## Moving Charges and Magnetism  Important Questions for CBSE Class 12 Physics Magnetic Force and Torque

### 1 Mark Questions

1.Using the concept of force between two infinitely long parallel current carrying  conductors, define one ampere of current. [All India 2014]
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Ans.Yes, the net magnetic force acting on a wire carrying a steady (constant) electric current! in an external magnetic field Band is given F=IdlB

3.Why should the spring/suspension wire in  a moving coil galvanometer have low  torsional constant?    [All India 2008]
Ans.Low torsional constant facilitates greater deflection (allpha) in coil for given value of current and hence, sensitivity of galvanometer increases,

4. Write two factors by which voltage sensitivity of a galvanometer can be increased.[Foreign 2008]
Ans.Voltage sensitivity of galvanometer can be increased by
(i) increasing the magnetic field (ii) decreasing the value of torsional constant,

5. Write two factors by which current sensitivity of a moving coil galvanometer can be  increased.   [Foreign 2008]
Ans.Current sensitivity of galvanometer can be increased by
(i) increasing the number of turns of coil.
(ii) increasing the magnetic field.

### 2 Marks Questions

6.A rectangular coil of sider I and b carrying a current I is subjected to a uniform magnetic field B acting perpendicular to its plane. Obtain the expression for the torque acting on it.[Delhi 2014 C]
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7.A coil of N turns and radius R carries a  current /. It is unwound and rewound to make a square coil of side a having same number of turns N. Keeping the current I same, find the ratio of the magnetic moments of the square coil and the Circular COil.    [Delhi 2013 C]
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8.A steady current I1  flows through a long  straight wire. Another wire carrying steady current I2 in the same direction is kept close and parallel to the first wire. Show with the help of a diagram, how the magnetic field due to the current Ir exert a magnetic force on the second wire. Deduce the expression for this force.   [All India 2011; Foreign 2008]
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9.How is a moving coil galvanometer  converted into a voltmeter? Explain giving the necessary circuit diagram and the required mathematical relation used.  [All India 2011 C]
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11.Define current sensitivity and voltage  sensitivity of galvanometer. Increasing the current sensitivity may not necessarily increase the voltage sensitivity of a galvanometer, justify your answer.  [All India 2009]
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### 3 Marks Questions

12.A metallic rod of length / is rotated with a frequency v with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius r about an axis passing through the centre and perpendicular to the plane of the ring. A constant uniform magnetic field B parallel to the axis is present everywhere. Using Lorentz force, explain how emf is induced between the centre and the metallic ring and hence obtained the expression for it.[Delhi 2013]
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14.Draw a labelled diagram of a moving coil galvanometer and explain its working. What is the function of radial magnetic field inside the coil?[Foreign 2012]
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15.Depict the magnetic field lines  due to two straight, long, parallel conductors carrying currents 1\ and 12 in the same direction. Hence, deduce an expression for the force per unit length acting on one of the conductors due to the other. Is this force attractive or repulsive?         [Delhi 2011 c]
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16.Find the expression for magnetic dipole moment of a revolving electron. What is Bohr magneton?
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17.State the underlying principle of working of a moving coil galvanometer. Write two reasons why a galvanometer cannot be used as such to measure the current in a given circuit. Name any two factors on which the current sensitivity of a galvanometer depends. [Delhi 2010]
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19.Write the expression for the magnetic moment (m) due to a planar square loop of side / carrying a steady current / in a vector form. In the given figure, this loop is placed in a horizontal plane near a long straight conductor carrying a steady current at a distance l as shown. Give reasons to explain that the loop will experience a net force but no torque. Write the

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20.Derive the expression for force per unit length between two long straight parallel current carrying conductors. Hence, define one ampere. [Delhi 2009]
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21.Deduce the expression for the torque experienced by a rectangular loop carrying a steady current I and place’4 in a uniform magnetic field B. In dicate the direction of the torque acting on the loop. [Foreign 2009]
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### 5 Marks Questions

23.(i)Draw a labelled diagram of a  moving coil galvanometer. Describe briefly its principle and working.
(a) Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer?
(b)Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain giving reasons. [All India 2014]
Ans.(i)

24.(i) State using a suitable diagram,  the working principle of a moving coil galvanometer. What is the function of a radial magnetic field and the soft iron core used in it? (ii) For converting a galvanometer into an ammeter, a shunt resistance of small value is used in parallel, whereas in the case of a voltmeter a resistance of large value is used in series. Explain why?    [Delhi 2014C]
Ans.(i)

(ii)

25.(i) Explain giving reasons, the basic  difference in converting a galvanometer into (a) a voltmeter and (b) an ammeter.(ii) Two long straight parallel conductors carrying steady currents and I2 are separated by a distance d. Explain briefly with the help of a suitable diagram, how the magnetic field due to one conductor acts on the other. Hence, deduce the expression for the force acting between the two conductors. Mention the nature of this force.[All India 2012]
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28.(i) With the help of a diagram,  explain the principle and working of a moving coil galvanometer.
(ii)What is the importance of radial magnetic field and how is it produced?
(iii)Why is it that while using a moving coil galvanometer as a voltmeter, a high resistance in series is required whereas in an ammeter a shunt is used?[All India 2010; Delhi 2009]
Ans.(i)

29.(i) Derive an expression for the force  between two long parallel current carrying conductors.
(ii) Use this expression to define SI unit of current.
(iii) A long straight wire AB carries a current /. A proton P travels with a speed v, parallel to the wire at a distance d from it in a direction opposite to the current as shown in the figure. What is the force experienced by the proton and what is its direction?[All India 2010; Foreign 2008]

Ans.(i)

30.(i) Two straight long parallel  conductors carry currents !x and I2 in the same direction. Deduce the expression for the force per unit length between them. Depict the pattern of magnetic field lines around them.
(ii) A rectangular current carrying loop EFGH is kept in a uniform magnetic field as shown in the figure.
(a) What is the direction of the magnetic moment of the current loop?
(b) What is the torque acting on the loop maximum and zero?

Ans.(i)