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Flashcards · 0625 · Topic 2

Thermal physics flashcards

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

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2.1 Kinetic particle model of matter

Particles in a solid
Very close together in a regular arrangement, in fixed positions. They only vibrate about fixed points.
Particles in a liquid
Close together in a random arrangement. They move around, sliding past each other.
Particles in a gas
Far apart in a random arrangement. They move fast and randomly, in straight lines between collisions.
Brownian motion
The random, erratic motion of visible smoke/pollen particles suspended in a gas or liquid, caused by collisions with the much smaller, fast-moving gas/liquid particles — evidence for the particle model.
Absolute zero
−273 °C (0 K): the lowest possible temperature, where particles have the least possible kinetic energy.
Converting between °C and kelvin. What is 27 °C in K?
T (in K) = θ (in °C) + 273. 27 °C = 300 K.
How do gas particles cause pressure?
They collide with the walls of the container. More frequent or harder collisions give a higher pressure.
A fixed mass of gas is heated at constant volume. What happens to its pressure, and why?
The pressure increases. The particles move faster, so they hit the walls more often and harder.
Boyle's law: pV = constantExtended
For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume. Halving the volume doubles the pressure.
Gas at 100 kPa is compressed from 2.0 m³ to 0.50 m³ at constant temperature. New pressure?Extended
p₁V₁ = p₂V₂: 100 × 2.0 = p₂ × 0.50, so p₂ = 400 kPa.

2.2 Thermal properties and temperature

Thermal expansion: everyday consequences and uses
Expansion gaps in bridges and railway tracks; bimetallic strips in thermostats; a tight metal jar lid loosens when warmed.
Order of thermal expansion for solids, liquids and gases (at constant pressure), and whyExtended
Gases expand most, then liquids, then solids. Gas particles are far apart with very weak forces between them; particles in solids are held most strongly.
What does a rise in an object's temperature tell you about its energy?
Its internal energy has increased.
Specific heat capacityExtended
The energy needed per unit mass per unit temperature rise: the energy to raise 1 kg of a substance by 1 °C. c = ΔE/(mΔθ), in J/(kg °C).
A 12 V heater takes 4.0 A for 5.0 min and warms a 1.0 kg block by 16 °C. Find c.Extended
E = IVt = 4.0 × 12 × 300 = 14 400 J. c = 14 400 ÷ (1.0 × 16) = 900 J/(kg °C).
Why is a measured specific heat capacity usually higher than the true value?Extended
Some energy is lost to the surroundings, so more energy is supplied than goes into the block. Insulating the block reduces this.
Melting/boiling
Energy is put in, but temperature stays constant while the state changes — the energy is used to overcome the forces between particles (increasing potential energy), not to speed the particles up.
Melting and boiling points of pure water at standard atmospheric pressure
Melts (and freezes) at 0 °C; boils (and condenses) at 100 °C.
Condensation
Gas particles lose energy, slow down and are pulled close together by the attractive forces, forming a liquid. Energy is transferred to the surroundings at constant temperature.
Why does evaporation cool a liquid?
The more energetic particles escape from the surface, so the average kinetic energy of the remaining particles falls, and the temperature falls.
Boiling vs evaporationExtended
Boiling: at one fixed temperature, throughout the liquid (bubbles). Evaporation: at any temperature, from the surface only.
Three factors that increase the rate of evaporationExtended
Higher temperature, larger surface area, and air moving over the surface.

2.3 Transfer of thermal energy

Conduction
Needs a medium (solids are best). Vibrating particles pass energy on to their neighbours.
Convection
Needs a fluid (liquid or gas). Heated fluid expands, becomes less dense and rises; cooler, denser fluid sinks to replace it.
Radiation
Needs no medium: works through a vacuum. Infrared is emitted by all objects. Dull black surfaces are the best emitters and absorbers; shiny/light surfaces are the worst emitters and best reflectors.
An experiment to compare thermal conductors
Heat one end of equal rods of different materials equally, with a drawing pin fixed by wax to the far end of each. The pin on the best conductor falls first.
How can a convection current in water be shown?
Drop a potassium manganate(VII) crystal at the bottom of one side of a beaker of water and heat gently beneath it. The purple colour rises, moves across the top and sinks on the far side.
Why are metals much better thermal conductors than other solids?Extended
In all solids, lattice vibrations pass energy along. Metals also have free electrons, which move through the metal and carry energy quickly.
Why do gases conduct thermal energy badly?Extended
The particles are far apart, so there is little particle-to-particle contact to pass vibrations along.
When does an object stay at a constant temperature?Extended
When it transfers energy away at the same rate as it receives energy. If it receives energy faster, its temperature rises; if it loses energy faster, it falls. The Earth's temperature depends on this balance.
Two factors that increase the rate at which a surface emits radiationExtended
A higher surface temperature and a larger surface area.
How does a car radiator transfer energy from the coolant to the air?Extended
By conduction through the thin metal tubes and fins, then by convection in the air moving over the large surface area. Mainly conduction and convection, not radiation.

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