Correct Answer: B

Rationale: The continuity equation for steady, incompressible flow states that the mass flow rate is the product of the fluid's density, velocity, and cross-sectional area. Hence, ṁ = ρvA.
Choice A is incorrect because the mass flow rate can be determined using the given formula.
Choice C is incorrect as Bernoulli's principle does not directly relate to the mass flow rate calculation.
Choice D is incorrect as the equation of state is not needed to calculate the mass flow rate in this scenario.

Correct Answer: D

Rationale: The force between two charges is calculated using Coulomb's Law, which states that the force is proportional to the product of the two charges and inversely proportional to the square of the distance between them. Given that one charge is twice as great as the other and the distance between them is 50 cm, we can calculate the repulsive force. The magnitude of the resulting repulsive force is 2.5 10^−3 N.
Choice A is incorrect as it does not match the calculated value.
Choice B is incorrect as it is significantly higher than the correct answer.
Choice C is incorrect as it represents 10^−3 N, which is lower than the calculated value.

Correct Answer: A

Rationale: A refrigerator operates on a Carnot cycle by transferring heat from a cold reservoir to a hot reservoir.
Choice A is correct because a refrigerator removes heat from a cold reservoir at a lower temperature.
Choice B is incorrect as a refrigerator does not violate the first law of thermodynamics but rather requires work input to transfer heat.
Choice C is incorrect as a refrigerator does not increase the total entropy of the universe.
Choice D is incorrect because a refrigerator does not operate isothermally at both the hot and cold reservoirs.

Correct Answer: D

Rationale: The change in length of the spring is 40 cm - 30 cm = 10 cm = 0.10 m. The force required to compress or stretch a spring is given by Hooke's Law: F = k x, where F is the force, k is the spring constant (20 N/m in this case), and x is the change in length (0.10 m). Substituting the values into the formula: F = 20 N/m 0.10 m = 2 N.
Therefore, the correct answer is 2 N.
Choice A (200 N) is incorrect because it miscalculates the force.
Choice B (80 N) is incorrect as it does not apply Hooke's Law correctly.
Choice C (5 N) is incorrect as it underestimates the force required.

Correct Answer: B

Rationale: The triple point is a unique temperature and pressure where all three phases (solid, liquid, and gas) of a pure substance can coexist in equilibrium. It is a constant for each substance and independent of container size.
Choice A is incorrect because the triple point is a fixed point regardless of the container size.
Choice C is incorrect as the pressure at the triple point is specific for each substance and will not be zero unless the substance has unique properties.
Choice D is incorrect since the temperature at the triple point is precisely defined and cannot be above the boiling point of the liquid phase.