Thermal Physics Notes PDF

Chapter -5 THERMAL PHYSICS Simple kinetic molecular model of matter Molecular model Solids ○ Molecules close together in regular pattern ○ Strong intermolecular forces of attraction ○ Molecules vibrate but can’t move about ○ Cannot flow, have fixed shape and cannot be compr...

Chapter -5 THERMAL PHYSICS Simple kinetic molecular model of matter Molecular model Solids ○ Molecules close together in regular pattern ○ Strong intermolecular forces of attraction ○ Molecules vibrate but can’t move about ○ Cannot flow, have fixed shape and cannot be compressed Liquids ○ Molecules close together in random arrangement ○ Weaker intermolecular forces of attraction than solids ○ Molecules move around each other ○ Flow, take the shape of their container and cannot be compressed Gases ○ Molecules far apart in random arrangement ○ Negligible/very weak intermolecular forces ○ Molecules move quickly in all directions ○ Flow, completely fill their container and can be compressed Brownian motion Gas molecules move rapidly and randomly This is due to collisions with other gas molecules Massive particles may be moved by light, fast- moving molecules The temperature of a gas is related to the average kinetic energy of the molecules. The higher the temperature, the greater the average kinetic energy and so the faster the average speed of the molecules. Total Kinetic energy and potential energies of all atoms or molecules in a material is called its internal energy. Measuring Temperature Thermocouple: Contains two different metals which meet The temperature difference between them causes a tiny voltage which makes a current flow; the greater the temperature difference the greater the current. Used for high temperatures which vary rapidly Liquid-in-glass thermometer: As temperature rises or falls, the liquid expands or contracts. Amount of expansion can be matched to temperature on a scale. Clinical thermometer. A thermometer used to measure body temperature that continues to show the highest temperature reached by a column of liquid until the thermometer is reset by shaking. Sensitivity, range and linearity: Sensitivity is the change in length per change in temperature. ○ To increase the sensitivity of a thermometer, use a bigger bulb or a narrower bore.  Range is the difference between maximum and minimum temperatures. To increase the range of a thermometer, use a wider bore or a longer stem. Linearity is when a given change in temperature causes the same change in length. Fixed points are used to calibrate thermometers. For example, the fixed points of the celsius scale are the melting point and the boiling point of water. Temperature is the measure of the average heat of the molecules in a substance that gives the degree of hotness or coldness of the substance. Thermal properties and temperature Thermal expansion of solids, liquids and gases  Solids and Liquids: When heated, particles vibrate more, causing them to push apart slightly, resulting in expansion.  Gases: Heating increases particle speed and collisions with container walls, which causes container expansion to maintain pressure. Applications  Bimetallic Strips: Made from metals with different expansion rates (e.g., copper and iron). Used in: o Fire Alarms: Bends to complete an electrical circuit when exposed to heat, triggering alarms. o Thermostats: Maintains temperature by bending to break or complete electrical circuits  Shrink-fitting: Cooling components contracts them, fitting tightly into other parts upon warming. Used in manufacturing for tight connections without fasteners.  Lid Removal: Expanding metal lids with hot water loosens them from glass jars, leading to easier opening. Precautions  Expansion joints are spaces left between rail tracks used in railways, and pipes to allow for thermal expansion without damage.  Internal Energy and Heating  Internal energy increases when an object is heated.  Different materials require varying amounts of heat to raise their temperatures due to differences in specific heat capacity.  Specific heat capacity (c) measures the amount of heat energyrequired to raise the temperature of a substance by 1 degree Celsius per unit mass. Thermal Capacity (Q) is the amount of energy required to raise the temperature of an object by 1oC. Q = mc Specific latent heat (L)is the amount of energy needed to change the state of 1kg of a substance. ○ Specific latent heat of fusion(Lf) is the energy needed to melt/freeze Q= m Lf ○ Specific latent heat of vaporization(Lv) is energy needed to boil/condense Q= m Lf where Q is the energy needed in J, m is the mass in kg, and L is the specific latent heat in Jkg-1.  Heat Transfer and Equilibrium  Heat transfers from higher to lower temperature bodies until thermal equilibrium is reached.  This transfer is caused by collisions between particles, making their average kinetic energies equal.  The anomalous expansion of water is an abnormal property of water whereby it expands instead of contracting when the temperature goes from 4oC to 0oC and it becomes less dense. Temperature, pressure and volume The temperature of a gas is related to the average kinetic energy of the molecules. The higher the temperature, the greater the average kinetic energy and so the faster the average speed of the molecules. At constant volume, if the temperature increases, the pressure increases because the molecules move faster so they collide harder and more frequently with the walls. At constant temperature, if the volume increases, the pressure decreases because the molecules collide less frequently with the walls. At constant pressure the volume of a given mass of gas varies directly with the absolute temperature Change of State  Heating can change a solid to a liquid (melting) and a liquid to a solid (freezing).  Pure substances melt and freeze at specific temperatures, such as water at 0°C.  Melting involves particles of a solid overcoming intermolecular forces to become a liquid.  Solidification (freezing) involves the transfer of potential energy from particles to surroundings as a liquid becomes solid.  Vaporisation requires substantial energy to overcome intermolecular forces in a liquid to become gas (vapour).  Condensation involves gas particles losing potential energy to their surroundings as they return to a liquid state. Melting and boiling The melting point is the temperature at which a given solid will melt when heated. The boiling point is the temperature at which a given liquid will turn into a gas when heated. Condensation is when some molecules in a gas do not have enough energy to remain as separate molecules, so they come close together and form bonds, becoming liquid. Freezing is when the molecules in a liquid slow down enough that their attractions cause them to arrange themselves into fixed positions, becoming solid. Evaporation  Occurs at any temperature below the boiling point.  Occurs at the surface of the liquid  Requires less heat and occurs due to energetic particles escaping.  Slower compared to boiling.  Happens only at the liquid's surface.  Thermal processes Conduction Thermal energy in solids and liquids can be transferred by conduction. Non-metals are usually poor conductors known as insulators. As a non-metal is heated up, the molecules vibrate more and cause adjacent molecules to vibrate more also, transferring heat energy from hot parts to cooler parts. Metals are usually good conductors. The electrons can leave the atoms and move freely among positively charged ions. As the metal is heated, the ions and electrons vibrate more. The free electrons collide with ions throughout the metal and transfer heat energy from hot parts to cooler parts. Convection Thermal energy in fluids (liquids and gases) can be transferred by convection. Convection occurs when molecules in a fluid with high thermal energy move to an area with low thermal energy. When part of a fluid is heated, it expands and becomes less dense. It therefore rises up to less dense areas in the fluid. Denser, colder fluid falls down to take its place. Examples of convection include water boilers and hot air balloons. Radiation Thermal energy is also transferred by infrared radiation which does not require a medium. Infrared radiation is part of the electromagnetic spectrum. Black bodies with a dull texture are the best absorbers and emitters of radiation. White bodies with a shiny texture are the best reflectors of radiation. The higher the temperature and the greater the surface area of a body the more infrared radiation emitted.