Thermal Physics PDF

Thermal Physics States of matter Three states of matter 1. Solids o Solids have a definite shape and definite volume o Solids aren’t compressible and can’t flow 2. Liquids o Liquids have no definite shape but have a definite volume o Liquids can flow to take the s...

Thermal Physics States of matter Three states of matter 1. Solids o Solids have a definite shape and definite volume o Solids aren’t compressible and can’t flow 2. Liquids o Liquids have no definite shape but have a definite volume o Liquids can flow to take the shape of a container but are not compressible 3. Gases o Gases have no definite shape and no fixed volume o Gases can flow to take the shape of their container and are highly compressible Change of states of matter o When a substance changes state, the number of molecules in that substance doesn’t change, so neither does its mass o The only thing that changes is its energy o Changes of state are physical changes, so they are reversible Melting and freezing o Melting occurs when a solid turns into a liquid o Freezing occurs when a liquid turns into a solid Boiling and condensing o Boiling occurs when a liquid turns into a gas. This is also called evaporation o Condensing occurs when a gas turns into a liquid Properties Solid Liquid Gas Arrangement of Closely Loosely packed Far apart molecules packed Shape Fixed shape No fixed shape No fixed shape Volume Fixed volume Fixed volume No fixed volume Rigid/flow Rigid Flow Flow Density High Medium Low Intermolecular force of Strong Weak Very weak attraction Vibration/movement Vibrate in Vibrate vigorously Move randomly in fixed position and slide past each all directions other Absolute zero o The amount of pressure that a gas exerts on its container is dependent on the temperature of the gas o The particles move with more energy as the temperature increases o As the temperature of the gas decreases, pressure on the container also decreases o There is a temperature at which the particles in a gas exert no pressure, and at this temperature, they must no longer be moving and hence, not colliding with the walls of the container. This temperature is called absolute zero(-273°C) o Absolute zero is defined as the temperature at which the molecules of a substance have zero kinetic energy In melting and boiling process, energy is given to break the bond between the molecules and increase the separation between the molecules o Kinetic energy remains the same o Potential energy increase In freezing and condensation process, heat energy is taken out to make the bond between the molecules and decrease the separation between the molecules o Kinetic energy remains the same o Potential energy decreases Brownian motion o Brownian motion is the random zig zag movement of particles in a liquid or gas, produced by large numbers of collision with small particles that are too small to see Evaporation o The process of changing a liquid to gas below the boiling point is called evaporation Factors affecting the rate of evaporation o Evaporation is increased by: ▪ Increase in temperature ▪ Increase in surface area ▪ Passing air (wind) across the surface ▪ Decrease in humidity Evaporation causes cooling o During evaporation, high energetic molecules escape from the surface of the liquid, leaving behind low energetic molecules which decreases the average kinetic energy and therefore cooling the object Difference between boiling and evaporation o Boiling ▪ Fast ▪ Bubbles are seen ▪ Occurs at fixed temperature ▪ Occurs throughout liquid o Evaporation ▪ Slow ▪ No bubbles ▪ Occurs at any temperature ▪ Occurs only at surface Transfer of heat energy Conduction o Conduction takes place in solids o Conduction is the transfer of heat from one region to another through particle vibration and movement of free electrons o Metals are the best thermal conductors because they have a high number of free electrons o Liquids and gases are poor conductors of heat o When a substance is heated, the atoms, or ions, start to move around (vibrate) more ▪ The atoms at the hotter end of the solid will vibrate more than the atoms at the cooler end ▪ As they do so, they collide into each other, transferring energy from atom to atom ▪ These collisions transfer internal energy until thermal equilibrium is achieved throughout the substance o Substance which allows heat transfer due to the presence of free electrons are known as conductors. E.g. Metals, like silver, copper, aluminum, iron, and nonmetals like graphite o Substances which do not allow heat energy to transfer due to the absence of free electrons are known as insulators. E.g. Wood, plastic, rubber o Vacuum is the poorest conductor or best insulator o Air is a poor conductor of heat, or insulator Convection ▪ Transferred through movement of particles ▪ Takes place due to density change ▪ Takes place only in liquids and gases Process o When part of a gas or liquid is heated, it expands, increasing its volume and decreasing its density o Since it is less dense, the gas or liquid around it rises  o The cooler, denser molecules above the heated ones sink and replace it o This is then again heated, repeating the process o This results in movement of molecules in a circular path, creating a convection current Sea breeze and land breeze o During the day, sea breeze occurs o Land is warm and sea is cool o Hot air rises from the land as it is heated and less dense o Air above the sea is cooler and it moves from sea to land o During the night, land breeze occurs Radiation o Heat transferred without any material medium is called thermal radiation o Heat radiations are infra-red radiation o Dull or black surfaces are best absorbers and best emitters of radiation o White surfaces are poor emitters of radiation o Silver or shiny surfaces are good reflectors of radiation Thermal expansion o Thermal expansion is defined as the increase in volume when heated o When materials are heated, the molecules move around faster and push further apart o Gas expands the most o Solids expands the least Application o Thermometer: - it relies on the expansion of liquid to measure temperature o Joint expansion o Fitting on an axel o Bimetallic strip (temperature activated switches) Consequences o Metal railway tracks warping o Cracks on road and bridges- to avoid cracks, give space for expansion to happen without causing damage Heat capacity The amount of heat energy needed to increase the temperature of a substance by one- degree Celsius is called heat capacity Specific heat capacity The amount of heat energy required to increase one kg of a substance by one-degree Celsius is known as specific heat capacity (J/kg°C) Formula for heat energy E = mcΔΘ