Heat transfer- overview

• Heat transfer takes place when there is a temperature difference

• Energy is naturally transferred from places of higher temperature to places of lower temperature. By definition, this energy is heat

• Processes: conduction, convection and radiation

• Conduction: transfer of heat via jiggling of molecules

• Convection: transfer of heat via the motion of a fluid

• Radiation: transfer of heat via light or electromagnetic waves

(do “Heat Transfer in familiar situations” worksheet)

• Greenhouse effect

• Newton’s law of cooling

Conduction

•Depends on what? (Do example (cup) first). •Relation to electrical conductivity •Insulators•Examples

•Temperature we feel•“Temp in middle” worksheet (Assignment 2)•Worksheet (on board?)

•Wood-metal, metal-wood-metal, area modification•Worksheet

•Calculation of heat flow through metal cylinder and sheet

•Demo

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Convection•Forced and free

•Demo for free convection•Examples

•Melting flashlight in orbit•Wind off lake•Cooling of lakes

•Depends on what?•An organized process (Rayleigh-Bernard Convection)•Do exercise “Why is not all of the air on the ground?”•Why warm air rises

•Microscopic view•Macroscopic view

•Why expanding air cools•Example of expanding alone not causing cooling•Microscopic view•Macroscopic view•http://phet.colorado.edu/en/simulation/gas-properties

Radiation•As electromagnetic waves•Depends on what?

•Temperature dependence•Emission, absorption and reflection (“black body”, perfect reflector)

•Good emitter = good absorber•Good emitter = poor reflector•Appearance of radiation of cold

•Examples•Incandescent light observation through diffraction grating•Field trip to watch liquid iron•Cream and coffee cooling problem•Frost when air is above freezing

•Solar power•Radiative equilibrium of the Earth

•Absorption demo•http://phet.colorado.edu/en/simulation/greenhouse•Greenhouse effect

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Newton’s law of cooling

•Rate of cooling depends on the temperature difference between the object and its surroundings•Applies to all methods of cooling

•Conduction•Convection•Radiation

•Examples

Conduction

Conduction is the transfer of heat where the jiggling of atoms or molecules in a solid or liquid works to jiggle their neighbors, who jiggle their neighbors, who jiggle their neighbors, who jiggle their neighbors...

Conduction

Metals are good conductors because they have free electrons which can travel through the metal and transport energy as they jostle and collide with atoms and other free electrons. Conductivity is a property of a material. That is - some materials are better conductors than others.

Conduction

This is an atomic scale structure called a quantum corral. It was made by placing iron atoms on a copper surface. The waves evident on the surface illustrate the electronic nature of a metal.

Insulators

Materials like wood, plastic, glass, etc. are not as good at conducting because their electrons are tightly bound. Again, the conductivity of a material is a property of the material.

Conduction

Equation goes here

Insulators

In some cases, insulators are made to be even better at insulating via the use of dead air space.

Convection

Convection is the transfer of heat via the motion of a fluid. You might think of it as being a mass migration of molecules.

Convection

Simulation of plasma jet convection

Convection

Convection plays a large role in weather. Because of the difference in heat capacity between the land and water, they heat and cool at different rates. Recall that water has a very high specific heat, so it takes a lot of heat to raise its temperature. In the morning, the land heats faster. Hot air rises and cool air moves in to take its place. This causes wind to blow from the water onto the land.

In the evening, the reverse happens. The water has warmed up slowly during the day and retains its heat better. The hot air now rises from the water and air from the cooler land region blows out to sea. These air currents are known as convection currents.

Convection

Convection is also an important factor inside the Earth. Convection of hot material from deep inside is buoyant and rises toward the surface. Near the surface the material cools, and then sinks. This cycle can take millions of years to complete. Geologists think it at least partly responsible for driving the tectonic plates.

Radiation

Heat can also be transferred by light or electromagnetic waves. When light strikes an object, the light is absorbed and reemitted. Dark colors tend to absorb more light and heat up more than light colors. They also tend to emit more heat via radiation. The ability of a material to emit heat as radiation is called its emissivity.

Radiation

Energy per second is called power and is measured in watts. The amount of energy per second striking one square meter of the Earth is called the solar constant.

Radiation

We can calculate how much power the sun puts out by measuring the amount of power striking a square meter on the Earth and then multiplying that amount by the total surface area of sphere surrounding the sun at the radius of the Earth.

Greenhouse effect

Greenhouse gases like carbon dioxide and methane blanket our planet and keep it warm. Near infrared light from the sun is able to pass through the atmosphere. The light loses energy when strikes the surface of Earth and warms it up. This lower energy light has a longer wavelength (far infrared) which cannot get back out through the atmosphere as easily, so it reflects back down to Earth. The atmosphere traps in the light and warms up. This warming is crucial to life on Earth. Too much warming can bring about major global climate change.

Greenhouse effect

The burning of fossil fuels greatly contributes to the greenhouse gases. Deforestation also contributes to greenhouse gases because in the day, plants take in carbon dioxide and emit oxygen.

Greenhouse effect

Cattle and landfills produce vast amounts of methane. There are also naturally occurring sources of greenhouse gases, such as volcanoes and bogs. Considerable amounts of methane are bound up in arctic permafrost and ocean beds. As the permafrost thaws and the water warms up, the methane is released.

Greenhouse effect

There are secondary effects of global warming that have dire consequences as well. The mountain pine beetle is invading northern forests. The native trees have no resistance

against them because the beetles were restricted to more southern climates until recently.

Greenhouse effect

Some species are having a hard time adapting to the rapid change in their environments. For instance, polar bears have relied on pack ice for hunting seals, which are a main food source. The bears are sometimes caught far out in the sea when the ice breaks up.

Newton’s law of cooling

Newton’s law of cooling

Say you get a cup of hot coffee at Starbuck's and then decide you want a biscotti. You'll have to stand in line a few minutes more. If you want to keep your coffee hot longer, is it a good idea to add creamer now or when you get back?

Newton’s law of cooling

It is very hot in the desert. Why would these Bedouin women wear dark robes?