Lecture 5 Continued Heat Transfer Phase Changes Conduction Convection Radiation Phase Changes

If you hold one end of a metal bar against a piece of ice, the end in your hand will soon become cold. Does cold flow from the ice to your hand? Yes In some cases yes No

A person walking barefoot on hot wooden coals can avoid serious burns because of wood’s good conduction. poor conduction. high specific heat capacity. ability to emit radiant energy.

Convection is The transfer of heat by the motion of fluid as it rises and sinks. The transfer of heat resulting from collisions between particles. a form of energy that requires no medium (matter) to travel through. Electromagnetic radiation. Both 3 and 4.

Convection Convection occurs in fluids (liquids and gases). Involves bulk motion of a fluid rather than interactions at a molecular level.

Convection Plate Tectonics is driven by convection within Earth’s mantle. Convection within the atmosphere drives weather patterns. Convection also occurs in the oceans.

When air rapidly expands, it will warm. cool. remain at a constant temperature.

Convection Warm air rises due to lower density. Air expands and cools as it rises. Air ceases to rise when its density = that of the surrounding air.

Convection Steam is not visible before it expands and cools. Hot steam quickly rises, expands and cools into a cloud of condensed water vapor.

Unequal heating over land and water causes convection in air.

Although warm air rises, why are mountaintops cold and snow covered, while the valleys below are relatively warm and green? Warm air cools when rising. There is a thick insulating blanket of air above valleys. Both of the above.

The Earth and Sun are 93 million miles apart The Earth and Sun are 93 million miles apart. How does the Sun’s energy reach the Earth? Convection of hot gases between the Sun and Earth. Conduction through interplanetary dust. Radiant energy (electromagnetic waves). All of the above.

Types of electromagnetic waves are defined by wavelengths

A high-temperature source gives off electromagnetic energy with relatively short wavelengths. long wavelengths.

Radiant Energy Shorter wavelengths = higher energy (hot) sources Longer wavelengths = lower energy (cool) sources. Blue light = hot stars Red light = cool stars

Which body emits radiant energy into space? Sun. Earth. Both of the above. None of the above.

Radiant Energy All objects emit radiant energy: Wavelengths are related to the Kelvin temperature of the body. At about 800 K, many materials begin to emit visible red light.

An increase in the surface temperature of a radiating body = an increase in the total amount of energy emitted.

A substance that is a good absorber of energy is A good emitter of radiant energy. A poor emitter of radiant energy.

Radiant Energy All objects absorb radiant energy. Good Emitters = Good Absorbers Infrared radiation (heat energy): Particles of matter vibrate more rapidly when they interact with infrared radiation.

A hot pizza placed in the snow is a Net absorber of energy. Net emitter of energy. Both of the above. None of the above. The same hot pizza placed in a hotter oven would be a net absorber.

Radiant Energy Temperature increases when an object absorbs more energy than it emits. Occurs when temperature of object is less than its surroundings (remember the 2nd law of thermodynamics).

Which is a more accurate statement? A black surface absorbs energy well (dark coloration is the cause of absorption). A surface that absorbs energy well appears black (dark coloration is the effect of absorption). Any object that does draw in and absorb energy will consequently appear dark.

A situation where a bright white surface can be made to appear completely black.

Cooling Newton’s Law of Cooling: the rate of cooling of an object is directly proportional to the temperature difference (ΔT) between the objects and its surroundings. Rate of Cooling ~ ΔT

Because a hot cup of tea is cooling more rapidly than a warm cup of tea, would it be correct to say that a hot cup of tea reaches room temperature sooner? Yes No

increase. decrease. remain unchanged. If the atmosphere somehow became more transparent to infrared radiation, Earth’s temperature would increase. decrease. remain unchanged.

Greenhouse Effect

Greenhouse Effect Atmospheric gases such as CO2 and water vapor retain heat that would otherwise escape into space.

Without the Greenhouse Effect, Earth’s average temperature would be -18°C. 0.035% CO2 can have a huge effect! Over the last 500,000 years, average temperatures fluctuated between 19°C and 27°C. Presently is 27°C and climbing. CO2 concentration is now 43% above preindustrial values.

Runaway Greenhouse Effect Venus: Atmosphere is 97% CO2. Surface temperatures approach 475°C.

Change of Phase High Average Internal Energy (molecular motion) Low

When a liquid changes phase to a gas, it absorbs energy. emits energy. neither absorbs nor emits energy.

Change of Phase Evaporation: process by which liquid turns to a gas. Molecules at surface of liquid gain enough kinetic energy to escape. Absorption of energy by escaping molecules = cooling effect on surroundings and liquid left behind.

Change of Phase Sublimation: a process by which molecules jump directly from the solid to the gas phase. Solid CO2 (dry ice) Water Ice

Change of Phase Condensation: changing of a gas to a liquid. Energy released to surroundings when water vapor condenses = warming effect.

You place a dish of water on a table and let it sit for several hours You place a dish of water on a table and let it sit for several hours. During this time, water is evaporating from the dish. water from the air is condensing in the dish. Both are occurring. None of the above.

Change of Phase Evaporation and Condensation occur simultaneously. If one exceeds the other, then the water level in a dish decreases or increases. If the rates are equal, then the water level in a dish remains constant.

Change of Phase Evaporation = a cooling process Condensation = a warming process

Change of Phase Condensation of steam releases a very large quantity of energy.

Change of Phase Why would the same air temperature feel greater in humid regions than in dry regions? Higher rate of evaporation in dry areas = cooling. Higher rate of condensation in humid regions = warming.

Refrigerators use the evaporation and condensation of a fluid to pump heat from the interior to the exterior.

Change of Phase Boiling: when liquid beneath the surface changes to a gas forming bubbles that rise to the surface. Pressure of vapor within bubbles must overcome pressure of surrounding liquid.

At boiling point temperatures, vapor pressure is great enough to form bubbles.

During the process of boiling, the temperature of water will Decrease. Increase. Remain constant.

Water stays at a constant 100°C during boiling. Energy entering water = Energy leaving water.

Change of Phase Boiling points are affected by pressure. Ex. lower atmospheric pressure at higher elevations = lower boiling temperature. Denver, Colorado = 95°C Monmouth, Oregon = 100°C

Change of Phase Pressure cooker inhibits boiling by holding in pressurized vapor above the water surface. Raises the boiling temperature above 100°C.

Are there conditions where the boiling and freezing of water can happen simultaneously? Yes No

Change of Phase In a vacuum: As pressure is reduced, so is the boiling point. Highest-energy water molecules escape leaving less energy behind (cooling process). As more and more water turns to vapor, the remaining water is cooled to 0°C – makes frozen bubbles.

lowers the boiling point of water. Your inventor friend comes up with a harmless and tasteless kind of salt that changes the boiling point of water. The usefulness of this salt for cooking food will be best if it lowers the boiling point of water. increases the boiling point of water.

Change of Phase Melting: when a solid changes to a liquid phase. Process involves absorption of thermal energy. Freezing: when liquid changes to a solid phase. Process involves a loss of thermal energy.

Ice and water in equilibrium The addition of impurities to water lowers its freezing point – foreign molecules interfere with ice crystal formation. Pure water at 0°C Ice and water in equilibrium Impurities cause fewer molecules to enter ice.

Energy and Change of Phase

Energy and Change of Phase Heat of Vaporization Vapor Water + Vapor Water Ice + Water Heat of Fusion Ice

Energy and Phase Changes Only one or the other can occur - not both at the same time: Change in temperature Change in phase

Energy and Change of Phase Heat of Fusion = energy required for complete melting = energy released during complete freezing. 334 J/g for water Heat of Vaporization = energy required for complete vaporization = energy released during complete condensation. 2256 J/g for water

Which will result in a more serious burn? Placing your hand in 100°C boiling water. Placing your hand in 100°C steam.