area of the substance is known as intensity of magnetization I.
Magnetic Susceptibility (c ) : -Intensity of magnetization ‘I’ is proportional to the intensity of magnetizing field
‘H’ in which the substance is placed. I a H
I = cm H Where cm is a constant called magnetic susceptibility.
If H = 1 & then c = I Intensity of magnetization developed in a substance when placed in uniform magnetic field of unit intensity is define as magnetic susceptibility. It is ability of the substance to acquire magnetization when placed in magnetic field.
4. Magnetic element of the earth’s magnetic field: -The quantities, which completely determine the magnetic field of the earth, are called magnetic elements of the earth.1) Declination 2) Dip or inclination 3) Horizontal component of the earth magnetic field
1) Angle of Declination: - Angel between the geographical meridian & magnetic meridian is called Declination ‘a’. It is about 17 °from the direction of the magnetic meridian & Geographical meridian, the declination can be measured. It is helpful in steering ship in the right direction with the help of mariners compass
2) Angle of Dip or inclination: -If a magnetic needle is free to rotate in a vertical plane, it does not remain horizontal but makes an angel with the horizon.
The needle must be in the magnetic meridian then the north pole of the needle is depressed below the horizontal, while the South Pole of the needle is pointing above the horizontal the direction of the needle give the direction of the total intensity of earth magnetic field.
5. (a)Work done W= MB(cosӨ1-cosӨ2)
(i) Ө1=0° and Ө2=90°
W= 1.5X0.22 (1-0)= 0.33J
(ii) Ө1=0° and Ө2=180°
W=2X1.5X0.22=0.66J
(b) Torque=MBsinӨ
(i)Ө=90°, torque=1.5x0.22=0.33Nm
(ii) Ө=180° torque=0
SECTION D (5 MARK QUESTIONS)
1. Diamagnetic substances: - Those substances which are repelled by a magnet are called diamagnetic substances. OR
Those substance which when placed in a magnetic field are feebly magnetized in a direction opposite to the magnetizing field are called diamagnetic substances. Eg: - Copper, Zinc, Bismuth, Silver, Gold, Lead, Glass, Marble, Water, Helium, Argon, Sodium Chloride, Inert Gasses.
Which is opposite to the external magnetic field. All substances show diamagnetic behavior.
Properties: -1) A magnet feebly repels a diamagnetic substance.
2) In the second property, weak magnetization produced in the substance opposite to the direction of the magnetic field.
3) Diamagnetic substances moves form the stronger magnetic field to weaker magnetic field.
4) M is small & negative so susceptibility c = M / H is small & negative. 8) Permeability m<1
5) A diamagnetic substance loses its magnetism as the external field is removed.
Paramagnetic Substances: - Paramagnetic substances are those substances, which are feebly attracted by the magnets. OR
Those substances which when placed in a magnetic field are feebly magnetized by the magnetizing field are called paramagnetic substances Eg. Aluminum, Sodium, Platinum, CuCl2, Mn, Cr, liquid O2.
Properties: -1) Paramagnetic substances are feebly attracted by the magnets.
2) When a paramagnetic substance is suspended in the uniform magnetic field then it becomes parallel to the magnetic field & magnetized weakly along the direction of the field.
3) Magnetic lines of forces prefer to pass through paramagnetic substances rather then the air.
4) As we remove the magnetic field from it the substance gets demagnetized.
.5) Paramagnetic substances move from the weaker field to stronger field.
6) M has a positive value, so susceptibility c = M / H is small & positive.
7) Permeability m> 1.
8) Magnetism is produced along the direction of the field.
Ferromagnetic: - The substances, which are strongly attracted by the magnets, are called ferromagnetic substances. Or Substances which when placed in a magnetic field are strongly magnetized along the direction of the external field called ferromagnetic substances. E.g. Fe, Ni, Co, Gadolinium (Gd), Dysprosium .
Properties: - Similar to those paramagnetic substances-1] Strongly attracted by a magnet.
2] Ferromagnetism is not found in liquids.
3] M has a large positive value.
4] Susceptibility c = M / H has very high positive value.
5] Magnetism still remains even after removal of the external field. 6] Ferromagnetism decreases with increase in temperature it is maximum at absolute zero & becomes zero at curie’s temp. 7] The temp at which magnetism is completely lost is called curie temperature 770 °C for iron, 356 °C for nickel & 1075 °C for Co.15 °C for Gadolinium. Above Tc substances becomes paramagnetic. 8] m >1.
2. Magnetic dipole moment (M) of the current loop: -Let a circular wire carry current in clockwise direction, then direction of magnetic field will be such that lower face will act as North pole and upper face will act as South pole. Thus a current loop behaves as a magnet i.e. magnetic dipole as shown in figure
Torque acting on a current carrying coil placed in the magnetic field B is given by t = IAB sinq - - -(i)
But, the torque acting on a magnetic dipole placed in the magnetic field is given by t = MB sin q - - -(ii)
Comparison of eqn. (i) and (ii) gives the magnetic dipole moment of current loop M B sin q = IA B sin q Hence dipole moment M = I A
If the loop has n turns, then M = n I A In vector form
Where n is number of turns of loop, I is the current and A is the area of loop.
Atom as a Magnetic Dipole: -In an atom, electrons revolve around the nucleus in circular orbits. The movement of the electron in circular orbit around the nucleus in anticlockwise is equivalent to the flow of current in the orbit in clockwise direction. Thus the orbit of electrons is considered as tiny current loop.
If an electron of mass ‘m’ revolves in anti-clockwise direction with velocity ‘v’ in circular orbit of radius ‘r’ as shown in figure, the angular momentum vector L acts along normal to the plane of orbit in upward direction and its magnitude is according to Bohr’s Theory is - - - - (1) Where n = 1, 2, 3 - - -