wavelength, frequency, and wave speed, along with the equation v = c / n
(which follows immediately from the definition of index of refraction), we
find that
- B Since the electron is naturally being accelerated toward a region containing
positive source charges, its potential energy decreases. (It’s like a ball
dropping to the ground; it is accelerated downward by the gravitational field,
and it loses gravitational potential energy.) Therefore, the answer is either
(A) or (B). Now since the potential due to a positive charge is higher than
the potential due to a negative charge (because positive numbers are greater
than negative numbers), the electron is accelerating toward a region of higher
electric potential, (B). Be careful: although (B) coincides with the fact that
an electron will naturally move from lower to higher potential, the problem
itself stated that it was being (maybe forcibly) accelerated from one region to
another region.
- B The transfer of heat due to a moving fluid (such as air) is known as
convection. [Choices (D) and (E), by the way, are not modes of heat transfer
and can therefore be eliminated immediately.]
- A The faster a spaceship passes by the station, the shorter its length is
observed to be. This is because as v increases, the relativistic factor γ
increases, so the amount of length contraction increases. Therefore, the
smaller the v, the smaller the value of γ, and the longer the ship will be
observed to be. Of the choices given, the speed in (A) is the smallest. [Note
that the spaceship traveling at the speed given in (E) will be observed to be
the shortest as it passes by the station.]
