Topic 2: Thermal physics

Thermal physics explains heat and temperature in terms of the tiny particles that make up all matter. By thinking about how these particles move, are spaced and are bonded, you can account for the three states of matter, why objects expand when heated, how much energy heating requires, and the three ways thermal energy travels.

The kinetic particle model and states of matter

All matter is made of particles in constant motion. The kinetic particle model describes three states. In a solid, particles are packed in a regular, closely spaced lattice and only vibrate about fixed positions; this gives solids a fixed shape and fixed volume. In a liquid, particles are still close together but can move past one another, so a liquid keeps a fixed volume but takes the shape of its container. In a gas, particles are far apart and move quickly in random directions, so a gas has neither fixed shape nor fixed volume and can be compressed easily. Heating a substance transfers energy to its particles. As they gain kinetic energy they move or vibrate faster, the average separation grows, and the substance can change state when the bonds between particles are overcome.

Brownian motion

Brownian motion is the small, random, jerky movement of tiny particles suspended in a fluid, such as smoke grains in air viewed under a microscope. The smoke grains are seen to jiggle about along unpredictable paths. This happens because the much smaller, fast moving air molecules collide with the visible grains from all sides. At any instant the collisions are unbalanced, so each grain is pushed first one way and then another. Brownian motion is important evidence for the kinetic particle model: it shows that a fluid is made of separate molecules that are themselves in constant, random motion, even though they are too small to see directly. Smaller suspended particles show larger, more obvious movements because they have less mass and respond more to each collision.

Gases: pressure, volume and temperature (Supplement)

Gas pressure is caused by the gas molecules colliding with the walls of their container. Each collision exerts a tiny force, and the many collisions per second produce a steady average pressure. Increasing the temperature gives molecules more kinetic energy, so they move faster and hit the walls harder and more often; at constant volume this raises the pressure. Reducing the volume of a fixed mass of gas at constant temperature crowds the molecules together, so collisions with the walls happen more often and the pressure rises. For a fixed mass of gas at constant temperature, pressure and volume are inversely related, which can be written as pV = constant, so p1 x V1 = p2 x V2. For example, halving the volume of a trapped gas at constant temperature doubles its pressure.

Thermal expansion of solids, liquids and gases

Most substances expand when heated and contract when cooled, because their particles vibrate or move more vigorously and take up more space on average. Gases expand the most for a given temperature rise, liquids expand more than solids, and solids expand the least. Useful applications include the liquid-in-glass thermometer, where a liquid expands up a thin tube as temperature rises, and the bimetallic strip, where two metals that expand by different amounts bend on heating and can switch a circuit in a thermostat or fire alarm. Expansion can also cause problems: bridges and railway lines need expansion gaps or rollers so they do not buckle in hot weather, and overhead power and telephone cables are left slightly slack so they do not snap when they contract in the cold.

Specific heat capacity (E = m c deltaT)

The specific heat capacity of a substance is the energy needed to raise the temperature of 1 kg of it by 1 degree C. It is measured in J/(kg degrees C). The thermal energy transferred is found using E = m c deltaT, where E is energy in J, m is mass in kg, c is specific heat capacity, and deltaT is the temperature change in degrees C. A large specific heat capacity, such as that of water (about 4200 J/(kg degrees C)), means a substance needs a lot of energy to warm up and releases a lot as it cools. Worked example: how much energy is needed to heat 2.0 kg of water from 20 degrees C to 70 degrees C? Here deltaT = 70 - 20 = 50 degrees C, so E = m c deltaT = 2.0 x 4200 x 50 = 420000 J (420 kJ).

Melting, boiling and specific latent heat (E = m L)

When a substance changes state its temperature stays constant even though energy is still being supplied. This is because the energy goes into breaking the bonds between particles rather than increasing their kinetic energy. Melting and boiling happen at fixed temperatures for a pure substance: the melting point and the boiling point. The specific latent heat of a substance is the energy needed to change the state of 1 kg of it without any change in temperature, measured in J/kg. It is calculated using E = m L, where L is the specific latent heat. There are two kinds: the specific latent heat of fusion for melting (or freezing) and the specific latent heat of vaporisation for boiling (or condensing). For example, melting 0.50 kg of ice with a specific latent heat of fusion of 340000 J/kg needs E = m L = 0.50 x 340000 = 170000 J.

Conduction

Conduction is the transfer of thermal energy through a material without the material itself moving along. When one end of a solid is heated, its particles vibrate more strongly and pass energy on to neighbouring particles through collisions. In metals there is a second, faster mechanism: free electrons gain kinetic energy at the hot end and carry it quickly through the material, which is why metals are excellent thermal conductors. Materials such as wood, plastic, air and other gases are poor conductors and are called thermal insulators. Conduction is most important in solids, because their particles are close together; gases conduct very poorly because their particles are far apart and collide less often.

Convection

Convection is the transfer of thermal energy through a fluid (a liquid or gas) by the movement of the fluid itself. When part of a fluid is heated it expands, becomes less dense, and rises; cooler, denser fluid sinks to take its place. This sets up a circulating convection current that carries energy around. Convection explains how a radiator warms a whole room, how a kettle heats water, and how sea breezes and other weather patterns form. Because it relies on the fluid being free to move, convection cannot happen in solids, where the particles are locked in place.

Radiation and its applications

Thermal radiation is the transfer of energy by infrared waves, which are part of the electromagnetic spectrum. Unlike conduction and convection, radiation needs no medium and can travel through a vacuum, which is how energy reaches the Earth from the Sun. All objects emit and absorb thermal radiation; the hotter an object, the more it emits. Dull, black surfaces are good emitters and good absorbers of radiation, while shiny, white or silvery surfaces are poor emitters and good reflectors. These ideas explain many designs: a vacuum flask uses silvered walls to reduce radiation, a vacuum to stop conduction and convection, and a stopper to limit convection, keeping drinks hot or cold; solar panels and cooling fins are often painted black to maximise energy transfer.