ATOC 181 Lecture Notes - Lecture 7: Sensible Heat, Evaporation, Latent Heat
7 Jun 2018
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LESSON 7
ENERGY: WARMING THE EARTH AND THE ATMOSPHERE
Introduction
• The sun has warmed the Earth for millions of years
• The temperature of the Earth system (surface and atmosphere) should have increased year after year,
centuries after centuries, etc…
• When talking about the Earth system, talking about Earth and the atmosphere
• If keep receiving heat and heat without losing it, the Earth system should be warmed up
• This is not the case. WHY?
o The Earth is not becoming hotter and hotter.
o Even if we hear that the surface temperature is increasing a little bit due to global warming, the
whole Earth system is not
• Radiative equilibrium: neither heat nor cool due to radiation that is exchanged and is in equilibrium
• Other heat exchanges also occur
Energy
• Amount of work required to change the state of a physical system
• Many different forms and definitions depending of what talking about
o For instance, when boil water, need to provide energy and do so by providing heat
• Ability to do work
o When go to Himalaya, acquire energy when going up and release it in form of speed or velocity
• Work is done when a force pushes, pulls or lifts matter
• Energy cannot be destroyed or created. It can change from one form to another
o A steam machine transformed heat into kinetic energy that will increase acceleration
• Forms of energy:
o Kinetic
▪ Ex: radiant
▪ can be used to change the speed of system
o Potential
▪ can be stored in water vapour when transformed to liquid
▪ Ex: chemical E
• Unit: Joule (J)
o Joule: work done to move an object by 1 mere when applying a force of 1N
• Power: Watt: 1W = 1J per sec.
o Watt: moving the object by 1m with a force of 1N in 1sec.
• For a light bulb, can see 60W or 100W and in that case, it means for 100W, the bulb provides 100J in
1sec.
o The higher the number of Watt, the more Energy is being released
• Temperature: measure of the molecule speed (average kinetic energy of all molecules)
o Here, kinetic is synonymous to speed
Temperature Scales
• Celcius (oC)
• Kelvin (k)
• 0 oC = 273k
• Can never reach 0k: no molecules moving at 0k. Can go as close as possible
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• The Kelvin temperature is used for scientific purpose
• A change of 1 oC = a change of 1k
• At 0k (absolute zero), molecular motion ceases. Never reached
Energy, Temperature and Heat
• Heat = energy transferred from one object to another because of temperature difference between them
• From hot/warm system to cold
• Received everywhere
• Can be transferred through different form
• In the atmosphere, can be transferred through
o Conduction
o Convection
o Radiation
o Advection
• Heat is also involved in change of phase
• To raise the temperature of 1kg of liquid water by 1 oC, need to provide an amount of energy (heat) equal
to 4 186J at room temperature and standard pressure
o This is called specific heat capacity of water
▪ in J per kg per kelvin (or oC)
• Specific is used to tell us that it is for 1kg
• Heat capacity: Ability of certain substance to absorb heat
• If do it in one sec, requires 4 186W or about 42 light bulbs of 100W each to raise 1kg of water by 1 oC
• For air, need 10 light bulbs of 100 W each to raise 1kg of air by 1 oC in 1 sec
• For dry air: to raise temperature of dry air, need 1005 J
• Need much more Energy to heat water than to heat dry air
• If have high heat capacity, need more Energy to warm the substance and take longer to warm the
substance
Sensible and Latent Heat
• Sensible heat: heat that is added or subtracted from a substance that changes its temperature but not
its physical state.
o As long as provide Energy, temperature increases till reach temperature where the phase will
change. Then, it stops increasing.
o Can heat water and increase its temperature, or heat ice (can have up to -100deg), but does not
boil or melt. Not change their physical state
o To change the temperature by 1 oC, need a certain heat capacity
• Latent (dormant) heat: amount of Energy in the form of heat that is required for, or released by, a
material to undergo a change of phase
o Ex: melting ice, or boiling liquid water. As long as has liquid water, temperature not change.
▪ Provide Energy to molecules of liquid water that is changing to molecules of water vapour.
▪ Temperature does not change, heat added is stored by water vapour and does not
increase temperature.
▪ keep the energy/heat until the water condenses. When condenses, releases heat back to
environment.
o Temperature remains constant whenever there is a change of state
o Energy supplied is used to change the state, not raise temperature
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
1sec: the higher the number of watt, the more energy is being released, temperature: measure of the molecule speed (average kinetic energy of all molecules, here, kinetic is synonymous to speed. Temperature scales: celcius (oc, kelvin (k, 0 oc = 273k, can never reach 0k: no molecules moving at 0k. Can go as close as possible: the kelvin temperature is used for scientific purpose, a change of 1 oc = a change of 1k, at 0k (absolute zero), molecular motion ceases. Energy, temperature and heat: heat = energy transferred from one object to another because of temperature difference between them, from hot/warm system to cold, received everywhere, can be transferred through different form. If have high heat capacity, need more energy to warm the substance and take longer to warm the substance. Then, it stops increasing: can heat water and increase its temperature, or heat ice (can have up to -100deg), but does not boil or melt.
