The motion of a particle of mass m in a constant magnetic field directed along the 2 axis can be described by the Lagrangian L = ½{m(i² + y² + 2²) + k(xÿ — yi) . (the constant k is proportional to the product eB of the electric charge of the particle and the magnetic field). (a) Write down the Euler-Lagrange equations, and use them to show that the three quantities I₁ := mȧ - 2ky, I : my + 2kx, I₂ := m², are conserved. (b) Show that L is invariant under a rotation by an infinitesimal angle € about the 2 axis, 8x = €y, dy = -x . Show that the corresponding Noether invariant is given by I = m(xy-yi) +k(x² + y²).

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The motion of a particle of mass m in a constant magnetic field directed along the 2 axis can be described
by the Lagrangian
L = ½{m(i² + y² + 2²) + k(xÿ — yi) .
(the constant k is proportional to the product eB of the electric charge of the particle and the magnetic
field).
(a) Write down the Euler-Lagrange equations, and use them to show that the three quantities
I₁ := mȧ - 2ky, I : my + 2kx, I₂ := m²,
are conserved.
(b) Show that L is invariant under a rotation by an infinitesimal angle € about the 2 axis,
8x = €y, dy = -x .
Show that the corresponding Noether invariant is given by
I = m(xy-yi) +k(x² + y²).
Transcribed Image Text:The motion of a particle of mass m in a constant magnetic field directed along the 2 axis can be described by the Lagrangian L = ½{m(i² + y² + 2²) + k(xÿ — yi) . (the constant k is proportional to the product eB of the electric charge of the particle and the magnetic field). (a) Write down the Euler-Lagrange equations, and use them to show that the three quantities I₁ := mȧ - 2ky, I : my + 2kx, I₂ := m², are conserved. (b) Show that L is invariant under a rotation by an infinitesimal angle € about the 2 axis, 8x = €y, dy = -x . Show that the corresponding Noether invariant is given by I = m(xy-yi) +k(x² + y²).
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