Equations Flashcards
What is temperature
A measure of how hot/cold something is. Measured in kelvin/celcius/farenheight
How to convert between calories and joules
1 cal = 4.2J
How do you calculate heat
Q = mcΔT
Q = quantity of heat (J) M = mass(kg) C = specific heat capacity (4.184j) ΔT = change in temperature (c/k)
How do you transfer from Fahrenheit to Celsius?
C = (F -32)5/9
What is heat?
Energy transferred from one system to another as a result of thermal interactions
What is the definition of isothermal?
Constant temperature within a system
What are the 3 measures of temperature?
Celcius
Kelvin
Fahrenheit
How do you convert between kelvin and celcius?
0C = 273 K
0 K = -273 C
What does adiabatic mean?
No heat transfers from system to surrounding
What is heat measured in?
Joules or calories
What is 1 Joule?
The energy required to raise 1g of water by 0.024 C
What is the SHC of water?
2.148J
What is 1cal in Joules?
4.2J
What is 1Kcal in Joules?
4200J
How do you calculate mass?
Volume x density
What equation is used in calorimeter to calculate SHC?
Q lost = q gained
Q = mcΔT
In what conditions can q=mcΔT be used?
When there is NO PHASE CHANGE
What is the main form of bonding in water and what happens to bonding as water changes phase from solid to gas?
Hydrogen
H bonds break
Why is ice less dense than water?
Because water forms 3.4 h bonds with other molecules whereas Ice forms 4+ h bonds with other molecules so is more spread out
What is van der waals bonding and why is it important?
Found in water due to dipole caused by uneven bonding, less strong than h bonding.
Important as it is expressed in the cohesion of water molecules and contributes to viscosity and surface tension.
What is the highest energy phase of water?
Water molecules in vapour
What is the equation of the latent heat of melting (and freezing)?
q= mHf
M= mass kg Hf= 334J
What is the latent heat of vaporisation (boiling/condensing)?
q = mHv
M = mass Kg Hv= 2260J
When temperature is below 0 -> 0 what equation is used to calculate quantity of heat?
Q= mcΔT
M= mass Kg C= SHC (ice) - 2.108J/gK ΔT = Change in temp C/K
When ice is melting what equation is used to calculate quantity of heat?
Q = mHf
M= mass Kg
Hf = 334J
What equation is used to calculate quantity of heat for liquid water?
Q = mcΔT
M =mass Kg
C = 2.184 J/gK
ΔT = Change in temperature C/K
What equation is used when calculating the quantity of heat for the vaporisation of water?
Q= mHv
M= mass kg Hv = 2260 j
What equation is used when calculating the quantity of heat of steam?
Q = mcΔT
M= mass Kg C = SHC (steam) - 1.996J/gK ΔT = Change in temperature C/K
What equation is needed if you are both heat in up and changing phase?
Q = mcΔT + mHv/f
M= mass Kg C = SHC (4.184 for water) ΔT = change in temp
M = mass Kg Hf = 334J Hv = 2260J
What is the triple point on a phase diagram?
The point where all states exist
What is sublimation?
Solid -> gas
No liquid stage
What is deposition?
Gas -> solid
No liquid transition
How is the latent heat of cooling different from the latent heat of heating?
It is exothermic (-ve value)
What does the phase diagram tell you?
The phase a material is in at a particular pressure and temperature.
Temp on x axis
Pressure on y axis
What is energy?
It is a quantifiable property that must be transferred to an object to provide heat or to perform work.
Either providing heat or doing work.
What is the principle of conservation of energy?
Energy can neither be created nor destroyed. Energy can only be transferred or transformed
What is kinetic energy?
The energy possessed by an object due to its velocity in relation to other objects.
Energy in motion.
What is potential energy?
The energy possessed by an object due to its position in relation to other objects.
Energy when not in motion.
How do you calculate kinetic energy (+units)?
1/2 mv^2
M = mass kg
V= velocity m/s
How do you calculate potential energy?
Energy pot = mgh
M = mass kg G = 9.81m/s^2 (acceleration due to gravity - constant) H = height m
What is the acceleration due to gravity?
9.81 m/s^2
What is the unit for energy?
Joules - J
How do you calculate terminal velocity?
V = √(2gh)
Velocity in m/s
G = 9.81 m/s^2 H = height m
How do you calculate force?
F = ma
Force = Newton’s M = mass kg A = acceleration m/s^2
How do you calculate weight?
F = mg
Force (N)
Mass (kg)
G = 9.81 m/s^2
How do you calculate pressure?
P = Fa
Pressure (pa)
F= force (N) A = area (m^2)
What is work?
It is energy.
Work transfers energy from one place to another or transforms energy from one to another
How do you calculate work?
W = F cos θ d
Work = joules
F = force (N)
cos θ = cos of the angle
D = distance (m)
How do you calculate work done against gravity?
W = mgh
Work (joules)
M = mass (g)
G = 9.81 m/s^2
H = height object raised (m)
How do you calculate work done on a gas?
W = -PΔV
Work in joules
-p = pressure pa( usually multiplied by 101325 as this is 1atm) ΔV = change in volume (litres /1000, GOOGLE THIS)
What 3 ways does heat transfer?
Conduction
Convection
Radiation
What is conduction?
Where molecules are touching and there is a progressive transfer of heat.
The transfer of energy in a substance by means of molecular excitation without any net external motion.
The transfer of heat by microscopic collisions of particles and movements of electrons within a body.
Give an example of a good thermal conductor
Metals - free electrons move through metal, increasing excitation
Give an example of a poor conductor and what are these known as?
Carpet, foam etc.
Insulators
What is the equation for conduction?
Q = k a/l ΔT
Q = heat flow j/s = W
K = thermal conductivity A = area of cross section (m^2) L = length (m) ΔT= temp difference (C)
What is convection?
The process in which heat is carried from place to place by the bulk movement of fluid (gas/liquid)
What is natural convection?
Is where a temperature difference causes the density at one place in a flier to be different from that at another.
What is forced convection?
The forced convection generated by a pump circulates radiator fluid through a car engine to remove excess heat
What does convection depend on and why?
Surface area
The higher the SA, the better the transfer and vise versa
What is the heat convection equation?
Q= hA ΔT
Q = heat flow W / j/s
H = heat transfer coefficient A = area contacting fluids (m^2) ΔT = temp difference (C)
What does the heat transfer coefficient depend on and why?(4 things)
Whether the flow is natural or forced convection
The geometry of the system
Rating of convection to conductive heat transfer
The rate of flow
What is radiation?
It is the process in which energy is transferred by means of electromagnetic waves.
The higher the temp of an object, the more thermal radiation it gives off.
What is thermal radiation?
Thermal radiation is electromagnetic waves (including light) produced by objects because of their temp.
The higher the temp of an object, the more thermal radiation it gives off.
Where on the light spectrum is radiation?
Infrared is where most occurs.
At what temp does the colour of the radio wave turn white?
1500 C
Why do we not see most thermal radiation?
Because it occurs at infrared wavelengths invisible to the human eye.
Why is infra-red useful in food testing?
To see if here are cool spots in food (blue in colour) as they are most likely not cooked.
When is the term black body used?
When referring to an object that absorbs all the electromagnetic waves falling on it (regardless of frequency or angle of incidence)
What kind of material would reflect radiation (i.e. keep water warm)?
Shiny metal, it reflects heat back into the water.
What is emissivity?
It is a measure of how effective a surface is at editing thermal radiation.
It is measured as the ratio of the thermal radiation from the surface to that of an equivalent black surface at the same temp.
Give an example of a material with low emissivity
Aluminium foil - it doesn’t emit radiation very well so will hold it in and keep things hot as a result
What level of emissivity would an ideal black body have?
The higher the emissivity, the closer it is to the ideal black body and the better thermal radiation absorbed.
What equation is used to calculate radiation?
Stefan Boltzmann Equation
𝑄 = 𝑒σ𝐴𝑇4
Q = power (Watts)
E = emissivity σ = SBC - 5.67 X 10^-8 W/m^2K^4 A = surface area (m^2) T = Absolute Temperature (K)
What is the Stefan Boltzmann constant and when is it used?
5.67x10-8
Used when calculating radiation
What is the equation for overall heat transfer rate?
Q = UAΔT
U = overall heat transfer coefficient A = area m^2 ΔT = temp change (C)
What is the calculation for thermal resistance for conduction and convection?
Conduction: R = L/kA(C/W) L = length (m) K= thermal conductivity A = area (m^2)
Convection:
R = 1/hA (C/W)
H = heat transfer coefficient
A = area (m^2)u
What is the equation for heat low from thermal resistance?
Q = ΔT/R
Q - j/s
ΔT = change in temperature (C) R = thermal resistance (diff Calc)
What type of cooking method is poaching?
Moist heat method
What is a moist heat cooking method?
A method of cooking using warm water/steam
What is poaching?
A moist heat method of cooking where food is submerged in a heated water-based liquid like stock, wine, milk or water.
What temperature is poaching carried out at?
71-82 C
What is poaching used to cook?
Delicate foods, e.g. fish, poultry, eggs etc
What are the 3 types of poaching?
Shallow - food being poached is partially submerged and heated by progressive conductors
Shallow in bags - food is put inside plastic bag and placed in poaching liquid
Deep poaching- food is fully submerged, and fats can be skimmed from the top of the liquid
What is simmering?
It is a moist heat method.
It is cooking food by submerging them in a water-based liquid below the boiling point (achieved by boiling and then reducing heat)
What is another word for simmering?
Gentle boil
What temp is simmering carried out at
85-96C
What is slow cooking?
A method of moist hat cooking which uses simmering
They are heated to cook food then reduced to keep it warm.
Healthy as more nutrients retained as the cooking vessel is covered
What temperature does simmering take place at?
Initial food cooked between 79-93 C.
Temp then reduced to 71-74 C to keep food warm
What is boiling?
Submerging food in a water-based liquid at its boiling point.
The bubbling rate is more violent than simmering so heat transfer rates is faster.
What food would you boil?
Large scale cooking or starchy materials e.g. potatoes, pasta.
High loss of water soluble vitamins
How does the moist heat method actually cook the food
There is a high conduction - the heat source heats the pan which heats the liquid, this then heats the food using convection.
What is steaming?
It is a moist heat method involving the steam produced from a boiling water-based liquid to cook food which is suspended above the liquid.
Retains more water-soluble vitamins than boiling as the food is not submerged in the liquid.
Not much energy required, therefore low cost.
What is the heat transfer method in steaming?
Both high conduction and high convection.
Heat transfers from heat source through the pan tot the liquid by conduction.
All of the cooking of the food by steam is then convection based.
Different to pressure cooking as not a sealed vessel.
What is pressure cooking?
Uses a water-based cooking liquid under increased temp and pressure to reduce the time it takes to braise food (braising uses heat, time and moisture to break own collagen and make tougher cuts of meat softer)
How is pressure applied during pressure cooking?
The lid is locked to the base and a weight placed on top of the steam vent to prevent steam escaping until a certain level is reached.
How does applying pressure reduce the cooking time?
It increases the boiling point therefore food cooks faster.
Why does increasing the boiling point reduce the cooking time?
The higher temp steam contains more thermal energy and the higher-pressure forces moisture into the food so cooks it faster.
Pressure cooking reduces cooking time by 30% compared to steaming and boiling and uses 50-75% less energy.
Retains more nutrients.
What is deep frying and what is it an example of ?
It is the process whereby food is immersed in hot fat/oil until cooked through. Different oils and fats can be used to alter the added saturated fats.
Example of dry heat method
What temperature is deep frying carried out at?
177-191C
What is the heat transfer method in deep frying?
It has high conduction and moderate convection int he fat - similar to boiling but fat in place of water
What are the positives and negatives of deep frying?
Positives:
Little to no change in the protein and mineral content.
The dietary fibre content of foods like potatoes are increased due to the formation of resistant starches.
Due to faster cooking, there is better retention of heat labile vitamins.
Negatives:
Unhealthy due to increase saturated fat
What is sautéing/shallow frying?
Sautéing - to brown/cook foods in a frying pan with a small amount of fat. Low-medium temps are used.
Shallow frying - where 1/3 of the food is immersed in oil, similar technique to above but using a higher heat.
What is the heat transfer method of sautéing/ shallow frying?
High conduction as the food is in direct contact with the heat of the pan.
What is baking?
It is an air based method.
A process whereby heated air passes over the food in an insulated chamber (oven) either by natural or forced convection.
What is roasting?
The term used for baking any solid structure e.g. veg
What is the heat transfer method of baking?
High convection - oven racks have gaps to allow better air flow.
Due to convection currents the best place to bake is generally the middle of the oven.
How does a fan forced oven work?
Uses a fan to push the air around a baffle and ensure the temperature is more even throughout the overnight
Lower temperatures can be used, reducing energy costs.
What is broiling?
A process whereby a thermal radiation heat source is above the food to be coked.
Grilling is the same but the heat source is below the food.
What is the heat transfer method of broiling/grilling?
High radiation with moderate conduction as the metal racks heat.
Healthy as no fat used
How does microwaving work?
Cooking method whereby microwaves are used to predominantly heat polar molecules in food. These molecules, like water, heat up and then heat the surrounding food. It’s a fast process as the heat is absorbed on a molecular level
What is the heat transfer method of microwaving?
High radiation due to the use of electromagnetic waves.
Retain a lot of nutritional value due to short cooking time.
Why cant you have metal in the microwave?
They don’t absorb the microwaves and will reflect them instead. It can lead to an electric charge building up which can lead to an electrical arc from the metal to the faraday cage around the microwave that stops the microwaves exiting and reflect it at your food
