Lecture 21 Flashcards
Temperature
Temperature is a property of an object (is a number indicates the hotness of coldness of the object) and related to the Average K.E of atoms and molecules in that object (is a measure of K.E)
A temperature is a number related to the average kinetic energy of the molecules of a substance. If the temperature is measured in Kelvin degrees, then its value is directly proportional to the average kinetic energy of the molecules of a substance.
KE = 1/2mV^2
V increase, T increase
T =kelvin
Heat
is a form internal energy and not property of an object (Total KE + potential energy of all molecules)
Arises from microscopic motion the atoms/molcules
is defined as: internal energy that flows as a result of temperature differences.
Ex: find The speed of oxygen molecules at zero oC:
Note: use Kelvin’s temp and 1atm (u) = 1.66 x10-27 Kg.
k= 1.38x10-23 J/K
use this equation.
Answer : 461 m/s
KE=1/2mV^2 = 3/2KT
is actual energy measured in Joules or other energy units
When you add heat to a substance, you are adding energy to the substance.
This added heat (energy) is usually expressed as an increase in the kinetic energies of the molecules of the substance.
If the heat (energy) is used to change the state of the substance by melting it, then the added energy is used to break the bonds betweenthe molecules rather than changing their kinetic energy.
the energy stored in chemical bonds, is considered a form of potential energy.
internal energy
depends upon on temp and mass. internal energy
Same T but vastly different amounts of heat energy. Higher no. of molecules in iceberg
When heat, (i. e., energy), goes into a substance two things can be happened:
1- The substance can experience a rise in temperature,
increase temp, increase volume, increase in kinetic energy
2- The substance canchange state (or phase). For example:
if the substance is ice, it can melt into water. this change does not cause a temperature rise. the energy used to break bonds.
When you add heat to a substance, you are adding energy to the substance.
This added heat (energy) is usually expressed as an increase in the kinetic energies of the molecules of the substance.
If the heat (energy) is used to change the state of the substance by melting it, then the added energy is used to break the bonds betweenthe molecules rather than changing their kinetic energy.
Heat
In nature, heat always flows (transfers) from regions of high T to regions of low T
From: Hot- to – cold- until common temperature reached
Heat transfer results in:
Heat gain can increase the temp.
Heat loss can decrease the temp. (taking heat away)
Heat transfers by
Conduction, Convection and Radiation.
Heat Units :
Heat is a form of internal Energy in a system.
Measured in calories and Joules
calorie (cal): is defined as the amount of heat necessary to raise a temperature of 1 g of water by one degree Celsius
1cal= 4.186 Joules
1kcal =4.186 x103 Joules
Example
After a heavy meal one found that he overrated by 500 kcal, to get rid of the extra calories he decided to do some work ( climbing a mountain ). What is the vertical height he must walk up if his mass is 60 kg?
Work done = mgh
500 kcalx 4.286 x103 = 60 kg x9.8m/s2xh
h= 3600 m
If human body efficiency is 20% therefore
0.2 x3600=720m
He needs to climb a mountain 720m high
Differences between Heat and Temperature
Temperature Scales(Units)
SI scale is Kelvin (K), but more normal to use Celsius (oC) or Fahrenheit (oF) Scales
Both scales are based on the freezing and boiling points of water.
water boils at 100 degrees celsius and freezes at 0 degrees celicus
Graph of volume vs Temp.
(Charles’s law) : At const. pressure, the volume of the gas increases with temp. At constant rate
-273 degrees is the absolute temp.
P=constant
the graph shows that any gas cooled to
-273 oC would have zero volume
At lower temp. negative volume has no meaning
-273 oC considered the lowest temperature possible.
This temperature is called absolute zero temperature.
Absolute zero forms the basis of the temperature scale known as the absolute or Kelvin scale.
T(K)=T(oC) +273
Scale conversion
TC = TK – 273.15degrees
TF= 9/5Tc + 32degrees
Tc=5/9(Tf-32degrees)
Thermometric properties
Thermometers: are instruments based on a change in some physical characteristic which can be used to measure temperature.
Examples:
Thermal Expansion
* e.m.f. of a thermocouple
* pressure of a gas at constant volume
* volume of a gas at constant pressure
* resistance
* colour
pyrometeror infrared-sensing thermometer.
PV= nRT
Various types of thermometers
use thermometric properties of various substances.
Both devices depend on the expansion as the temperature increases
Liquid crystals can be constructed to change colour at a very specific temperature.
Length of a liquid column
Electric properties of the thermistor (semiconductor) can be used to measure temp.
Infra Red Radiation Thermometer
Measures the temperature of the infrared heat given by the eardrum and surrounding tissues
Gay-lussac law
Pressure of a gas at constant volume
Pressure increases
measurably
with temperature
PV=nRT
Charles law
Volume of a gas at constant pressure
Volume increases measurably
with temperature
Dv= vob DT (b is Coeff. of Vol. Expansion)
Thermal expansion of mercury
example:
What is the change in length of a column of mercury 3.0 cm long if its temperature increases from 37 oC to 40 oC. ( the coefficient of linear expansion is a= 60x10-6/oC).
DL= LaDT
= (3 cm)(60x10-6/oC)(40oC - 37oC)
DL= 5.4x10-4 cm
Heat and Human body
Core temperature (Tc)
Core temperature (Tc), core body temperature, is the temperature of the internal organs, such as the liver, located deep within the body.
human body maintains its core temperature within a very narrow range of 36.5-38.5
Peripheral Temperature Measure through:
Oral:3 min, 0.4oC lower than core T
Axillar:0.9oC lower than core T
The skin on the forehead(temporal):
lower by 0.3 to 0.6 oC
rectal temperature
Rectal measurements closer to core temp.
Rectal 10cm from sphincter 0.3-0.8 degrees Celsius higher than core
Physiological state
Daily variation in body temperature, ranging from about 37.5oC from 10 a.m. to 6 p.m., and
falling to about 36.4 oC from 2 a.m. to 6 a.m
isotherms
core of deep body temperature shrinks in cold, so peripheral shell of cooler tissue left
Isotherm: distribution of temperature
Body temperature assessments and heat content
mass-weighted average temperature of body tissues
H=sp x mass x temp.
