Flashcards · 5054 · Topic 2

Thermal physics flashcards

15 cards on Topic 2, subtopics 2.1 to 2.3. Pick a subtopic or study them all.

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Particles in solids, liquids and gases: spacing and movement: Solid: close, regular, vibrate in place. Liquid: close, irregular, slide past each other. Gas: far apart, random, fast with straight paths between collisions.
Absolute zero, and converting °C to K. What is 25 °C in kelvin?: Absolute zero, −273 °C = 0 K, is the lowest possible temperature (particles have least kinetic energy). T = θ + 273, so 25 °C = 298 K.
Explain why a sealed rigid can of gas has a higher pressure when warm: Gas pressure is the force per unit area from particles hitting the walls. Warmer particles move faster, so they hit the walls more often and with greater force.
Gas at 90 kPa and 50 cm³ is compressed to 30 cm³ at constant temperature. New pressure?: p₁V₁ = p₂V₂ (fixed mass, constant temperature): p₂ = 90 × 50 ÷ 30 = 150 kPa.

What happens to particles when a substance expands on heating? Which state expands most?: They move or vibrate more, so their average spacing increases; the particles do not get bigger. Gases expand most, then liquids, then solids.
Specific heat capacity: The energy required per unit mass per unit temperature increase. c = ΔE/(mΔθ), in J/(kg °C).
A heater warms 2.0 kg of water by 10 °C in 5.0 min (c = 4200 J/(kg °C)). Power, if no losses?: E = mcΔθ = 2.0 × 4200 × 10 = 84 000 J. P = 84 000 ÷ 300 = 280 W.
Latent heat: The energy transferred for a substance to change state, at constant temperature. It overcomes the forces between particles, raising their potential energy, not their kinetic energy.
Boiling vs evaporation: Boiling: one fixed temperature, throughout the liquid, bubbles, fast. Evaporation: any temperature, surface only, no bubbles, slow.
Why does evaporation cool a liquid, and what makes it faster?: Only the fastest particles leave the surface, so the remaining liquid has a lower average kinetic energy. Faster with higher temperature, larger surface area and moving air.

Conduction vs convection: Conduction: energy passes between neighbouring particles that stay in place (lattice vibrations; free electrons in metals). Best in solids. Convection: the heated fluid itself moves; warm, less dense fluid rises.
Thermal radiation: what it is, and the best and worst surfaces: Infrared electromagnetic waves; needs no medium. Dull black: best emitter and absorber. Shiny silver or white: worst emitter and absorber, best reflector.
What does the rate of emission of infrared radiation from a surface depend on?: Its surface temperature (a hotter surface emits more), its surface area (a larger area emits more), and its colour and texture (dull black emits most).
Explain a metal pan with a plastic handle, and a heater that warms a whole room: The metal base is a good conductor, so energy passes quickly to the food; the plastic handle is a poor conductor, so it stays cool to hold. The heater warms nearby air, which becomes less dense and rises; cooler, denser air sinks to replace it, setting up a convection current.
How does thermal insulation in buildings reduce energy transfer?: Materials such as loft insulation, cavity-wall foam and double glazing trap air, which is a poor conductor, and stop it moving, so conduction and convection are reduced.