SPH3U1 Lesson 03 Energy

Transcription

1 THERMAL ENERGY AND LATENT HEAT LEARNING GOALS Students will learn: Heat changes the amount of thermal energy in an object Temperature is a measure of the average thermal energy in an object Heat capacity is the link between heat and temperature THERMAL ENERGY James Prescott Joule ( ) spent much of his honeymoon studying waterfalls. He noticed that the water at the bottom of a waterfall had a higher temperature than at the top. How might this happen? Thermal energy and heat play significant roles in our lives from the furnaces that heat our homes to winds generated by the uneven heating of the Earth s surface. Even most of the food that we consume is converted into thermal energy. Thermal Energy Heat Calculating Heat The amount of heat released or gained during a temperature change can be found from, Specific Heat Capacity A beaker of 250 g of water is heated over a bunsen burner from room temperature (20 0 C) to boiling point (100 0 C). The heat capacity of water is 4180 J/kg 0 C. How much heat does the water gain? 1

2 Temperature HOW HEAT SPREADS FROM ONE REGION TO ANOTHER Consider two samples of water: 100 g at 50 C and 500 g at 50 C. Consider two other samples of water: 500 g at 50 C and 500 g at 90 C. 2

3 METHODS OF HEAT TRANSFER Conduction Convection Radiation CONDUCTION Collision of atoms and electrons transfers heat Particles with more kinetic energy transfer some of their energy to neighbouring particles with lower kinetic energy increases the kinetic energy of the neighbouring particles. Note: Conduction is not the main form of heat transfer in liquids and gases because their molecules are spaced further apart. MOLECULAR VIBRATION FREE ELECTRON DIFFUSION 3

4 Molecular Vibration Free Electron Diffusion This explains why metals heat up faster: CONDUCTORS AND INSULATORS CONVECTION Transferring heat by a circulating path of fluid particles. Occurs in liquids and gases Does not occur in solids because the molecules are not free to move around Heating Water Since Earth s surface is over 70 percent water, water has a large effect on Earth s climate. Therefore, regions closer to large bodies of water tend to experience more moderate weather conditions than regions farther from them. 4

5 RADIATION Energy is transferred by means of electromagnetic waves. Radiation does not require a medium to transfer heat. (can occur in a vacuum) Sun releases electromagnetic waves (heat is contained in the waves as infra-red) Hotter objects radiates more heat. Any substance at a higher temperature than its surroundings will emit radiant energy, usually as infrared radiation. The warmed matter then transfers some of its thermal energy to substances at lower temperatures or re-emits it as IR. EMITTERS AND ABSORBERS CONSERVATION OF ENERGY Law of Conservation of Energy Consider a Pile Driver 5

6 CONSERVATION OF ENERGY WHEN MIXING The amount of heat gained by a cold substance is equal to the amount of heat lost by a hot substance. A beaker containing 250 g of water at 25 0 C is poured into another beaker that initially contains 350 g of water at 85 0 C. What is the final temperature of the mixed water? 6

7 LATENT HEAT AND CHANGES OF STATE LEARNING GOALS Students will learn: It takes heat to change an object s state. During a change of state, the temperature of a substance stays constant. HEATING / COOLING CURVE We learned that when you add heat to a substance, the temperature increases. When you remove heat, the temperature decreases. BUT, an interesting thing occurs when an object is undergoing a change of state. During a change of state, the temperature remains constant. The heat being added or removed is going into breaking or creating the bonds between the particles in the different states. If you measured the temperature of a solid substance to the point where it melts to a liquid, then continued heating the liquid until it boiled and turned entirely to a gas, you would get the following graph of temperature versus time: Consider the stages: 1. The substance is a solid. It increases temperature from the heat added. The heat to do this is given by 2. The substance has reached melting point. The temperature stops increasing as the heat is going into breaking the lattice bonds to change the solid to a liquid. The heat to do this is given by L f is called the specific latent heat of fusion. It is the amount of heat needed to change 1 kg of the solid at its melting point to a liquid at the same temperature. 3. The substance is a liquid. It increases temperature from the heat added. The heat to do this is given by Note that the heat capacity of the liquid state is not always the same as the heat capacity of the solid state. Water is an example of this. 7

8 4. The substance has reached boiling point. The temperature stops increasing as the heat is going into breaking the bonds between the liquid particles to change the liquid to a gas. The heat to do this is given by L v is called the specific latent heat of vapourization. It is the amount of heat needed to change 1 kg of the liquid at its boiling point to a gas at the same temperature. 5. The substance is a gas. It increases temperature from the heat added. The heat to do this is given by Note that the heat capacity of the gas state is not always the same as the heat capacity of the solid or the liquid state. Water is an example of this. Latent heats of fusion and vapourization can be found in many textbooks. See table 1 on p 291 in your text book. A 300 g block of ice at C is heated until it eventually becomes 300 g of water vapour at C. How much total heat does this take? Data you have to look up (pages 281 and 291) heat capacity of solid water = J/kg 0 C melting point of ice to water: 0 0 C heat capacity of liquid water = J/kg 0 C boiling point of water to steam: C heat capacity of steam = 200 J/kg 0 C latent heat of fusion of water = J/kg latent heat of vapourization of water = J/kg Solution: This must be solved in 5 steps. Each step has its own calculation. 1. Heat the ice from to 0 0 C: = 2. Heat to melt the ice to water at 0 0 C = 3. Heat liquid water from 0 0 C to C. 4. Heat to boil the water to steam at C 5. Heat water vapour from C to C. Total energy needed: x 10 4 J x 10 4 J x 10 5 J x 10 5 J x 10 3 J TOTAL: 9.4 x 10 5 J 8

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