13. A particle of mass m and charge q is circling on an orbit of radius R normal to an external magnetic field Bext. This moving charge would create a new magnetic field. What is this new magnetic field at the center of the orbit? (A) (B)(C)(D)(E) None of the above.

4. A pendulum is formed by pivoting a long thin rod of length L and mass M about a point on the rod. Apparently, the period (T) of this pendulum (for small oscillations) depends on the position of the pivot. Suppose the pivot is at a distance d from the center of the rod. What d would give a minimum T? (The moment of inertia of a thin rod about its center is ML²/12) (A) L/√6 (B) L/6 (C) L/√12 (D) L/12 (E) None of the above.

13. A particle of mass m and charge q is circling on an orbit of radius R normal to an external magnetic field B_ext. This moving charge would create a new magnetic field. What is this new magnetic field at the center of the orbit?
(A)
(B)
(C)
(D)
(E) None of the above.

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