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- Describe qualitatively the thermal expansion of solids, liquids and gases.
When matter is heated, its particles gain energy, which is exerted as kinetic energy.
In solids, the particles vibrate harder and faster, creating more space between the particles, causing them to expand. This is most visible in metals. This process is thermal expansion.
In liquids, the particles move around faster, weakening the intermolecular forces of attractions, and are thus held less closely together. The liquid expands. If you want, you can test this out yourself, by measuring and comparing the volume of the same mass of water, before and after heating. A common example is the traditional thermometer – as the bulb of the thermometer heats up, the heat is conducted to the liquid. This causes the liquid to expand, forcing it to rise up the thermometer.
In gases, particles move faster as they are heated. If they are heated under constant pressure, the gas particles collide harder with the container surfaces, forcing them out, and allowing the gas to expand. This can be seen when warming the gas in a gas syringe.
If gases are heated at a constant volume, however, they do not expand – the gas pressure simply increases.
Note that the cooling down of substances tends to have the opposite effect – the particles lose kinetic energy, come closer together, and thus contract.
- Explain in terms of motion and arrangement of molecules the relative order of magnitude of the expansion of solids, liquids and gases.
When considering thermal expansion, gases expand the most, followed by liquids, and solids expand the least. This is because gases have the weakest intermolecular forces of attraction, allowing their molecules to move the furthest apart, and solids have the strongest intermolecular forces, limiting the range of motion of the particles.
- Identify and explain some of the everyday applications and consequences of thermal expansion.
- We often use hot water to warm up the lid of a jar. This expands the lid (metals expand more than glass), making it easier to remove.
- Liquid in thermometers expand and contract as the temperature changes. The volume of the liquid at a given temperature is how we read the temperature off of a thermometer.
- Overhead cables have to be slack so that on cold days when they contract, they won’t snap or detach.
- Expansion joints – these are found on most large bridges. They look like two metal combs, their teeth interlocking, and have small gaps between each other. When heat causes the bridge to expand, the two sides of the expansion joint move towards each other. As the temperature cools, they gradually retract. This gives the bridge room for expansion and contraction, preventing the cracking/ deformation of the bridge. The expansion joints have interlocking ‘teeth’ because this minimizes the bump that motorcyclists feel as they ride over it.
- Bimetallic strips in thermostats. This requires a little more explanation, so I’ve written a paragraph about it below.
Thermostats are devices used to adjust the temperature of a heating or cooling system.
In order to understand how they work, you’ll need to know a little about expansion coefficients.
Thermal expansion is expressed, in numbers, as the change in length, area, or volume per unit temperature change.
For wires, as the cross-sectional area is often tiny and thus negligible, we don’t have to concern ourselves with calculating the area or volume change – we can just measure the change in length of the wire per unit temperature change. This value would be the coefficient of linear expansion,
For sheets, such as metal sheets, its thickness is negligible when compared to its area, so we don’t have to calculate its volume change. We normally use the change in area per unit temperature. This is the coefficient of superficial expansion.
For other substances, like materials in 3D shapes, or liquids or gases, we use the coefficient of cubical expansion. This is the change in area per unit temperature change.
Bimetal thermostats have a bimetallic strip. This is a strip in which there are two metals, with different coefficients of linear expansion, placed side by side. Therefore, when the strips warm up, one of the metals linearly expand more than the other, causing the bimetallic strip to bend. When it becomes hot enough, the strip bends enough to close the circuit, and the air conditioner turns on, cooling down the room. Once the room has reached the desired temperature, the strip slowly unbends, opening the circuit and turning off the air conditioner. The same mechanism can be used for heaters – when it is warm, the strip bends away from the circuit, and is it grows colder, the strip straightens out until it closes the circuit and the heater can turn on again.
When you adjust the temperature on a thermostat, you’re adjusting how far the bimetal strip has to bend/ straighten out to close the gap.
- Describe qualitatively the effect of a change of temperature on the volume of a gas at constant pressure.
This has already been explained in point 1.
As the temperature increases, the gas molecules gain kinetic energy and move faster. This causes them to collide with the container surfaces harder, forcing the surfaces outwards and allowing the gas to expand.
In other words, at constant pressure, the volume is directly proportional to the temperature in Kelvin (K).
In formula form,
PV/T = k;
Where P is the pressure in Pascals (Pa),
V is the volume in m3,
T is the temperature in Kelvin (K)
And k is a constant.
If we rearrange the formula to PV = kT, it becomes apparent that as T increases so does V.
Technically, you don’t have to know anything about this particular formula, but I personally understand information more easily when they’re presented in a neat, mathematical way, so I thought it might help you guys too 😊
Notes submitted by Sarah.
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