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Published on: 23/05/2021
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Questions + Answers key
Take MCQ Physics Test1.
If two magnets having magnetic moments M and M\(\sqrt { 3 } \) are joined to form a cross (i.e. X). The combination is suspended freely in a uniform magnetic field. In equilibrium position, the magnet having magnetic moment M makes an angle \(\theta \) with the field. Calculate the value of \(\theta \)
2.
A short bar magnet has a magnetic moment of 0.48 J/T. Give the direction and magnitude of the magnetic field produced by the magnet at a distance of 10cm from the centre of the magnet on
(i) the axis,
(ii) the equatorial lines (normal bisector) of the magnet.
3.
A current of 5 A is flowing from South to North in a straight wire. Find the magnetic field due to a 1cm piece of wire at a point 1m North-East from the piece of wire.
4.
A cyclotron when being used to accelerate positive ions? (Mass = 6.7 x 10-27kg, charge = 3.2 x 10-19C) has a magnetic field of ( \(\frac { \pi }{ 2 }\)) T. What must be the value of the frequency of the applied alternating electric field to be used in it?
5.
Two long wires carrying currents I1 and I2 are arranged as shown in the figure. One carrying current I1 is along the X-axis. The other carrying current I2 is along a line parallel to Y-axis, given by x = 0 and z = d. Find the force exerted at point O2 because of the wire along the X-axis.

1.
If magnet of magnetic moment M makes an angle \(\theta \) with the field, then other magnet of magnetic moment M\(\sqrt { 3 } \) makes an angle \(\left( { 90 }^{ \circ }-\theta \right) \) with the field.
In equilabrium, \(\tau _{ 1 }=\tau _{ 2 }\)
\(\Rightarrow MBsin\theta =M\sqrt { 3 } Bcos\theta \ \Rightarrow \ \frac { sin\theta }{ cos\theta } \sqrt { 3 }\)
\( \\ \Rightarrow tan\theta =\sqrt { 3 } \Rightarrow \ \theta ={ 60 }^{ \circ }\)
2.
(i)

The direction of magnetic field is from S to N-pole of magnet.
(ii)

The direction of magnetic field is from N to S-pole of magnet.
3.
here, I = 5 A, dl = 1 cm = 0.01 m, r = 1 m, \(\theta ={ 45 }^{ 0 }\)
[\(\because \) direction is North-East]
\(\therefore \ dB=\cfrac { { \mu }_{ 0 } }{ 4\pi } .\cfrac { Idl \ sin\theta }{ { r }^{ 2 } } \)
\(={ 10 }^{ -7 }\times\cfrac { 5\times0.01\times sin\quad { 45 }^{ 0 } }{ ({ 1) }^{ 2 } } \)
= 3.54 x 10-9 T
Its direction is vertically downwards.
4.
Frequency of alternating electric field cyclotron is given by
\(f=\frac { qB }{ 2\pi m } \)
Here, q = 3.2 x 10-19 C,
m = 6.7 x 10-27 kg and B = \(\frac { \pi }{ 2 } T\)
\(\\ \therefore \quad f=\frac { q(\frac { \pi }{ 2 } ) }{ 2\pi m }\)
\( \\ f=\frac { (3.2\times { 10 }^{ -19 })\times (\frac { \pi }{ 2 } }{ 2\times \pi \times 6.7\times { 10 }^{ -27 } }\)
f = 1.2 x 107 cycle/s
5.
Here, first we have to find the direction of magnetic field at point O2 due to the wire carrying current I1. Use Maxwell's right hand grip (cork screw) rule, the direction of magnetic field at point O2 due to current I1 is along Y-axis.
Here,the wire at point O2 is placed along Y- axis. Now, by the formula,
F = I2 (I x B)
Angle between I and B is 0o ,both are at Y-axis, i.e
F = IlB sin 0o = 0
So,the force exerted at point O2 because of wire along X-axis is zero.
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