MNSR 22 - Heat Transfer

Question 1

Thermometric Property

Answer 1

Any physical property of a substance that changes in a
-uniform (but not necessarily linear)
-and predictable manner as T changes

Usually defined for a specific temperature range.

e.g. volume, color, electrical resistance, voltage, pressure
- Mercury/Alcohol thermometer
- Thermocouple (resistance of 2 different metals)
- Gas-filled bulb thermometer
- Liquid crystal thermometer (crystal color change)

As the mercury/alcohol heats up in the thermometer it will expand in a linear fashion for this temp region, and after calibration a thermometer can be made.

Question 2

Human Body Temp

Answer 2

Core body temp 37°C

Temps at extremities vary
Oral Cavity - 0.4°C (lower than core )
Axilla/Armpit - 0.9°C (lower than core)
Rectum - 0.3°C - (higher than core)

Question 3

External factors affecting body temperature

Answer 3

Time of day
Age
Smoking
Hot/cold drinks
Hormones
Diurnal cycle
Menstrual cycle

Question 4

Absolute Zero

Answer 4

Charles’ Law:
Volume of an ideal gas is directly proportional to the absolute temperature at constant pressure.
Extrapolate graph to 0, temperature -273.15°C is at the x-axis.
Absolute zero - start of Kelvin scale
Not possible to reach absolute 0, no such thing as ideal gas, can come really close though

Question 5

Convert Celsius to Kelvin

Answer 5

Add 273.15

Question 6

Convert Celsius to Fahrenheit

Answer 6

(𝐶x9/5)+32 = 𝐹

To convert Fahrenheit to Celsius
(𝐹−32)(5/9) = 𝐶

Question 7

Thermal Inertia

Answer 7

Object is slow to heat up due to a high specific heat capacity
Not strictly true, also related to thermal conductivity & density of material

Question 8

Convection Cycle (James)

Answer 8

liquid heated, goes up, less dense, cools down, more dense, goes down

Question 9

Conduction

Answer 9

Heat transfer by DIRECT CONTACT
𝐻𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑒𝑑 = 𝑄/𝑡 = 𝜅𝐴∆𝑇/𝐿
- 𝐻𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑒𝑑 is the rate of heat conduction
- 𝜅 is the conductivity of the material (“kappa”), unit W/m.K, different for every material
- A is the cross-sectional area
- ∆𝑇 is the temperature difference
- L is the length of the bar
- Q is the total amount of energy

Question 10

Heat Q conducted depends on

Answer 10

• time during which conduction takes place
• temp difference between ends of bar
• cross sectional area of the bar
• length of the bar

Question 11

Thermal Conductivity

Answer 11

• Conductor = depends on material, with electricity something that allows electricity/heat to pass through it easily
  e.g. copper (thermal conductor)
• Insulator = does not allow electricity/heat to pass through easily
  e.g. air (thermal insulator, why there is an air pocket in double-glazed windows)
  A lot of electrical conductors are good conductors of heat

Water has thermal conductivity 24 times greater than air, why a person will cool down much more quickly in water at 10°C than in air at 10°C

Question 12

Convection

Answer 12

• Heat transfer by the bulk movement of a fluid.
• Does not have to be a liquid, also applies to gases.

𝐻_𝑐𝑜𝑛𝑣𝑒𝑐𝑡𝑖𝑜𝑛 = 𝑄/𝑡 = 𝑞𝐴∆𝑇
- 𝑞 is the convective heat transfer coefficient, unit W/m2.K
- A is the cross-sectional area
- ∆𝑇 is the temperature difference
- L is the length of the bar

Question 13

Convection Effects

Answer 13

• Wind chill - moving cold air cools you down
• Fan-oven Effect - convection oven, moving hot air will cause food/substance/person to heat up more quickly

Question 14

Radiation

Answer 14

Transfer of heat energy by EM radiation.
No medium required.

𝐻𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 = 𝑄/𝑡
= 𝑒𝜎𝐴(𝑇𝑏𝑜𝑑𝑦^4−𝑇𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔𝑠^4 )
- e = emissivity (0<e<1)
- 𝜎 = Stefan Boltzmann constant, 5.7x10-8 W/m2.K4

Question 15

q) Wind-chill accelerates _____________ cooling by continually removing the heated layer of ________ in contact with the skin surface.

Answer 15

Convective, Air