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Saturday, 12 November 2016
Problem set 24
The Hamiltonian of a simple pendulum consisting of a mass m attached to a massless string
of length l is H = \frac{p_\theta^2}{2ml^2}+ mgl(1- \cos\theta). If L denotes the Lagrangian, the value
\frac{dL}{dt} is:
Two bodies of equal mass m are connected by a massless rigid rod of length l lying in the XY-plane
with the centre of the rod at the origin. If this system is rotating about the z-axis with a
frequency \omega, its angular momentum is
ml^2\omega/4
ml^2\omega/2
ml^2\omega
2ml^2\omega
Angular momentum, L=I\omegaI=\sum_im_ir_i^2
Here, r_i=\frac{l}{2}I=m\frac{l^2}{4}+m\frac{l^2}{4}=m\frac{l^2}{2}L=\frac{ml^2\omega}{2}
Hence, answer is (B)
An infinite solenoid with its axis of symmetry along the z-direction carries a steady current I.
The vector potential \vec A at a distance R from the axis
is constant inside and varies as R outside the solenoid
varies as R inside and is constant outside the solenoid
varies as \frac{1}{R} inside and as R outside the solenoid
varies as R inside and as \frac{1}{R} outside the solenoid
Let r is the radius of solenoid.
Magnetic filed inside the solenoid is B=\mu_0 n I, where n is number of turns per unit length, and, I is current
flowing through the wire. Magnetic filed outside the solenoid is zero.
Vector potential inside the solenoid:
Now, according to Stoke's law, we have
\oint \vec A\cdot d\vec l=\int_S\left(\nabla\times\vec A\right)\cdot d\vec S
But \left(\nabla\times\vec A\right)=\vec B\oint \vec A\cdot d\vec l=\int_S\vec B\cdot d\vec S=\Phi_B=flux
Let us calculate flux \Phi_B. For this let us consider a circular path of radius R centered along its axis. The flux
through the circular area is
\Phi_B=\pi R^2B=\pi R^2\mu_0 n I
Hence,
\oint \vec A\cdot d\vec l=\pi R^2\mu_0 n I
But, \oint \vec A\cdot d\vec l=2\pi R A_\Phi, this is because \vec A is along the direction of current which is
circumferential.
2\pi R A_\Phi=\pi R^2\mu_0 n IA_\Phi=\mu_0 n I\frac{R}{2}
Vector potential outside the solenoid:
Let us consider the point R>r. The flux through circular area is
\Phi_B=\pi r^2B=\pi r^2\mu_0 n I\oint \vec A\cdot d\vec l=\pi r^2\mu_0 n I
But, \oint \vec A\cdot d\vec l=2\pi R A_\Phi2\pi R A_\Phi=\pi r^2\mu_0 n IA=\mu_0 n I\frac{r^2}{2R}
Hence, \vec A varies as R inside and as \frac{1}{R} outside the solenoid
Hence, answer is (D)
Consider an infinite conducting sheet in the xy-plane with a time dependent cunent density
Kt\hat i, where K is a constant. The vector potential at (x, y,z) is given by
\vec A=\frac{\mu_0K}{4c}(ct-z)^2\hat i. The magnetic field \vec B is
When a charged particle emits electromagnetic radiation, the electric field \vec E and the
Poynting vector \vec S = \frac{1}{\mu_0} \vec E\times\vec B at a large distance r from the emitter vary as
\frac{1}{r^n} and \frac{1}{r^m} respectively. Which of the following choices for n and m are correct?
n = 1 and m=1
n = 2 and m=2
n = 1 and m=2
n = 2 and m=4
When a charged particle emits electromagnetic radiation E\propto\frac{1}{r} and B\propto\frac{1}{r}. As \vec S=\frac{\vec E\times \vec B}{\mu_0}, S\propto\frac{1}{r^2}
Hence, answer is (C)
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