CBSE Class 12-science Answered
Question no. 6
Direction of propagation of electromagnetic wave is determined from its phase term .
For the given electromagnetic wave , phase term is " k z - ω t " .
Hence , given electromagnetic wave is propagating in z-axis direction , or in the direction of unit vector k .
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Question no. 7
Velocity of charged particle has components along x-axis and z-axis.
Since the magnetic field is along z-axis , component along x-axis gives magnetic force that makes the charged particle to move in circular motion.
velocity component along z-axis moves the charged particle along z-axis.
Hence , overall effect is helical path for the charged particle and the charged particle makes displacement along z-axis.
Time taken by the charged particle to move 2 m above x-y plane , i.e., along z-axis is ( distance / speed ) = ( 2 / 4 ) = 1/2 sec
Hence ansere is :- (B) Helical path, 1/2 s
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Question No. 8
Figure shows the electric dipole p is palced along z-axis.
Work done to move the charge q from A ( a, 0, 0) to B (0, 0, a ) is difference of potential energy between A and B.
Electric potential V at a point due to dipole p is
where K = 1/(4πεo) is Coulomb's constant, θ is angle between the axis of dipole and the line joining dipole and the point where potential is measured.
r is the distance between dipole and the point where potential is measured.
Potential energy UA of charge q at A is zero because angle θ = 90° and cosθ = 0 .
UA = 0
Potential energy UB of charge q at B is
Hence work done is ( UB - UA ) = q × K × ( p / a2 )