Basics Flashcards
What is specific heat capacity equation, and SI units
Q = mcΔT (Δh = cΔT for one mol) J = kg . J/kg.k . K
What factors affect SHC?
Pressure, temperature, material
What’s the difference between thermal conductivity and thermal diffusivity?
Conductivity - heat flow through the material (property of material),
q = -KAΔT/ΔΧ, constant.area.temp/length, W/MK
Diffusivity - how quickly a material will lose stored heat, D = K/ρC, constant/density.SHC, M2/S
How would you measure thermal diffusivity and conductivity?
Conductivity - heat input needed to maintain a material at steady temp, power in one end of insulated wire and measure temp at both ends = ΔT/ΔΧ
Diffusivity - rate at which a pulse travels through a material with known dimensions
Describe thermal expansion
Materials heat up and expand volumetrically - will be relative to bond strength
Polymers - very high expansion
Metals - medium expansion
Ceramics - low expansion
Describe DSC
Differential scanning calorimetry - heat sample and reference at constant rate and measure temps of both.
Plot temp vs time, will see phase transitions etc
Define electrical resistance
R = PL/A, resistance = resistivity.length/area
What happens to resistance with heat?
Heating a material will increase its resistivity - more lattice vibrations = more ion collisions = reduces current
What is the dielectric effect and why is it bad for insulators?
When a material heats up it can emit electrons or ions, this increases number of charge carriers = lower resistivity
What is a capacitor, and what is meant by capacitance?
Capacitor stores charge, capacitance = Q/V (charge/Pd) = εo.A/D (permittivity of free space.area/distance between plates)
What is meant by relative permeability?
Replace the vacuum separating capacitor plates with a polarisable insulator, when Pd applied dipoles align in insulator increasing capacitance - allows reduction in capacitance size when high dielectric strength materials are used
What’s the difference between soft and hard magnets?
Soft - easily magnetised and de-magnetised, narrow BH loops, used in transformers/generators
Hard - harder to magnetise and demagnetise but higher saturation levels and remnance, large BH loop, used in energy harvesting systems
Sketch the BH relationships (not loops) for paramagnetic, diamagnetic and ferromagnetic materials
Dia magnetic - almost 0
Para magnetic - low straight line
Ferromagnetic - steep nonlinear relationship
Draw the BH loop for Fe and label
Steep non-linear line ending at a saturation point, high remnance and coercivity, BH loop encompassing -Ve saturation and coercivity
How do you measure the BH loop of a sample?
Use a permeameter - have a large magnetic pole with primary coils, induce current in coils to induce magnetic field - measure induced field in sample
What is the mechanism of thermal conductivity?
Phonons transference between particles (lattice vibrations)
Which way do phonons travel on a material? And explain phonon scattering
In all directions, but generally moving down the temperature gradient
Phonon scattering - when phonons are scattered due to a defect in the material (inclusions, grain boundary, dislocations etc) or due to phonon-phonon collisions
Explain which material is a better conductor: stainless steel or zinc
Zinc because it is a pure metal - has a more regular structure and doesn’t include alloying elements = less phonon scattering = more free mean paths = higher conductivity = better conductor
What is meant by a free mean path?
A path which a phonon can follow without encountering a scattering event
How can you find the probability of finding an electron at a particular point?
Use the wave equation
When there is no charge on a metallic structure where can the electrons be said to be?
In an electron cloud in the structure, but each nuclei having 1 electron around it
Why are metals conductors?
Have high conductivity, caused by overlapping bands within the material (continuous bands) so electrons can move easily in the band
What is meant by the density of states? And draw it
In metals there is a continuous energy band, this is represented by the density of states, g(E), and is a function of the energy level
g(E) vs E, curve up to saturation
How do electrons fill density states?
Fill lowest energy states first
What is the fermi level?
It’s the energy level at which electrons sit at 0k. Electrons can only be on or above this level
What happens when electrons are heated? (From 0k)
They move from fermi level (at 0k). Continuous bands so electrons move between levels easily (mobile and delocalised)
What is the electron movement when no field is applied?
Random with no net movement
How can scattering affect conductivity?
Electrons are energy packets so scattering events disrupt them. Electrons are most effective heat transport = conductivity reduces with scattering events
What can be a scattering event cause?
- Phonon interactions
- Solute atoms have biggest effect
- Grain boundaries and dislocations also act as scattering event (alloys have less conductivity)
What is a thermocouple and how is it made?
Two metals with different fermi levels, electrons move from one metal to another resulting in an equalisation of fermi levels (creating equilibrium so electrons stop moving)
fermi levels change with temp meaning Pd changes with temp and this can be measured
What are the requirements of a thermocouple?
- Two metals with different fermi levels
- electrical contact between the metals
- high oxidation resistance (as oxides are insulators)
- no phase changes before melting (as changes Fe)
What is the thermoelectric effect? And when does it occur in different materials?
Changing relationship between Temp and Pd
Conductors - only takes places at interface
Insulators - can occur in element
What is the difference between thermal conductivity in metals and non-metals?
Metals have phonon movement, free electrons and electron clouds = fast movement of heat
Non-metals - only phonon movement transfers heat, heat transfer rate = phonon movement rate
What is the kinetic theory?
K = 1/3 . Cv . V . L Cv = SHC, V = mean velocity, L = mean free path
At what speed do phonons travel at?
Speed of sound
Define electrical conductivity
L = K/σΤ L = lorentz Constant T = absolute temp K = electrical conductivity σ = thermal conductivity
What are the carriers for electrical conductivity?
Charged particles (ions or electrons), electrons are the main carrier
When is electron movement directional?
When an electrical field is applied
How do scattering events interrupt electron movement and effect speed of electron movement?
They stop electrons from moving, then electrons are accelerated again until next scattering event
Speed = K distance between events
What are the material requirements for high electrical conductivity?
- Low defect density (less grain boundaries)
- high valence electrons
- continuous energy bands
- high thermal conductivity
What are the modifications made to make a conductor suitable for high working temperatures?
Thermoplastic insulating coating is replaced by ceramic coating (much higher Tg and Tm)
Describe and explain the requirements for resistors and heating elements
Resistors - resistance must be known and not change with temp much = low thermal coefficient of resistivity (as resistance increases temp during service)
Heating elements - high thermal coefficient if resistivity is needed, coating applied to prevent oxidisation
