1-3 物態（上）

In this chapter, we will discuss 3 states of matter, the conversion between them, and the transfer of energy between an object and its surroundings during a change of state. First, let us learn about the relationship between energy and change of state. 0:00 Opening 0:58 Change of state 3:04 Latent heat 7:06 Example 1 9:52 Example 2 14:10 Example 3 Show all topics https://bit.ly/physics_concepts_eng ------------------------------ Download examples: Chinese https://bit.ly/phy_con_ex_1-3-1_c English https://bit.ly/phy_con_ex_1-3-1_e ------------------------------ Supplementary information: [I heard that microwave can heat water to above 100 °C without boiling?]    • Superheating Water   DO NOT TRY THIS AT HOME!!! Indeed, a liquid may exceed its boiling point without boiling, and this phenomenon is known as "superheating". When water reaches its boiling point, normally it "cannot contain any more energy" and absorbing more energy would cause it to become gas and fly away. However, this transformation can be suppressed by the surface tension of the water and the pressure of the atmosphere. At this state, a slight disturbance would be enough for the water to break the suppression and boil. If the water is heated using a fire or an electric kettle, convection would occur in the water and this is enough disturbance for the water to boil, and so superheating would not occur. However, if you use a microwave oven to heat the water, the water basically does not move in the process, so the energy may keep accumulating and cause overheating. After heating, if you put something (such as a spoon) into the water, the disturbance caused can make the water suddenly boil and may injure you. Aside from going above the boiling point without boiling, it is also possible to go below the melting point without freezing. This phenomenon is called "supercooling". Superheating and supercooling are not common and occur only under special circumstances. [Effects of pressure on state of matter] More detailed investigation revealed that temperature is not the only factor that can cause matter to change its state. Changing the pressure around an object may also cause a change of state. Taking both temperature and pressure into consideration, we can draw a "phase diagram" to show the condition for changing states. The following takes the phase diagram of water as an example: https://commons.wikimedia.org/wiki/Fi... From the diagram, we can see that at a pressure of 100 kPa (about the air pressure at sea level), water becomes solid at 0 °C and becomes gas at 100 °C. However, at a higher pressure, water would not boil at 100 °C but instead boils at a higher temperature. On the other hand, at a temperature lower than 100 °C, it is possible to turn water into gas by lowering the pressure. [Is there a 4th state of matter?] Yes, and there are more than 4 of them. For example, when the temperature and pressure of matter both exceed the "critical point", it becomes a "supercritical fluid". This is a state of matter between liquid and gas: Like a gas, it does not have a fixed volume; like a liquid, it can dissolve substances. https://commons.wikimedia.org/wiki/Fi... Besides states between liquid and gas, there are also states between liquid and solid, such as liquid crystal. An object at liquid crystal state can flow freely like a liquid, but at the same time has a highly ordered atomic arrangement like a solid. https://commons.wikimedia.org/wiki/Fi... If you raise the temperature of a gas to an extremely high level, it can transform to a higher-energy state called a "plasma". When you see lightening in the sky, the glowing part of the lightening is air at plasma state. https://commons.wikimedia.org/wiki/Fi... "Solid", "liquid", "gas" and "plasma" are now usually called "the 4 common states of matter". So you may ask... what are the "uncommon states"? Well, you will need a whole piece of paper just for making a list of them... List of states: https://en.wikipedia.org/wiki/List_of... [Change of temperature in gases] If the temperature of a gas changes, the heat it gains or loses cannot be simply calculated from Q = mcΔT. What we need to know is not only the initial and final temperatures, but the initial and final values of either two of temperature T, volume V and pressure P (i.e. we need to know the initial and final positions of the gas in the phase diagram). Then, we will need to solve a rather advanced system of equations to find the heat absorbed Q (Q is negative if heat is lost): PV = nRT U = (3/2)PV ΔU = Q - W W = ∫PdV Solving this system of equations requires maths skills at a higher level, so I will not go into details here. ------------------------------ Acknowledgements: Royalty Free Music from https://audiohub.com #DSE #physics #thermodynamics #物理 #熱和氣體 #熱力學