L 17 - Thermodynamics [2] it applies to all living and non-living thingsit predicts the direction of natural processes- why ice melts (rather than getting colder!)
- why gases expand to fill entire volumes
Thermal Expansion Expansion occurs in all dimensionsExpansion 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!
Does the whole get bigger or smaller when 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
Heating a Bi-metal strip when heat is applied to the bi-metallic strip, both metals expand, but by different amounts! result:
Bi-Metal strip thermal switch
Thermocouples- measure Temp. The temperature sensor you stick into your turkey is a thermocoupleit is composed of 2 wires composed of different metals welded together - when it gets hot it makes electrical current flow
Thermocouples protect you! a thermocouple is used in gas heaters and dryers to protect against explosionsa thermocouple is placed in the pilot lighta circuit senses the current and allows the main gas valve to openif the pilot light is out, the circuit prevents the main gas valve from opening
Internal energy, Temperature and Heat have energy because they are moving.the sum of all the energies of all the molecules is the system’s internal energythe temperature of the system is a measure of how much internal energy it has, Temperature Internal Energy
Heat what is it ? ? ? 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 BIf 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
But what about refrigerators? A refrigerator is a device that moves heat from something cold (inside) to something hot (the room).
Refrigerators and the 2nd Law Does this violate the 2nd law? NO, because it is not a spontaneous processRefrigerators 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 gasesdoes not apply to solidswhen water is boiled, hot liquid rises and mixes with cooler liquid, thus the heat is transferredHot air rises:- want heat into lower level of house (winter)
- cooled air into upper levels (summer)
Conduction
heat conduction
Thermal Conductivity The effectiveness of a material in conducting heat is characterized by a parameter called the thermal conductivitythere are good thermal conductors (metals) and poor ones (insulators)
Grandma’s silver spoons
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