Download L 17

L 17 - Thermodynamics [2]
Science that studies the relationships
between heat and work
 it applies to all living and non-living things
 it predicts the direction of natural
processes

 why ice melts (rather than getting colder!)
 why gases expand to fill entire volumes
Thermal Expansion
L1
Metal bar at T1
L2
Metal bar at T2 > T1
• Expansion occurs in all dimensions
• Expansion must be taken into account when
designing roads and bridges in climates that
vary significantly from winter to summer – all
materials expand, steel, concrete, asphalt . . .
winter/summer expansion gaps
Areas and volumes expand too!
cold
hot
cold
hot
Does the whole get bigger or
smaller when heated?
cold
hot
Both the inner and outer diameters
increase when the ring is heated
Bi-Metal strips
• thermal expansion of metals is put to good use
in a bi-metallic strip.
• this is two strips of different metals bonded
together
metal A
metal B
Heating a Bi-metal strip
• when heat is applied to the bi-metallic
strip, both metals expand, but by different
amounts!
• result:
this can be used as a switch – in your coffee maker!
Bi-Metal strip thermal switch
when a preset temperature is reached, the
bi-metallic strip opens up
Thermocouples- measure Temp.
• The temperature sensor you stick into your
turkey is a thermocouple
• it is composed of 2 wires composed of different
metals welded together - when it gets hot it
makes electrical current flow
350.0 °F
Thermocouples protect you!
• a thermocouple is used in gas heaters and
dryers to protect against explosions
• a thermocouple is placed in the pilot light
• as long as the pilot light is on, the
thermocouple is hot and current flows
• a circuit senses the current and allows the
main gas valve to open
• if the pilot light is out, the circuit prevents
the main gas valve from opening
Internal energy, Temperature
and Heat
• in a gas the molecules
have energy because
they are moving.
• the sum of all the energies of all the
molecules is the system’s internal energy
• the temperature of the system is a
measure of how much internal energy it
has, Temperature
Internal Energy
Heat  what is it ? ? ?
System A
at temp TA
System B
at temp TB
• Heat is the energy that flows from one system to
another because of their temperature difference.
• Heat stops flowing when the two systems come
to the same temperature.
• Heat was considered to be an actual fluid
(caloric), but it is NOT a fluid- it is energy!
Heat Flow and the laws of
thermodynamics
• System A has a certain amount of internal
energy and so does system B
• If energy is transferred and the internal energy
of B decreases by some amount then internal
energy of A must increase by the same amount.
[the first law]
• If the temperature of A is less than the
temperature of B then heat flows from B to A (hot
to cold).  [the second law]
1st and 2nd Laws of
Thermodynamics
• the 1st law says that energy is conserved
 whatever energy system A gains must
be accounted for by the energy that B lost.
(assuming that the systems are isolated so
that they do not interact with any other
systems)
• The 2nd law specifies the direction of heat
flow hot to cold (ice melts!)
Reverse heat flow- why ice melts
Notice that the 1st law does
not say that ice must melt.
The first law does not prohibit
that the ice from getting colder
and the water warmer!
The 2nd law specifies the direction of heat flow
in natural (spontaneous) processes.
But what about refrigerators?
• A refrigerator is a device that moves heat
from something cold (inside) to something
hot (the room).
needs to
be plugged
in to work!
[energy in]
Refrigerators and the 2nd Law
• Does this violate the 2nd law? NO,
because it is not a spontaneous process
• Refrigerators require energy input (work)
(electricity) to operate.
• Heat does not flow spontaneously from
cold to hot, but it can be made to flow
backwards if there is an input of WORK.
• It uses electrical energy to pump heat from
cold to hot.
Heat flow
• HEAT  the energy that flows from one
system to another because of temperature
differences.
• But how does it flow? Three ways:
• convection
• conduction
• radiation
Convection
• heat is carried from place to place by the
bulk movement of either liquids or gases
• does not apply to solids
• when water is boiled, hot liquid rises and
mixes with cooler liquid, thus the heat is
transferred
• Hot air rises:
• want heat into lower level of house (winter)
• cooled air into upper levels (summer)
Conduction
• heat is transferred directly through a material,
with no bulk movement of stuff
• only energy moves
iron is a particularly
poor conductor of heat
heat conduction
HOT
COLD
Heat Flow
Thermal Conductivity
• The effectiveness of a
material in conducting
heat is characterized
by a parameter called
the thermal
conductivity
• there are good
thermal conductors
(metals) and poor
ones (insulators)
Material
Copper
Silver
Steel (stain.)
wood
glass
wool
Goose down
styrofoam
Thermal
conductivity
400
420
14
0.15
0.8
0.04
0.025
0.01
Grandma’s silver spoons