1 L 17 - Thermodynamics [2] Science that studies the relationships between heat and work and the conversion of one into the other Science that studies the relationships between heat and work and the conversion of one into the other it applies to all living and non-living things it applies to all living and non-living things it predicts the direction of natural processes it predicts the direction of natural processes  why ice melts (rather than getting colder!)  why gases expand to fill entire volumes

2 350 C But first, some practical matters Thermocouples AMPS two different metals

3 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 °F

4 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

5 Thermal Expansion 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... Metal bar at T 1 Metal bar at T 2 > T 1 L2L2 L1L1

6 Thermal expansion for Copper* change in Temp = 100 – 22 = 78 degrees C * for concrete  = 10 x per degree C

7 winter/summer expansion gaps

8 expansion gaps on bridges

9 Thermal Expansion problems

10 Areas and volumes expand too! cold hot cold hot

11 Does the whole get bigger or smaller when heated? cold hot Both the inner and outer diameters increase when the ring is heated

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13 H 2 O is Weird Stuff ! density in g/cm 3 expansion density = mass/volume

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15 Fire sprinklers use thermal expansion

16 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

17 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!

18 Bi-Metal strip thermal switch when a preset temperature is reached, the bi-metallic strip opens up ON OFF

19 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

20 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! System A at temp T A System B at temp T B

21 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]

22 1 st and 2 nd Laws of Thermodynamics the 1 st 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 2 nd law specifies the direction of heat flow  hot to cold (ice melts!)

23 Reverse heat flow- why ice melts Notice that the 1 st 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 2 nd law specifies the direction of heat flow in natural ( spontaneous) processes.

24 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]

25 Refrigerators and the 2 nd Law Does this violate the 2 nd 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.

26 How does Heat flow ? HEAT  the energy that flows from one system to another because of temperature differences. There are 3 processes by which heat flows convection conduction radiation