Thermal physics flashcards

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Describe the arrangement and motion of particles in a solid, a liquid and a gas.
Solid: closely packed in a regular pattern, vibrating about fixed positions. Liquid: close but irregular, able to slide past each other. Gas: far apart, random, fast motion.
How does the particle model explain why a solid has a fixed shape and volume but a gas does not?
Strong forces hold solid particles in fixed positions, so shape and volume are fixed. Gas particles have negligible forces between them, so they spread out to fill any container.
What is Brownian motion and what does it show?
It is the random jerky motion of visible particles such as smoke or pollen in air or water. It is caused by collisions with fast, invisible, randomly moving molecules, so it is evidence that molecules exist.
How does a gas exert pressure on its container?
Gas particles collide with the walls and rebound. Each collision gives a tiny force; the huge number of collisions per second gives a steady force per unit area.
Why does the pressure of a fixed mass of gas in a rigid container rise when it is heated?
The particles gain kinetic energy and move faster. They hit the walls more often and with greater force, so pressure rises. Volume stays the same.
What is absolute zero and how do you convert between °C and kelvin?
Absolute zero, 0 K or −273 °C, is the lowest possible temperature, where particles have minimum kinetic energy. T(K) = θ(°C) + 273.
Convert 27 °C to kelvin and 350 K to °C.
27 °C + 273 = 300 K. 350 K − 273 = 77 °C.
(Extended) State Boyle's law and the equation for it.
For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume: pV = constant, so p₁V₁ = p₂V₂.
(Extended) A gas at 100 kPa and 6.0 cm³ is squeezed to 2.0 cm³ at constant temperature. What is the new pressure?
p₂ = p₁V₁/V₂ = 100 × 6.0 ÷ 2.0 = 300 kPa.
(Extended) Use the particle model to explain why gas pressure rises when its volume is reduced at constant temperature.
Particles have the same speed but less room, so they hit the walls more often. More collisions per second per unit area means a greater pressure.
Why do solids, liquids and gases expand when heated, and which expands most?
The particles vibrate or move more and take up more space on average. Gases expand most, then liquids, then solids, because the forces between particles are weakest in gases.
Describe two practical uses or problems of thermal expansion.
Uses: bimetallic strips in thermostats; liquid-in-glass thermometers. Problems: rails and bridges buckle unless expansion gaps are left; overhead cables sag in the heat.
How does a liquid-in-glass thermometer work, and how can its sensitivity be increased?
The liquid expands up a narrow tube as temperature rises. A narrower tube, a larger bulb or a liquid that expands more makes it more sensitive. Range depends on the liquid's melting and boiling points.
What is a thermocouple and what is one advantage over a liquid-in-glass thermometer?
Two different metals joined at two junctions give a voltage that depends on the temperature difference. It responds quickly, can measure very high temperatures and gives an electrical output.
What is the difference between heat energy and temperature?
Temperature measures how hot something is (average kinetic energy of particles, in °C or K). Thermal energy is the total energy transferred or stored, measured in joules; a bath has more than a cup at the same temperature.
(Extended) Define specific heat capacity and give the equation.
It is the energy needed to raise the temperature of 1 kg of a substance by 1 °C (or 1 K): c = ΔE/(mΔθ), so ΔE = mcΔθ. Unit: J/(kg °C).
(Extended) How much energy is needed to heat 2.0 kg of water by 15 °C? (c = 4200 J/(kg °C))
ΔE = mcΔθ = 2.0 × 4200 × 15 = 126 000 J.
(Extended) How can you measure the specific heat capacity of a metal block in the lab?
Insulate the block and fit a heater and thermometer. Record the heater's voltage, current and time (E = IVt), and the temperature rise. Then c = E ÷ (mΔθ).
What is the difference between melting, boiling and evaporation?
Melting: solid to liquid at the melting point. Boiling: liquid to gas throughout the liquid at the boiling point. Evaporation: liquid to gas at the surface only, at any temperature.
Why does temperature stay constant while a substance melts or boils?
The energy supplied goes into breaking bonds between particles (increasing potential energy), not into increasing their average kinetic energy, so temperature stays constant.
(Extended) Define specific latent heat and say how it differs for fusion and vaporisation.
The energy needed to change the state of 1 kg of a substance without a temperature change: L = ΔE/m. Latent heat of vaporisation is much larger than that of fusion, as particles must separate completely.
(Extended) How much energy melts 0.50 kg of ice at 0 °C? (specific latent heat of fusion 334 000 J/kg)
ΔE = mL = 0.50 × 334 000 = 167 000 J.
(Extended) Why does evaporation cause cooling, and what increases its rate?
The fastest, most energetic molecules escape, so the average kinetic energy of those left falls. Rate rises with higher temperature, larger surface area and moving air over the surface.
How does thermal energy transfer by conduction?
Particles vibrate more at the hot end and pass energy on to neighbours by collisions. Metals are the best conductors; gases and liquids are poor conductors.
(Extended) Why are metals better conductors than non-metals?
Metals have free (delocalised) electrons which move through the metal and carry energy quickly, as well as particle vibrations. Non-metals rely on vibrations alone.
Explain how convection transfers thermal energy in a fluid.
Heated fluid expands, becomes less dense and rises. Cooler, denser fluid sinks to take its place, setting up a convection current. It cannot happen in solids or a vacuum.
What is thermal radiation and which surfaces are the best absorbers and emitters?
Infrared waves emitted by all objects, needing no medium (travels through a vacuum). Dull black surfaces absorb and emit best; shiny white or silver surfaces absorb and emit least and reflect best.
List ways a vacuum flask reduces heat loss.
Vacuum between the walls stops conduction and convection. Silvered walls reduce radiation. A plastic stopper and insulating case reduce conduction and convection through the top.
How do you reduce heat loss from a house, and which mechanism does each method target?
Loft insulation and cavity wall foam reduce conduction; trapped air reduces convection; double glazing reduces conduction; reflective foil behind radiators reflects radiation.

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