Which of the following must be true about an ideal gas that undergoes an isothermal expansion? (there may be more than one correct choice.)

(a) no heat enters the gas.
(b) the pressure of the gas decreases.
(c) the internal energy of the gas does not change.
(d) the gas does positive work.

The voltage applied across a given parallel-plate capacitor is doubled. how is the energy stored in the capacitor affected?

People with normal vision cannot focus their eyes underwater if they aren't wearing a face mask or goggles and there is water in contact with their eyes. in a simplified model of the human eye, the aqueous and vitreous humors and the lens all have a refractive index of 1.40, and all the refraction occurs at the cornea, whose vertex is 2.60 cm from the retina. with the simplified model of the eye, what corrective lens (specified by focal length as measured in air) would be needed to enable a person underwater to focus an infinitely distant object? (be careful-the focal length of a lens underwater is not the same as in air! assume that the corrective lens has a refractive index of 1.62 and that the lens is used in eyeglasses, not goggles, so there is water on both sides of the lens. assume that the eyeglasses are 2.05cm in front of the eye.)

Calculate a pendulums frequency of oscillation (in hz) if the pendulum completes one cycle in 0.5s.