Conceptual Physics Change of Phase 03a Evaporation, Boiling

Demo # 1 – Cloud Formation What did you see form as the glove is pulled out from the jar? What has happened to the pressure in the jar? What is the role of the match in making a cloud? What would happen if I pulled the glove more slowly? As pressure decreases, temperature decreases as well.

Evaporation Question: Why do we sweat when we are warm? Which would feel more cold on a cool day? Standing in front of a fan while dry or soaking wet? – Explain! Evaporation is the process of a liquid changing to a gas and cools the surface left behind

Demo #2 - Evaporation and Pennies

If you win RPS, exchange a penny. When you get to 10 pennies, you have enough kinetic energy to evaporate, and you leave the game. What are you taking with you as you leave the game? – Thermal energy! What will happen if we start out with 7 pennies? – Evaporation will occur more quickly! How would our starting water temperature change? – Starting water temperature will have been greater; warmer. What would happen if we change the evaporation point to 13 pennies and started at 5? – Evaporation will occur more quickly than with 7 pennies. Activity: States of matter phet simulation Demo #2 - Evaporation and Pennies

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point. Evaporation

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point. – When a liquid evaporates, it takes thermal energy with it. Evaporation

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point – When a liquid evaporates, it takes thermal energy with it. – When a liquid evaporates, the liquid left behind has lower thermal energy. Evaporation

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point – When a liquid evaporates, it takes thermal energy with it. – When a liquid evaporates, the liquid left behind has lower thermal energy. – A liquid in life will fully evaporate because thermal energy is added to the system Evaporation

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point – When a liquid evaporates, it takes thermal energy with it. – When a liquid evaporates, the liquid left behind has lower thermal energy. – A liquid in life will fully evaporate because thermal energy is added to the system. Evaporation

Demo #3 – Water vs. Alcohol Rub two different patches of your forearm, one with water and one with rubbing alcohol, and then blow on the patches. – Explain the differences felt. Rubbing alcohol evaporates at a lower temperature How will your skin’s temperature actually measure? – Temperature of the chalkboard demo.

Relative Humidity Lab Record the temperature of the room (this is your dry bulb temp) Wrap a cotton ball with a rubber band around the thermometer and dip the cotton in room temp water Place the thermometer in front of a fan and record the resulting temp (this is your wet bulb temp) What is warming and what is cooling? Why is the wet bulb temperature what you expect it to be? Why would the wet bulb temp not change if humidity is 100%? Why would the wet bulb temp might be different outside compared to inside? How would the wet bulb temp be different if you used rubbing alcohol instead of water?

Question: What are some ways to cool off a hot drink? – Blowing on it. Why? Increases KE of molecules thus increasing the rate of evaporation. – Increase surface area for evaporation. Why? More molecules able to escape at one time. – Put silverware in it to act like a radiating antenna. Why? Metal is a good conductor of heat; will carry thermal energy away from hot drink. Evaporation

Boiling What is the process of boiling? – Boiling is the change in phase from a liquid to a gas. It takes thermal energy to break the bond between the molecules in the liquid.

Question: What are the air bubbles in a pot of boiling water? – Water turning to gas. – Greatly increase the volume! What is adding the thermal energy? – Burner. How is thermal energy leaving this system? – Evaporation! In something that is boiling or evaporating, what part is loosing or gaining thermal energy? – Water is losing thermal energy, air is gaining thermal energy. Boiling

Question: What will happen to the water vapor bubbles if the pressure is increased? – The water has to get to a higher boiling point to withstand the added pressure. – Over 100 degree ᵒC What is the benefit of a pressure cooker? – Food cooks faster. Boiling

Question: So what will happen to the boiling point if we remove air pressure? – Boiling point will be lowered. – Bubbles are easier to form. Which has a higher boiling point, Colorado springs or San Diego? – San Diego – more air pressure – boiling point at 100 ᵒC. – Colorado Springs – around 94 ᵒC. What about in a near vacuum? Boiling

Question: When water is boiling at sea level, does it ever go above 100 ᵒC? – No! Heat of fusion – – Idea that water stays at 100 C when boiling Boiling

Question: What will happen to the cup if I put it over a flame? What will happen if I put water in the cup and then over a flame? What parts of the cup will burn? Demo #4 – Paper Cup

Space Craft Heat Shields Space Shuttle: – Thermal Protection System… tiles. – Insulator – prevents heat transfer. Apollo Capsule: – High melting point. – Surface melts and vaporizes away, referred to as “ablation,” releasing thermal energy. – Entry angle: b/w 5⁰ and 10⁰ scibuff.com

What do you see coming out of the tube? What will happen if I add thermal energy to the steam? What will be the temp of the water? The steam? The steam with more heat transferred to it? Demo #5 – Steam and Candle