On this essay I am going to investigate about how thermal physics interfere with the construction of railways. I was given the possibility to choose between explaining how thermal physics is used in the design of railways or roads and bridges. I chose to explain how it is used in railways because I find it particular more interesting. After that, I will explain what Thermal Expansion is. Posteriorly I will be discussing the physics role when designing railways. To complete my One World Essay, I am going to discuss 2 world aspects: Environmental and Economical. I will be talking through the pros and cons. I think both these world aspects have a lot to do with the topic being studied, including its effect.
In today’s days heat, energy and temperature have a significant effect on our lives. We are highly sensitive to hot and cold. We think about it in almost everything we do. Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. Heat (thermal energy) causes increased atomic motion in materials, and this change in the kinetic energy of the atoms and molecules of a material will cause it to increase in volume. The equation of Thermal Expansion is ΔL = αLΔT. Being ΔL a particular length measurement. ΔT is the rate of change of the linear dimension per unit change in temperature. L is the original length. The coefficient of thermal expansion is different in every material: α.
Expansions in Solids
Materials generally change their size when subjected to a temperature change while the pressure is held constant. In the special case of solid materials, the pressure does not have a reasonable affect on the size of an object. Common engineering solids usually have coefficients of thermal expansion that do not vary significantly over the range of temperatures where they are designed to be used, so where extremely high accuracy is not required, practical calculations can be based on a constant, average, value of the coefficient of expansion.
Expansions in Liquids
Liquids expand at different rates, depending on their composition. Liquids also expand by different amounts at different temperatures. How much a volume of a given liquid will expand as its temperature rises from one degree to the next can be determined by experiment or by consulting tables. The expansion of freezing water can cause rock and masonry to crack, and will also crack plumbing pipes and engine blocks.
Expansions in Gases
Unlike liquids, under equal pressures, all gases expand at the same rate. A gas expands by the same proportion as the temperature rises, provided external pressure remains the same. The effect of heat on the expansion of gases is stated in Charles’ Law. A gas can be made to expand without the addition of outside heat if the pressure confining the gas is reduced. This principle is stated in Boyle’s Law. Energy for the expansion is drawn from the expanding gas itself, causing a lowering of temperature in the gas. Steam engines and turbines, rockets, and internal combustion engines are powered by the expansion of gases. Bread rises in baking because heat expands the carbon dioxide gas it contains.
How does Thermal Physics help in the construction of Railways?
Image 1- Fishplate Image 1- Fishplate Due to Thermal Expansion, railways end up by changing their line shape and size. Engineers have a way of solving these problems. They build fishplates (Image 1). A fishplate is a bar that is bolted to the ends of two rails to join them together in a track. They were made with intuit of maintaining alignment and electrical continuity. Another way of preventing railways of changing their line shape is leaving small gaps between them. This helps by allowing expansion and contraction of the metal due to temperature changes. If there weren’t gaps, the metal would buckle or develop stress fractures as it expanded and contracted.
Image 2 shows Adelaide Darwin Railway Line in Australia. We can see that due to the fishplates bolted on the ends of the rails and the gaps between the railways, it didn’t suffer consequences in it’s alignment. It was not affected by thermal heat. It maintained straight without any warping. Image 2 – Adelaide Darwin Railway Line, Australia
Image 2 – Adelaide Darwin Railway Line, Australia Image 3 shows an example of a railway in Melbourn, England. Unlike image 2, in image 3 we can see that the railway has suffered alterations due to thermal expansion. Due to the heat, the railway warped. The first railways that where built where made of wood. Because of the immense weight of the trains, these railways soon frayed. Image 3 – Warped Railway, Melbourn, England
Image 3 – Warped Railway, Melbourn, England On today’s days, due to evolutions, this doesn’t happen any more. Currently most of the track use hot rolled steel or iron and steel. These tracks are made of a top quality of steel alloy due to the fact that they are exposed to an intensive pressure.