Understanding Thermodynamics: Heating, Cooling, and Energy Conservation

1. We start from a piece of ice at -30 °C, supply heat at a constant rate and monitor the temperature… Straight line with constant slopes – warming up of ice, water and steam. The slopes are inversely proportional to their specific heats. Horizontal lines – no temperature change over long time intervals. Phase transitions – melting and vaporization.

2. It takes some amount of energy to melt and vaporize substances. Those energies per unit mass of the materials are called heat of fusion Lfand heat of vaporization, Lv. Since no temperature change occurs during the phase transitions, those energies are sometimes called latent heats of fusion/vaporization.

3. Positive energy is required to melt/vaporize. The same amount of energy, however, is released when the substance solidifies/condenses.

4. Hot compress. Paraffin compress – I hope you never had one applied to you! Typical melting temperature is from 49 to 71 °C (120 to 160 °F). So, if you melt it, wrap it in a cloth and apply to yourself (or your kid) it is going to stay at its melting temperature, around 140 °F till its heat of fusion is transferred to your body…

5. How does it compare? So, if you melt it, wrap it in a cloth and apply to yourself (or your kid) it is going to stay at its melting temperature, around 60 ºC (140 ºF) till its heat of fusion is transferred to your body… For water cooling down: To give away (reject) the same amount of heat, 1 kg of water must cool down by ~35 ºC, from 60 ºC to 25 ºC (140 to 80 ºF).

6. Heat of vaporization of water is huge, Lv = 2257kJ/kg, compared with its specific heat of c =4.2kJ/kg°C. It takes a kettle 5 minutes to heat 1 kg of water from the room temperature of 20 °C to the boiling temperature of 100 °C. How much time it would take the same kettle to evaporate the same amount (1 kg) of water at the same rate of energy supply (neglecting all energy losses)? Energy to heat to 100 °C is Energy to vaporize the water is So the total evaporation will take

7. First law of thermodynamics Kinetic energy: Potential energy (U): Conservation of energy in mechanics: mechanical energy of a closed system is conserved

8. External forces and work they do. work of the external forces The system is not totally on its own (not closed or isolated). External forces can do work on it and it can do work on external objects. Wsys is the work done by the system (in this case – against friction forces). If Wsys > 0 the energy decreases.

9. 1st law of thermodynamics. In thermodynamics we introduce a new type of energy – internal energy of the system, U, which is a sum of energies of microscopic components of the system. In thermodynamics we usually assume that kinetic and potential energy of the system do not change, or their changes are negligible: We unify Wext and Wsys by introducing the work W, which is positive if done by the system and negative if done by external forces

10. 1st law of thermodynamics. We unify Wextand Wsys by introducing the work W, which is positive if done by the system and negative if done by external forces How do we convert work into internal energy? This is easy!

11. Potential energy of the weights is converted into work (external work, negative W ) and into an increase in the internal energy of water (positiveDU ) External work done by the hands (negative W) is converted into positive internal energy (positiveDU) Are not we forgetting something, though?

12. In order to increase internal energy of a system, we do not necessarily have to have some work done on it. There is a simple and common alternative – transferring heat, Q, from some hot object (heater) The heat Q is positive if the energy is transferred to the system.

13. change in the internal energy of the system net heat transferred to the system First law of thermodynamics work done by the system The change in the internal energy of a system depends only on the net heat transferred to the system and the net work done by the system, and is independent of the particular processes involved. The equation is deceptively simple… One of the forms of the general law of conservation of energy. BUT! Mind the sign conventions, prepositions and multiple meanings/implications!

14. Heat is energy in transit! Internal energy relates to energetic contents of an object or a system. Whereas Heat is energy in transit!

15. change in the internal energy of the system net heat transferred to the system work done by the system The heat, Q is considered positive, when it is transferred TO the system. – The system gains energy. The work, W, is considered positive if it is done BY the system. – The system loses energy. WHY? – The history heavily influenced by the heat engines.

16. change in the internal energy of the system net heat transferred to the system work done by the system Statement #1 – conservation of energy The energy is transferred to and from the system by means of heat transfer and mechanical work. It causes change in the energetic content of the system – its internal energy. The total amount of energy is conserved, though.

17. change in the internal energy of the system net heat transferred to the system work done by the system Implication: both heat, Q, and mechanical work, W, are means of energy exchange between the system and the outside world. The both are kinds of energy in transit. That’s why they are on the same side in the equation. Nevertheless, the heat, Q, specifically relates to the energy transferred due to temperature difference alone. While, W, incorporates all other sorts of energy transfer, most commonly mechanical work.

18. change in the internal energy of the system net heat transferred to the system work done by the system Sometimes things may become confusing: solar heaters vs. solar batteries. We will try to restrict ourselves to unambiguous situations… Mechanical work is rather simple and straightforward.

19. change in the internal energy of the system net heat transferred to the system work done by the system Consequence: heat, Q, and mechanical work, W, are interchangeable in terms of their effect of internal energy of the system…

20. change in the internal energy of the system net heat transferred to the system work done by the system Statement #2 – internal energy, U, is a function of internal state of the system, a thermodynamic state variable – a quantity, whose value does not depend on how a system got to a particular state. In principle, you can measure internal energy of the system -, by measuring the sum of energies of all molecules, - the same way as you can measure its temperature and pressure – both the state variables, well defined in thermodynamics equilibrium. Usual ambiguity with setting the zero level of energy, though… What is energy of Uranium at absolute zero temperature?

21. NET heat transferred to the system Continuous processes – we differentiate with respect to time to define rates of energy flow, measured in Watts. Gasoline burning in an automobile engine releases energy at a rate of 160 kJ per second. Heat is exhausted through the car’s radiator at a rate of 51 kJ per second and out of the exhaust at 50 kJ per second. An additional 23 kJ per second goes to frictional heating within the machinery of the car. What fraction of the fuel energy is available for propelling the car?