Science Physical Science Flashcards

A sample of an unknown gas at STP occupies 11.2 liters and has a mass of 16 grams. Which of the following is most likely the identity of this gas?

Answer: Methane (CH₄)

At STP, one mole of any ideal gas occupies 22.4 L. If 11.2 L is half a mole, then the molar mass = 16 g ÷ 0.5 mol = 32 g/mol. However, methane (CH₄) has a molar mass of 16 g/mol, meaning 16 g = 1 full mole occupying 22.4 L — but wait, 11.2 L = 0.5 mol, so mass = 0.5 × 16 = 8 g for CH₄. Re-examining: 16 g at 11.2 L means molar mass = (16/11.2) × 22.4 = 32 g/mol, which matches O₂ (32 g/mol). The correct answer is Oxygen (O₂).

Two identical metal spheres are placed in thermal contact inside an insulated container. Sphere A is at 80°C and Sphere B is at 20°C. After reaching thermal equilibrium, which statement BEST describes the entropy of the system?

Answer: Entropy increased because heat flowed spontaneously from hot to cold

Entropy is a measure of disorder in a system. When heat spontaneously flows from a hot object to a cold one, this is an irreversible process that increases the total entropy of the system, even in an isolated container. The Second Law of Thermodynamics states that entropy of an isolated system always increases during spontaneous processes. Although total energy is conserved, the distribution of energy becomes more disordered.

A physicist observes that when light of frequency 8.0 × 10¹⁴ Hz strikes a metal surface, electrons are ejected with maximum kinetic energy of 1.2 eV. When light of frequency 6.0 × 10¹⁴ Hz strikes the same surface, no electrons are ejected. Which phenomenon does this BEST illustrate?

Answer: The photoelectric effect and the concept of a work function threshold

The photoelectric effect demonstrates that electrons are only ejected when incident light exceeds a minimum threshold frequency, regardless of intensity. This minimum frequency corresponds to the metal's work function — the minimum energy needed to liberate an electron. At 6.0 × 10¹⁴ Hz, photon energy is below the work function so no emission occurs, while at 8.0 × 10¹⁴ Hz, the excess energy above the work function becomes kinetic energy of ejected electrons. Einstein's explanation of this earned him the Nobel Prize.

A 2 kg block slides down a frictionless incline of height 5 meters, then travels across a rough horizontal surface with a coefficient of kinetic friction of 0.4. Using g = 10 m/s², approximately how far does the block travel along the horizontal surface before stopping?

Answer: 12.5 meters

First, find the block's kinetic energy at the bottom of the incline using conservation of energy: KE = mgh = 2 × 10 × 5 = 100 J. On the horizontal surface, friction decelerates the block. The friction force = μ × m × g = 0.4 × 2 × 10 = 8 N. Using the work-energy theorem: friction force × distance = KE, so 8 × d = 100, giving d = 100/8 = 12.5 meters.

In nuclear fission of uranium-235, the products include barium-141, krypton-92, and free neutrons. If this reaction takes place in a nuclear reactor, what is the PRIMARY role of the moderator material (such as graphite or heavy water)?

Answer: To slow down fast neutrons so they can more effectively trigger further fissions

Neutrons released in fission are 'fast neutrons' with high kinetic energy. U-235 undergoes fission most efficiently when struck by 'thermal' (slow) neutrons. The moderator slows fast neutrons through elastic collisions without absorbing them, increasing the probability that each neutron will induce another fission event. Control rods (not the moderator) absorb neutrons to regulate the reaction rate. The moderator's job is specifically to thermalize neutrons.

A wave traveling through medium A enters medium B and its speed decreases. Assuming the frequency remains constant, which of the following correctly describes what happens to the wave?

Answer: Wavelength decreases and the wave bends toward the normal at the boundary

The wave speed, frequency, and wavelength are related by v = fλ. If the speed decreases and frequency stays constant (frequency is set by the source, not the medium), then wavelength must also decrease proportionally. Additionally, when a wave slows down entering a new medium, it refracts (bends) toward the normal — this is analogous to Snell's Law in optics, where light bends toward the normal when entering a denser (slower) medium.