Discussion
Second sound in superfluid helium is :
Answer: A
Second sound is the wave motion of entropy and temperature.
In the two-fluid model of He II, the liquid is a mixture of a superfluid component, which carries no entropy, and a normal component, which carries all of it. In ordinary (first) sound both components move together, giving a pressure and density wave. In second sound they move in opposite directions with no net mass flow, so the total density and pressure stay almost constant while the local entropy, and with it the temperature, oscillates. It is a temperature wave, excited by a heater rather than a loudspeaker.
The pressure and density waves describe first sound, and the motion of the normal component alone is not second sound, which needs the counterflow of both components.
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