Physics 102 - Heat transfer




Chapter 16




heat matters...

 

 

Heat transfer

 

Heat transfer takes place when there is a temperature difference, from a higher temperature to a lower temperature. In this section, we will study different ways that heat can transfer: conduction, convection and radiation.

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...

 

 

 

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 metals are better conductors than others.

 

 

 

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 electron nature of a metal.

 

 

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.

 

Is it possible to boil water in a balloon?

 

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.

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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 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 remitted. 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.

 

 

Energy per second is called power and is measured in watts or kilowatts. 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 multiply that amount of power by the total surface area of sphere surrounding the sun at the radius of the Earth.

The amount of energy per second striking one square meter of the Earth is called the solar constant. To be more accurate, the solar constant is actually defined as the electromagnetic radiation per unit area incident on the atmosphere on a plane perpendicular to the sun's rays. This means it is not effected by clouds or the latitude of the place on the Earth.

 

 

If you want to calculate how large a solar panel you need to power your home, you also need to consider the efficiency. Say your average solar power per unit area is S, and the power required for your home is P, and the efficiency of your solar panels is e. How can you calculate how big the solar panels need to be?

 

 

 

 

The wavelength of light is how far it is between crests of the waves. The smaller the wavelength, the higher the frequency is. The frequency is the number of oscillations per second. High frequency light has more energy than low frequency light. Note that not all wavelengths of light penetrate Earth's atmosphere equally well.

 

Blackbody spectra

Every object absorbs and radiates light. A blackbody spectrum is a diagram showing the wavelengths of light emitted by a particular object and are typically used in astronomy to study the light from stars. The peak frequency is directly related to the surface temperature of the star.

 

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.

 

 

Man made contributions to the greenhouse gases include the burning of fossil fuels, raising cattle which produce vast amounts of methane as well as methane-producing landfills. Deforestation contributes to greenhouse gases because in the day, plants take in carbon dioxide and emit oxygen. 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.

 

 

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.

 

 

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.

 

 

Deep ocean currents known as the Global Conveyor Belt are driven by differences in water density and temperature. When water freezes at the poles, salt doesn't freeze with it. This makes the water very dense and cold, so it sinks below the warmer water coming in. The current moves extremely slowly, much slower than normal surface currents. Scientists estimate it takes 1000 years to complete its circuit. It enriches the surface waters by dredging up nutrients from the deep. It is feared that global warming may shut down the Global Conveyor belt by melting the polar ice and reducing the salinity of the water. Warmer, lower density water would not sink down to complete the circuit.

how stuff works

Newton's law of cooling

The rate of cooling of an object depends on the difference between the temperature of the object and the ambient surroundings. This is true whether the cooling mechanism is that of conduction, convection, radiation or a combination of these.

Say you get a cup of hot coffee at Starbuck's and then decide you need 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? Think of three good reasons...

 

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

 

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