Week 4_Session 2.pdf

Week 4: Materials and Energy Materials and Energy 1 Materials and Energy Rojin Eghbali  Types of Energy Mechanical Chemical Electrical Nuclear Light/Radiant Heat/Th...

Week 4: Materials and Energy Materials and Energy 1 Materials and Energy Rojin Eghbali  Types of Energy Mechanical Chemical Electrical Nuclear Light/Radiant Heat/Thermal 2  Why is Heat so important to material behaviour? Most processes require a certain amount of work to be done Heat often provides most, or even all, of the energy to do that work Understanding some details will make it easier to understand how those processes work 3  Materials and Heat Latent Heat Sensible Heat Latent Heat is the energy Sensible Heat is the required for a phase change; vibration of atoms; Latent Heat is associated with At Absolute Zero (0K /- the change in entropy 273°C), atoms in a solid (randomness) of the atomic would cease to oscillate. arrangement. 4  Sensible Heat The average vibrational energy of each atom is equal to 3kBT; kB is Boltzmann’s constant (is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas). 5  How is thermal energy stored in materials? We can think about atoms in a solid being connected by springs (A good mental picture, but not exactly the real situation …) Atomic vibration stretches one spring and compresses the other Energy is at a maximum when Δa is at its maximum Energy is at a Minimum when Δa is equal to zero. 6  Bonding energy and interatomic spacing – Positive nucleus of one atom is attracted to the negative electrons of the other. – Nuclei and electrons of each atom also repel each other. Attractive – The equilibrium distance between atoms is caused by a Fnet=Fatt + Frep balance between repulsive and attractive forces. – Equilibrium separation occurs when the total interatomic energy of the pair of atoms is Force r at a minimum – or when no net force is acting to either attract or repel the atoms Repulsive 7  Binding energy The minimum energy, is the binding energy, or the energy required to create r or break the bond. E   Fdr  Energy r att Force r rep 8  Atomic Motion in Solids A longitudinal wave Two transverse waves Heat is the motion of atoms. Atomic motion in solids is limited by the bonds with neighbouring atoms. Vibrations form standing waves of various wavelengths. Each atom has 3 distinct wavelengths. 9  Heat Capacity (Specific Heat) Heat capacity, Cp is a material property that characterizes the amount of energy required to increase the temperature of a given mass of that material The subscript ‘p’ denotes a measurement at constant pressure. For gases, the measurement is more commonly made at constant volume – Cv. For liquids and solids, the difference is negligible. 10  Heat Capacity (Specific Heat) The average amplitude of each vibration results in an energy kBT; The average vibrational energy of each atom is equal to 3kBT; If the volume occupied by an atom is Ω, then the energy per unit volume is 3kBT/Ω. The specific heat is the change in energy per change in temperature: Since atomic volumes do not vary much, ρCpis called volumetric heat capacity. 11  Example: Heat Capacity Estimate the energy required to raise one cubic meter of aluminum from 20ºC to it’s melting point (660ºC). ρ=2710 kg/m3; Cp=2600 J/kg K; 12  Temperature-varying Properties Many material properties change with temperature; These curves exhibit similar trends, but different materials have different critical temperatures; E.g. Temperature to cause https://www.engineeringtoolbox.com/metal- similar reductions in temperature-strength-d_1353.html strength. 13