Thermal Energy and Heat

1. Thermal Energy and Heat

2. Temperature • Particle level kinetic energy • A measure of the average kinetic energy of the atoms or molecules of a substance • Temperature increases if the motion of the particles increases

3. Thermal Energy (Q) • Particle level Mechanical (total) Energy • Total kinetic and potential energy of a system’s particles

4. Heat • Particle level WORK • The transfer of thermal energy

5. Methods of Heat Transfer Conduction • Heat transfer through a material by collision of atoms • Particles in flame gain kinetic energy and start moving faster – they collide with neighbouring atoms making them move more rapidly and heat spreads • Occurs well in metals

6. Methods of Heat Transfer Convection • the process of transferring heat by a circulating path of fluid particles – called a convection current • Hot fluid spreads out and moves upwards, cooler fluid takes its place creating a current

7. Methods of Heat Transfer Radiation • Energy transfer by electromagnetic waves – no particles are necessary • i.e. visible, radio, micro, UV, infrared, X-rays

8. Calculating Heat • We use Specific Heat Capacity (c) Unit: J/(kg ·oC) • A measure of the amount of energy needed to raise the temperature of 1.0kg of a substance by 1.0oC • Different for every substance • cwater – 4.18 x 103 J/(kg ·oC)

9. Calculating Heat Q = mcΔt • Q = heat (J) • m = mass (kg) • c – specific heat capacity J/(kg ·oC) • Δt – change in temp – Δt = tf - ti

10. Ex 1: What is the mass of a bucket of water that requires 8.4 x 104 J of heat to increase its temperature from 12oC to 22oC? Ans: m = 2.0 kg

11. Latent Heat

12. What has more energy? Water at 10oC or water at 50oC? Cooler particles move slower (less kinetic energy) Hotter particles move faster (more kinetic energy)

13. What has more energy? Ice at 0oC or water at 0oC? Solid – Highly structured, particles are trapped, little motion Liquid – Less structured, particles can move, more motion Liquids are higher energy

14. Latent Heat • the energy released or absorbed during a change in state • “latent” means “hidden” – the energy is hidden in the bonds between molecules • there is no temperature change during a change in state

15. Specific Latent Heat – the heat required for a particular mass of substance to change state

16. Q = ml Q = heat energy (J/kg) m = mass (kg) l = specific latent heat (J)

17. Specific Heat of Fusion • the quantity of heat required to melt/freeze 1kg of a substance without changing the temperature

18. Specific Heat of Vaporization • the quantity of heat required to vaporize/condense 1kg of a substance without changing the temperature

19. Using the latent heat values from your sheet: How much energy is required to turn 29.0g of solid oxygen at -219oC into liquid oxygen at -219oC? Ans: Q = 403J How much energy is required to turn a 50g, 0oC ice cube into 0oC water? Ans: Q = 1.7x104J

20. Consider an ice cube melting • Below 0oC the ice is solid, heat energy from the surroundings increases the temperature of the ice • As the ice melts, the heat energy is used to break the forces of attraction that hold the ice in a solid crystal lattice (no temperature change) • Once the ice is melted, the heat energy from the surroundings can be used to increase the temperature of the water

21. 0C Ice Add Latent Heat 0 C Water

22. Consider an ice cube melting • As the water reaches 100oC, the heat energy will stop raising the temperature and be used to break the forces of attraction in order to turn the liquid into a gas

23. 0C Ice Add Latent Heat 0 C Water

24. Applications of the Latent Heat of Water Water has one of the largest specific latent heats of fusion (melting) of all substances: 333kJ/kg • this means that 333 kJ of heat must be absorbed to melt 1kg of ice • this makes ice a good refrigerant in coolers When water freezes it releases energy into the surroundings (333kJ for every kg) • farmers use this to protect their crops from frost: they water the plants and as the water freezes it gives 333kJ/kg worth of heat E to the plant to warm it up

25. Applications of the Latent Heat of Water A burn from steam is worse than a burn from boiling water! • when water condense on a person's skin, 2268 J of energy is transferred to the skin

26. Sample Problem: • How much energy is required to turn 60 g of -15oC ice into 50oC water?

27. Principle of Heat Exchange When heat is transferred from one body to another, the amount of heat lost by the hot body equals the amount of heat gained by the cool body Qlost + Qgained = 0 OR: m1c1Δt1 (heat lost) + m2c2Δt2 (heat gained) = 0

28. Principle of Heat Exchange • When heat is transferred from one object to another, it usually flows from the hotter object to the cooler one

29. A gunsmith is making homemade bullets of melted lead. After he creates the 2.3kg bullet, it has a temperature of 1680oC. He submerges it into 25kg of 20oC water to cool it down and let it harden. What is the final temperature of both the lead and the water? Ans: Tf =26oC • cwater – 4.18 x 103 J/(kg ·oC) • clead– 1.60 x 102J/(kg ·oC) Heat Transfer

30. Solution

31. More on latent heat:

32. Graph of Temperature vs Heat absorbed to melt an ice cube

33. A: Heat is absorbed by the surroundings to raise the temp. of the ice • B: Temperature doesn't change – energy is used to break forces of attraction within the solid to turn it into a liquid • C: Heat is absorbed by the surroundings to raise the temp of the water. • D: Temperature doesn't change – energy is used to break forces of attraction within the liquid to turn it into a gas • E: Heat is absorbed by the surroundings to raise the temperature of the gas