Heating Processes Unit 1.1 PDF

Heating Processes Unit 1.1 Units, significant figures & uncertainties Unit 1 Topic 1.1 Part A Kinetic Particle Model The kinetic particle model says that all matter consists of very small particles which are constantly moving or in a continual state of motion. The particles collide...

Heating Processes Unit 1.1 Units, significant figures & uncertainties Unit 1 Topic 1.1 Part A Kinetic Particle Model The kinetic particle model says that all matter consists of very small particles which are constantly moving or in a continual state of motion. The particles collide and exchange energy. The degree to which the particles move is determined by the amount of energy they have and their relationship to other particles. The particles might be atoms, molecules or ions. This theory helps explain the flow or transfer of heat. Particles in solids, liquids and gases Temperature and Energy All substances have a temperature All substances have energy when T  0 Kelvin (above absolute zero) The temperature of a substance is related to the average amount of energy of the individual particles. Energy is transferred from particles with lots of energy to those with less energy when they collide. Heat & Temperature – Video Heat and temperature involve the motion of molecules in substances But… Heat and temperature are different Heat is thermal energy transferred between two systems at different temperatures that come in contact. Symbol Q, and it has units of joules (J) QCAA Definition for Heat: the energy transferred from one system to another because of a difference in temperature. Temperature is related to the average kinetic energy of the molecules. Symbol T, and is measured in degrees Celsius ​˚C or Kelvin K QCAA Definition for Temperature: the degree or intensity of heat present in a substance or object, especially as expressed according to a comparative scale, e.g. the Celsius temperature scale Energy & Temperature again Objects do not have heat, in the same way they have a temperature What they have is internal energy ‘U’ This is the sum of all of the energies Kinetic energy of molecules Rotational kinetic energy Potential energy due to forces between molecules Energy & Temperature again Thermal energy, is the total kinetic and potential energy of the moving atoms and stretched bonds inside an object. It does NOT include chemical or nuclear potential energy in bonds. Thermal energy can also be regarded as that portion of internal energy that changes when the temperature of the system changes. Heat Transfer Heat can be transferred between objects in multiple ways Conduction Occurs when particles of matter are in direct contact. The adjacent atoms of higher energy vibrate against one another, which transfers the higher energy to the lower energy atoms, or higher temperature to lower temperature. A CONDUCTOR allows heat to MOVE THROUGH it. This mechanism of heat transfer occurs through particle collisions. In conduction, there is no net transfer of physical matter between the objects. The changes in temperature are the result of the gains and losses of kinetic energy during collisions. Conduction Convection Convection is the process of heat transfer from one location to the next by the movement of fluids. The moving fluid carries energy with it. The fluid absorbs heat energy and carries that energy from one place to another. The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up. Convection Convection is the process of heat transfer from one location to the next by the movement of fluids. The moving fluid carries energy with it. The fluid absorbs heat energy and carries that energy from one place to another. The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up. Convection Radiation Radiation is the transfer of heat by means of electromagnetic (EM) waves. Any particles with heat energy will vibrate and create EM waves, transforming thermal energy into radiant energy. When these waves encounter other matter the radiant energy will cause the atoms to vibrate, transforming the radiant EM energy back to heat energy. This form of heat transfer does not involve particle contact and can occur in a vacuum. Radiation Temperature scales Temperature The kelvin temperature scale is the SI unit of thermodynamic temperature (symbol K without the degree sign) This scale has as its zero point at absolute zero, the theoretical temperature at which the molecules of a substance have the lowest energy. The difference between the freezing and boiling points of water is 100 degrees in both kelvin and Celsius, so that the kelvin has the same magnitude as the degree Celsius. Temperature Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin (K). A difference or interval of temperature may be expressed in kelvins or in degrees Celsius Converting Temperature units: Kelvin and Celsius TK = TC +273 Change in temperature When a system absorbs or loses heat, the average kinetic energy of the molecules will change. Therefore, heat transfer results in a change in the system's temperature as long as the system is not undergoing a phase change. Complete worksheet 1: Temperature & Heat Part B Specific Heat Capacity (c) Relates to how well a material can store internal energy. If you like, Specific Heat is thermal inertia, just like inertia is a resistance to motion, specific heat is a resistance to a change in temperature Specific Heat Capacity We can calculate the heat released or absorbed using the specific heat capacity c, the mass of the substance m, and the change in temperature ΔT, in the equation: Q=mcΔT Q is the amount of heat energy (J) m is the mass (kg) c is the specific heat capacity (Jkg-1°C-1) ΔT is the temperature change (T​final​−T​initial) - °C or K Calculating specific heat - examples 1. Calculate the mass of a liquid water sample if the addition of 120 kJ of heat results in a temperature change of 12°C. (water has a specific heat of 4.18 x103 Jkg-1°C-1 ) Convert to SI units Q = 120 kJ = 120 000 J c = 4.18 x103 Jkg-1°C-1 ΔT = 12°C Q=mcΔT m = Q/cΔT m = 120 000/ (4180 x 12) m = 2.3889 Therefore the mass of the water is 2.4 kg. (2 sf) Specific Heat Capacity 1 ∆𝑇 1 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡 = = ∆ 𝑄 𝑚𝑐 1 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡 𝑐= 𝑚𝑎𝑠𝑠 Example 2. How much heat is required to heat a cup (250g) of water for Dave’s coffee from 18C to 87C? mH20 = 250g = 0.25 kg Tinitial= 18C Tfinal = 87C c = 4180 J/kg°C Solution: Q = m c T Q = 0.25 kg (4180J/kg°C)(87 C - 18 C) =72105 J = 72kJ Should you use degrees C or K? A. You must use °C B. You must use K C. It doesn’t matter D. I reject your units and substitute my own! Should you use degrees C or K? A. You must use °C B. You must use K C. It doesn’t matter (you are measuring temperature difference) D. I reject your units and substitute my own! Complete worksheet 2: Specific Heat Capacity Unit 1 Topic 1.2 Part C Temperature and phase changes Transitions between solid, liquid, and gaseous phases typically involve large amounts of energy compared to the specific heat. The energy required to accomplish the phase changes is called the latent heat of fusion and latent heat of vaporization. Temperature and phase changes When a substance changes from one state, or phase, of matter to another we say that it has undergone a change of state Vaporisation Fusion Temperature and phase changes - Video During a change in state the heat energy is used to change the intermolecular bonding between the molecules. In the case of melting, added energy is used to break the inter-molecular bonds between molecules. In the case of freezing, energy is subtracted as the molecules bond to one another. These energy exchanges are not changes in kinetic energy. They are changes in bonding energy between the molecules. Temperature and phase changes If heat is coming into a substance during a phase change, then this energy is used to break the intermolecular bonds between the molecules of the substance. Example: ice melting into water. Immediately after the molecular bonds in the ice are broken the molecules are moving (vibrating) at the same average speed as before, so their average kinetic energy remains the same, and, thus, their temperature remains the same. Temperature and phase changes Specific Latent Heat The specific latent heat (L) of a material is a measure of the heat energy (Q) per mass (m) released or absorbed during a phase change. This is unique for each substance and specific phase change. defined through the formula Q = mL Q is the amount of heat energy (J) m is the mass (kg) L is the latent heat of (vaporisation = Lv / fusion = Lf) (J/kg) Often just called the "latent heat" of the material. SI unit joule per kilogram [J/kg]. Specific Latent Heat Problems When there is only a phase change, you only need to consider the Q = mL relationship Usually there is both a phase change and a temperature change, so you need to use both Q = m L and Q=m c T - Qlost = Qgained Examples 1. How much heat must be added to 15g of solid tungsten at its melting point to completely liquify it? Tungsten has a latent heat of fusion of 193 kJ/kg. Change to SI units m = 0.015 kg Lf = 193 000 J/kg Q = mL = 0.015 x 193 000 Q = 2895 J Therefore 2900 J of energy must be added (2 sf) Examples 2. A 430g sample of ice at -12C is heated to water at 56C Sketch the heating curve for the water and identify the sections of the curve that the problem covers. (cwater = 4.18 x 103 J/kg.K, cice = 2.05 x 103 J/kg.K, LFusion = 3.34 x 105 J/kg) How much thermal energy is required? Q = m c T Q = m c T m = 430 g = 0.43kg Q = 0.430 x (4.18 x 10^3 Q = 0.430 x (2.05 x 10^3 Lf = 3.34 x 10^5 J/kg J/kg.K)(12C) J/kg.K)(56C) =10578 J = 11kJ =100653 J = 101kJ Q = mL Total Q = = 0.430 x 3.34 x 10^5 10578+143620+100653 Q = 143620 J = 144 kJ Q = 254851J = 255kJ Examples 3. On a hot day, Kat places two ice cubes at 0 C (m=15g) into a glass and pours 375mL of her favourite caffeinated cola beverage (T=5 C) over the ice cubes. What is the temperature of her drink after the ice cubes have melted? mice = 15g Tice =0C mcola = 375g Tcola =5C ccola = 4.18 x 103 J/kgC Lf = 3.34 x 105 J/kg Examples The energy lost by the cola = energy required to melt the ice - Qcola= Qice - mcT = mL - mc(Tf – Ti) = mL mcola cTinitialcola  mice L Tf  cola mcola c 375 g 4.186 J/g°C ( 5°C)  15 g 335 J/g  Tf  co la 375 g (4.186 J / g°C ) Tf = 1.8°C Complete worksheet 3: Latent heat Review Video – click here Part D Energy conservation in Calorimetry Calorimetry: the measuring of heat absorbed or released in chemical reactions. Thermal equilibrium The molecules in an object are constantly in motion and colliding with each other. Every time molecules collide, kinetic energy can be transferred. When the two systems are in contact, heat will be transferred through molecular collisions from the hotter system to the cooler system. The thermal energy will flow in that direction until the two objects are at the same temperature. When the two systems in contact are at the same temperature, we say they are in thermal equilibrium. Part D Energy conservation in Calorimetry Thermal equilibrium Part D Energy conservation in Calorimetry The thermal energy will flow in that direction until the two objects are at the same temperature. When the two systems in contact are at the same temperature, we say they are in thermal equilibrium. Zeroth Law of Thermodynamics If two objects at thermal equilibrium are in contact, there is no net heat transfer between the objects; therefore, they are the same temperature. Zeroth Law of Thermodynamics Part E Energy in Systems Thermal energy is the internal energy present in a system in a state of thermodynamic equilibrium due to its temperature.  Types of Systems Mass Heat Heat Heat Open system Closed system Isolated system  Open Systems Energy out Boundary Energy in Open Mass out System Mass in  Closed Systems Energy out Boundary Energy in Closed No mass System in or out Isolated Systems Boundary No energy in or out Isolated No mass System in or out Thermal Equilibrium Rate of change in temperature changes with the difference in temperature between two objects. The bigger the difference in temperature, the greater the rate of temperature change. As the difference in temperature decreases, the rate of change will decrease, forming the graph below. Examples 1. If 350g of water at 10C is added to 0.4kg of water at 75C, what is the final temperature of the mixture? (cwater = 4.18 x 103 Jkg-1K-1) – Qlost = Qgained – mhotcThot = mcoldcTcold (apply equations) – mhotc(Tf – Ti hot) = mcoldc (Tf – Ti cold) (expand equations) – 0.4 x 4180 (Tf – 75) = 0.35 x 4180 (Tf – 10) (substitute values) – 1672Tf + 125 400 = 1463Tf – 14630 (expand brackets) 1463Tf + 1672Tf = 125 400 + 14630 (gather like terms) 3135Tf = 140 030 (simplify) Tf = 44.666666 (correct sig figs) Tf = 45C (2 sf) Examples 2. Two 50.0g ice cubes at 0C are placed into a full 3.0 L jug of 25C milk. Calculate the final temperature of the mixture. (cmilk = 3.93 x103 Jkg-1K-1 and Lice = 3.34 x 105 Jkg-1, cwater= 4.18 x103 Jkg-1K-1 ) – Qlost = Qgained (however melting ice AND warming water from ice - mL + mcT) –mmilkcmilk(Tf – T i milk) = miceLf ice +micecwater(Tf – T i water) –3 x 3930 (Tf – 25) = 0.1 x 3.34x105 + 0.1 x 4180 (Tf – 0) – 11 790Tf + 294 750 = 33 400 + 418Tf – 11 790Tf – 418Tf = – 33 400 - 294 750 – 12 208Tf = – 261 350 Tf = 261 250/12 208 Tf = 21.408 Tf = 21 C Complete worksheet 4: Equilibrium Mechanical work Work is the energy interaction between a system and its surroundings. More specifically: work is the energy transfer associated with force acting through a distance. W = Work (J) F = Force (N) s = displacement (m) There are several ways of doing work, each in some way related to a force acting through a distance. First Law of Thermodynamics This applies the conservation of energy principle to heat and thermodynamic processes. The total energy in an isolated system is constant. The change in internal energy of a system (ΔU) is equal to the Heat added to the system (Q) plus the work done on the system (W). ΔU= Q + W Law of conservation of energy: The total energy of an isolated system in a given frame of reference remains constant over time. First Law of Thermodynamics ΔU= Q + W When thermal energy is added to the system +Q When thermal energy removed/wasted from system –Q If work is done on the system +W Work is done by the system –W. Law of conservation of energy: The total energy of an isolated system in a given frame of reference remains constant over time. Thermal Efficiency Thermal efficiency is the ratio of output work to input heat for engines. This expresses the percentage of heat that becomes useful work. Thermal efficiency is represented by the symbol η (eta), and can be calculated as a percentage using the equation: Thermal Efficiency For two given engines with same input heat, the one with the higher thermal efficiency will give more work output and is hence more desirable. In general, the higher the thermal efficiency the better. Examples If a system produces 3300J of usable energy when 5800J of energy is put into it, calculate the efficiency of the system. 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑢𝑝𝑢𝑡 100 η= × % 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑛𝑝𝑢𝑡 1 η = 3300/5800 x 100 η = 56.89655 η = 57% (2 sf) Complete worksheet 5: Work and efficiency References https://www.khanacademy.org/science/chemistry/thermodynamics-chemistry/internal-energy-sal/ a/heat http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html#c1 http://zonalandeducation.com/mstm/physics/mechanics/energy/heatAndTemperature/changesOfP hase/changeOfState.html https://www.khanacademy.org/science/physics/thermodynamics/laws-of-thermodynamics/a/what-i s-the-first-law-of-thermodynamics https://physics.info/heat-latent/summary.shtml https://www.britannica.com/science/thermal-energy https://physics.nist.gov/cuu/pdf/sp811.pdf https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/thermo0.html https://www.hoddereducation.co.uk/media/Documents/International/Physics-for-the-IB-Diploma_3- 1-Thermal-Concepts.pdf