Heat Transfer Chapter 16

Recall… The spontaneous transfer of heat is always from warmer objects to cooler objects. If we have several objects at different temperatures then they will undergo a equalization of temperatures (so that they end up having a common temperature) is brought about in three ways: by conduction, convection, and radiation

Conduction

If I hold an Iron nail in a flame, what will happen to the nail? Think about what’s happening at the molecular level as well as the temperature.

Conduction The transfer of heat energy by molecular and electron collisions within a substance (especially a solid).

How well a solid object conducts heat depends on the bonding within its atomic or molecular structure. Solids have one or more “loose” outer electrons. These “loose” outer electrons conduct heat and electricity well.

Metals having the “loosest” outer electrons This allows them to carry out collisions throughout the metal, this property makes them an excellent conductor of heat and electricity.

Best Conductors… Silver Copper Aluminum Iron

Poor Conductors of Heat… Wool Wood Straw Paper Cork Styrofoam

The microscopic structure of paper: Micrograph of paper autofluorescing under ultraviolet illumination. The individual fibres in this sample are around 10 µm in diameter.

These are poor conductors because the outer electrons in the atoms of these materials are firmly attached. Poor conductors are referred to as insulators.

Most liquids and gases are poor conductors of heat.

The good insulating properties of things like wool, fur, and feathers are due to the air spaces they contain.

Ever hear the someone say “keep the cold out of your house”? A better way to put this is to say that they want to prevent the heat from escaping There is no cold that flows into a house, unless a cold wind blows into it If the home becomes colder, it is because heat flows out.

No insulator can totally prevent heat from getting through it An insulator just reduces the rate at which heat penetrates. Insulation slows heat transfer

Convection

Liquids and Gases Transmit heat mainly by convection The transfer of heat energy in a gas or liquid by means of currents in the heat fluid. The fluid moves, carrying energy with it.

Convection involves the overall motion of a fluid Convection occurs in all fluids (liquids & gases)

As the fluid is heated from below, the molecules at the bottom begin moving faster, spreading apart more, becoming less dense, and are buoyed upward. Denser, cooler fluid moves in to take the place of the now-warmed fluid at the bottom. Convection keep the fluid stirred up as it heats – warmer fluid moving away from the heat source and cooler fluid moving towards the heat source.

Weather Video Extreme Winter Weather Explained http://www.youtube.com/watch?v=cL0TEsHO5tI

The steam at the top of the pressure cooker is a dramatic example of cooling. Molecules in a region of expanding air collide more often with receding molecules than with approaching ones. As a result the air cools.

Radiation

Energy from the Sun passes through space and then through the Earth’s atmosphere & warms Earth’s surface.

Radiation The transfer of energy by means of electromagnetic waves. This includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

Frequency is the rate of vibration of a wave. High-frequency vibrations produce short waves and low-frequency vibrations produce longer waves.

Emission of Radiant Energy

All substances at any temperature above absolute zero emit radiant energy. The frequency peak of the radiant energy is directly proportional to the absolute (Kelvin) temperature of the emitter.

The surface of the Sun has a high temperature and therefore emits radiant energy at a high frequency (much of which is in visible portion of the electrometric spectrum).

The radiation emitted by Earth is in the form of infrared waves. This radiant energy emitted by Earth is called terrestrial radiation. This is caused by radioactive decay

The Sun is energized by thermonuclear fusion

March 8th, 2012 https://www.youtube.com/watch?v=SfrOQo1drbI

Absorption of Radiant Energy

Everything that emits energy must also absorb it Good emitters of radiation are also good absorbers and vice versa

What would happen if a good emitter wasn’t a good absorber?

If a good emitter were not also a good absorber black objects would remain warmer than lighter-colored objects and the two would never reach a common temperature. Objects in thermal contact, given a sufficient amount of time, reach the same temperature

A blacktop pavement and dark automobile may remain hotter than their surroundings on a hot day, but at nightfall they cool off fast. Everything eventually reaches a thermal equilibrium. A dark object that absorbs a lot of radiant energy must also emit a lot as well.

Reflection of Radiant Energy

Absorption and reflection are opposite processes. A good absorber of radiant energy reflects very little radiant energy, including visible light

Where do the colors come from?

The Sun's light contains a perfect blend of seven colors that gives the color white which makes sunlight a white light. As demonstrated by mathematician and scientist Sir Isaac Newton (1642-1727), when sunlight (white light) passes through a prism, it scatters the different colors of light according to their wavelength, showing a continuous band of colors. This band of colors appears in the same pattern as the colors of a rainbow. Taken from http://www.tellmewhyfacts.com/2006/11/where-do-objects-get-their-colors-from.html

Therefore when the sunlight strikes the surface of an object, that object based on its properties will absorb some of these colors and the color that is reflected from its surface gives it its color. In other words, the color we see of an object is actually the light (color) - coming from that object - which was not absorbed by the object. Taken from http://www.tellmewhyfacts.com/2006/11/where-do-objects-get-their-colors-from.html

A surface that reflects very little or no radiant energy looks dark. So a good absorber appears dark and a perfect absorber reflects no radiant energy and appears completely black.

An example of this would be the pupil of the eye It allows light to enter with no reflection which is why it appears black.

Openings appear black because the light that enters them is reflected back and forth on the inside walls many times and is partly absorbed at each reflection. Very little or none of the light remains to come back out of the opening and travel to your eyes

Good reflectors are poor absorbers Clean snow is a good reflector and therefore doesn’t melt rapidly in the sunlight But when the snow get dirty, it absorbs radiant energy from the Sun and melts faster.

Cooling at Night by Radiation

Bodies that radiate more energy than they receive become cooler. This occurs when solar radiation is absent

An object left out in the open at night radiates energy into space and due to the absence of any warmer bodies in the vicinity, receives very little energy from space in return. That object gives out more energy than it receives

Newton’s Law of Cooling

The rate of cooling of an object – whether by conduction, convection , or radiation – is approximately proportional to the temperature difference ΔT between the object and its surroundings Rate of Cooling ~ ΔT This also holds true for warming

The Greenhouse Effect

Earth and its atmosphere gain energy when they absorb radiant energy from the Sun. This warms the surface of the Earth, which in turn emits terrestrial radiation. Absorption and emission continue at equal rates to produce an average equilibrium temperature.

The average temperature over the last 500,000 years has fluctuated between 19°C and 27°C Earth’s temperature increases when either the radiant energy coming in increases or there is a decrease in the escape of terrestrial radiation.

The Greenhouse Effect The warming of the lower atmosphere, the effect of atmospheric gases on the balance of terrestrial and solar radiation.

Our present environmental concern is that excess carbon dioxide and other greenhouse gases will trap too much energy and make the Earth too war.

Solar Power

This input of energy is called the solar constant. The amount of radiant energy received each second over each square meter at right angles to the Sun’s rays at the top of the atmosphere is 1400 joules. Or 1.4 kilowatts per square meter This input of energy is called the solar constant.

The amount of solar power that reaches the ground is affected by the atmosphere and reduced by nonperpendicular elevation angles of the Sun

The solar power received in the US (on average) is about 13% solar constant (0.18 kilowatts per meter squared) This amount of power falling on the roof area of a typical American house is twice the power needed to comfortably heat and cool the house year-round

On a larger scale, the problems of utilizing solar power to generate electricity are greater. First of all no energy arrives at night This calls for supplemental sources of energy or efficient energy-storage devices.

Variations in weather, particularly in cloud cover, produce a variable energy supply from day to day and from season to season. Even in clear daylight hours, the Sun is high in the sky for only part of the day.

Videos… http://www.youtube.com/watch?v=--X9zfgZtS0 http://www.youtube.com/watch?v=nr-grdspEWQ