lecture 22

Question 1

Heat & Temperature

Answer 1

Temperature is a property which indicates the relative “hotness” of an object.

Heat is a measure of thermal energy which is transferred between systems due to a temperature difference.

Heat energy is always transferred
from a higher temperature to a lower temperature

Question 2

Measurement of Heat & Temperature

Answer 2

A thermometer is used to measure temperature.

it is calibrated to a reference material and its thermometric property. i.e a mercury thermometer and the thermal expansion of the mercury.

That way an increase in the height of a column of mercury can be related to a change in temperature.

Question 3

Thermometric Property

Answer 3

A thermometric property is any physical property of a substance that changes in a uniform (but not necessarily, linear relationship) and predictable manner as T changes.

e.g. volume, colour, electrical resistance or voltage, pressure. Leads to the following types of thermometer:

Mercury/Alcohol thermometer

Thermocouple – resistance of two different metals

Gas-filled bulb thermometer

Liquid crystal thermometer – colour change of crystals

Question 4

Thermometric Property

Answer 4

As the mercury or alcohol heats up in the thermometer, it will expand.

It will expand in a linear fashion for this temperature region and after calibration, a thermometer can be made.

Question 5

Human body temperature

Answer 5

Humans have a core body temperature of around 37°C.
Though the temperature at extremities will vary.
Oral Cavity - 0.4°C lower than core
Axilla (arm-pit) - 0.9°C lower than core
Rectum 0.3°C - higher than core

Question 6

External affecting body temperature

Answer 6

time of day
age
Smoking
hot/cold drinks
hormones

Question 7

Absolute Zero

Answer 7

The volume of an ideal gas is directly proportional to the absolute temperature at constant pressure – Charles’ Law

If you extrapolate the graph back, a temperature of -273.15°C is found at the x-axis.

This value is absolute zero and the start of the Kelvin scale.

Question 8

Temperature Scales

Answer 8

We know the Celsius scale, 0°C and 100°C are the freezing and boiling points of water at STP, respectively.

To convert Celsius to kelvin, add 273.15

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

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

Question 9

Heat Capacity

Answer 9

is the amount of energy required to increase the temperature of a substance by 1 degree Celsius

Question 10

Specific Heat Capacity, c,

Answer 10

is the amount of energy required to increase the temperature of 1kg of a substance by 1 degree Celsius.

𝑄=𝑚𝑐∆𝑇

Q is the amount of energy (J), m is the mass of the substance (kg), c is the specific heat capacity (J/kg.K), ∆𝑇 is the change in temperature °C.

Question 11

Example

Answer 11

How much heat energy is required to raise the temperature of 3L of water from 20 to 80°C? SHC of water is 4200
J/kg.K
𝑄=𝑚𝑐∆𝑇
m = 3 kg (1L of water is 1 kg)
∆𝑇 = 80-20 = 60°C
𝑄=3(4200)(60)=7.56𝑥10^5 𝐽

Question 12

Heat Transfer

Answer 12

High vapour pressure means liquid wants to change to gas quickyly

Conduction done by direct contact (touch hot substance)

Convection = liquid gets heated, goes up and cools down and does back down

Radiation= heat transfer occurs through electromagnetic waves without involving particles.

Question 13

Conduction

Answer 13

Heat transfer by direct contact.
𝐻_𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑒𝑑=𝑄/𝑡=𝜅𝐴∆𝑇/𝐿

𝜅 is the conductivity of the material (writing a k is fine, “kappa” is the Greek letter). Unit W/m.K

A is the cross-sectional area

∆𝑇 is the temperature difference

L is the length of the bar

Question 14

Thermal conductivity

Answer 14

Depends on the material, with electricity something that allows electricity to pass through it easily is termed a “conductor” and something that does not allow electricity to pass through it easily is termed an “insulator”. The same terminology is

a lot of electrical conductors are also good conductors of heat and a lot of electrical insulators are also bad conductors of heat

Example of thermal conductor: Copper
Example of thermal insulator: Air – this is why there is an air pocket in double glazed windows

Question 15

Thermal conductivity

Answer 15

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

And why double-glazed windows are so effective.

Question 16

Convection

Answer 16

Heat transfer by the bulk movement of a fluid.
It does not have to be a liquid, this also applies to gases.

warm molecules move up and cool molecules move down

Question 17

Convection

Answer 17

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 18

Convection effects

Answer 18

Wind-chill: Moving cold air cools you down
“Fan oven effect”: Principle behind a convection oven, moving hot air will cause the food/substance/person to heat up more quickly

Question 19

Radiation

Answer 19

Transfer of heat energy by means of electromagnetic (EM) radiation.
No medium required

Question 20

Radiation

Answer 20

Question 21

example

Answer 21

Wind-chill accelerates _____________ cooling by continually removing the heated layer of ________ in contact with the skin surface.
Conductive, air
Convective, air
Convective, perspiration
Radiative, air
Radiative, perspiration