Category Science

Most things get bigger when they heat up and smaller when they cool down (water is a notable exception: it expands when it heats up and when it freezes too). Mechanical thermostats use this idea (which is called thermal expansion) to switch an electric circuit on and off. The two most common types use bimetallic strips and gas-filled bellows.

A thermostat has two pieces of different metals bolted together to form what’s called a bimetallic strip (or bimetal strip). The strip works as a bridge in an electrical circuit connected to your heating system. Normally the “bridge is down”, the strip carries electricity through the circuit, and the heating is on. When the strip gets hot, one of the metals expands more than the other so the whole strip bends very slightly. Eventually, it bends so much that it breaks open the circuit. The “bridge is up”, the electricity instantly switches off, the heating cuts out, and the room starts to cool.

But then what happens? As the room cools, the strip cools too and bends back to its original shape. Sooner or later, it snaps back into the circuit and makes the electricity flow again, so the heating switches back on. By adjusting the temperature dial, you change the temperature at which the circuit switches on and off. Because it takes some time for the metal strip to expand and contract, the heating isn’t constantly switching on and off every few seconds, which would be pointless (and quite irritating); depending on how well-insulated your home is, and how cold it is outside, it might take an hour or more for the thermostat to switch back on once it’s switched off.

Radiation is a means of transferring heat that does not cause particles to vibrate. Instead, it travels in waves, called infra-red rays. Infra-red radiation has a longer wavelength than light but travels at the same speed. Unlike other methods of heat transfer, radiation can work in a vacuum.

Radiation, process by which energy, in the form of electromagnetic radiation, is emitted by a heated surface in all directions and travels directly to its point of absorption at the speed of light, radiation does not require an intervening medium to carry it.

Thermal radiation ranges in wavelength from the longest infrared rays through the visible-light spectrum to the shortest ultraviolet rays. The intensity and distribution of radiant energy within this range is governed by the temperature of the emitting surface. The total radiant heat energy emitted by a surface is proportional to the fourth power of its absolute temperature (the Stefan-Boltzmann law).

The rate at which a body radiates (or absorbs) thermal radiation depends upon the nature of the surface as well. Objects that are good emitters are also good absorbers (Kirchhoff’s radiation law). A blackened surface is an excellent emitter as well as an excellent absorber. If the same surface is silvered, it becomes a poor emitter and a poor absorber. A blackbody is one that absorbs all the radiant energy that falls on it. Such a perfect absorber would also be a perfect emitter.

The heating of the Earth by the Sun is an example of transfer of energy by radiation. The heating of a room by an open-hearth fireplace is another example. The flames, coals, and hot bricks radiate heat directly to the objects in the room with little of this heat being absorbed by the intervening air. Most of the air that is drawn from the room and heated in the fireplace does not reenter the room in a current of convection but is carried up the chimney together with the products of combustion.

In solids, heated particles also begin to vibrate in their positions, but while the substance remains solid, they cannot move upwards. Instead, one moving particle bumps into the next and so transfers some energy. This continues until the heat energy is transferred throughout the solid. If enough heat energy is transferred to the solid, the particles move so rapidly that they break free from each other and the substance melts to become a liquid.

Conduction is one of the three main ways that heat energy moves from place to place. The other two ways heat moves around are radiation and convection. Conduction is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules. Conduction occurs more readily in solids and liquids, where the particles are closer to together, than in gases, where particles are further apart. The rate of energy transfer by conduction is higher when there is a large temperature difference between the substances that are in contact.

Think of a frying pan set over an open camp stove. The fire’s heat causes molecules in the pan to vibrate faster, making it hotter. These vibrating molecules collide with their neighboring molecules, making them also vibrate faster. As these molecules collide, thermal energy is transferred via conduction to the rest of the pan. If you’ve ever touched the metal handle of a hot pan without a potholder, you have first-hand experience with heat conduction!

Some solids, such as metals, are good heat conductors. Not surprisingly, many pots and pans have insulated handles. Air (a mixture of gases) and water are poor conductors of thermal energy. They are called insulators.

Conduction, radiation and convection all play a role in moving heat between Earth’s surface and the atmosphere. Since air is a poor conductor, most energy transfer by conduction occurs right near Earth’s surface. Conduction directly affects air temperature only a few centimeters into the atmosphere.

During the day, sunlight heats the ground, which in turn heats the air directly above it via conduction. At night, the ground cools and the heat flows from the warmer air directly above to the cooler ground via conduction. On clear, sunny days with little or no wind, air temperature can be much higher right near the ground that just a short way above. Although sunlight warms the surface, heat flow from the surface to the air above is limited by the poor conductivity of air. A series of thermometers mounted at different heights above the ground would reveal that air temperature falls off rapidly with height.

Heat energy is always on the move. It flows from a hotter object towards a cooler one until both are the same temperature. In liquids and gases, heat energy usually moves by convection. This means that the molecules nearest to the heat source begin to move more rapidly and spread apart, so that this area of the fluid is less dense. As the less dense part of the liquid or gas rises, denser parts sink to take its place. Convection, process by which heat is transferred by movement of a heated fluid such as air or water.

Natural convection results from the tendency of most fluids to expand when heated—i.e., to become less dense and to rise as a result of the increased buoyancy. Circulation caused by this effect accounts for the uniform heating of water in a kettle or air in a heated room: the heated molecules expand the space they move in through increased speed against one another, rise, and then cool and come closer together again, with increase in density and a resultant sinking.

Forced convection involves the transport of fluid by methods other than that resulting from variation of density with temperature. Movement of air by a fan or of water by a pump is examples of forced convection.

Atmospheric convection currents can be set up by local heating effects such as solar radiation (heating and rising) or contact with cold surface masses (cooling and sinking). Such convection currents primarily move vertically and account for many atmospheric phenomena, such as clouds and thunderstorms.

An orchestra has instruments that produce sounds in different ways, but all cause air to vibrate to carry the sound to listening ears. String instruments have vibrating strings that are bowed or plucked. Wind instruments cause a column of air to vibrate when the player blows into them. Instruments that create sounds by being struck or shaken are percussion instruments. Brass instruments resonate when air is blown into them.

Welcome to the world of classical music instruments! Musical instruments are grouped into different families based on the way the instrument makes its sound. There are four main families of instruments: strings, woodwinds, brass, and percussion. Here is how an orchestra is often set up:

The Strings

The four most commonly used instruments in the string family are the violin, the viola, the cello and the double (string) bass. They are all made by gluing pieces of wood together to form a hollow “sound box.” The quality of sound of one of these instruments depends on its shape, the wood it is made from, the thickness of both the top and back, and the varnish that coats its outside surface.

Four strings made of gut, synthetics, or steel are wrapped around pegs at one end of the instrument, tightly stretched across a “bridge,” and attached to a tailpiece at the other end. The pegs are used to tune the instrument (change the length of the string until it makes exactly the right sound). The strings are tuned in perfect “fifths” from each other – 5 notes apart.

The player makes the strings vibrate by plucking them, striking them, strumming them, or, most frequently, by drawing a bow across them. The bow is made of wood and horsehair. The instrument sounds different notes when the performer presses a finger down on the strings on the instrument’s neck, changing the length of the portion of the string that vibrates. The shorter the vibrating part of the string, the higher the sound produced.

The Woodwinds

Instruments in the woodwind family used to all be made of wood, hence the name, but now they can be made of wood, metal, plastic or some combination of materials. They are all tubes with an opening at one end and a mouthpiece at the other end. They each have rows of holes that are covered by metal caps called keys. Pressing on different keys produces different musical notes – the sound changes depending on where the air leaves the instrument (through one of the key holes or out the far end). There are three ways in which the woodwind family creates sound: by blowing air across the edge of or into the mouthpiece (flute or piccolo), by blowing air between a single reed and a fixed surface (clarinet and bass clarinet), or by blowing air between two reeds (oboe, English horn, bassoon, and contrabassoon).

The Brass

Brass instruments are essentially very long pipes that widen at their ends into a bell-like shape. The pipes have been curved and twisted into different shapes to make them easier to hold and play. Instruments in the brass family produce their sound when the player “buzzes” her or his lips while blowing air through the mouthpiece, kind of like making a “raspberry,” creating a vibrating column of air within the instrument. Most brass instruments have valves attached to their long pipes. When the player presses down on the valves, they open and close different parts of the pipe, increasing the length of the pipe when played and creating a lower sound. In addition to the valves, the player can select the pitch from a range of overtones or harmonics by changing his or her lip aperture and tension (known as the embouchure). The mouthpiece can also make a big difference in tone. Brass musicians can also insert mutes into the bell of their instrument to change the timbre of its sound.

The Percussion Family

The percussion section provides a variety of rhythms, textures and tone colors to orchestral music. Instruments in the percussion family make sound in one of three ways, by striking, shaking, or scraping. Percussion instruments can also be tuned or unturned. Tuned instruments play specific pitches or notes, just like the woodwind, brass and string instruments. Unturned instruments produce a sound with no definite pitch, like the sound of hitting two pieces of wood or metal together. Percussion instruments are an international family, representing musical styles from many different cultures. There are numerous kinds of percussion instruments, such as rattles, castanets, or tambourines. Keyboard instruments are a special class of percussion instrument.

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When we describe an object as hot, we are really comparing it with something else. The word does not mean anything by itself. We can say that we feel hot after exercise, that a cup of coffee is hot and that the surface of the Sun is hot, but we mean something quite different each time. An object can be hot compared with an ice cube but cold compared with boiling water. In fact, both the ice cube and the boiling water contain heat energy, but their temperatures are quite different. Temperature is a measure of how hot something is compared with an agreed scale. A small object with a temperature of 100°C may not have as much heat energy as a very large object with a temperature of 0°C.

I have often noted that when patients are running a fever with an infection, their face may be flushed and hot to the touch while their hands and feet may be cold and clammy. I think that the flushed face sensation causes one to feel hot when ill. And I think the cold hands and feet cause one to feel cold when ill. These can happen at the same time, and is truly miserable. Even though the body has a fever, the extremities (hands and feet) can be cool because of substances like adrenaline being pumped into circulation by the body in response to physiological insult (infection).

When running a fever, a person will often start getting dehydrated because of a lot of water loss (sweat, faster breathing). Adrenaline will increase a person’s blood pressure and heart rate. This is usually experienced as a flushed face. One of adrenaline’s functions is to constrict the blood vessels so as to raise the blood pressure. If the body is running dehydrated, this vessel constriction in the smaller vessels (the like in the hands and feet) may prevent good blood circulation in those parts of the body. Thus, the extremities can get feel cool to the touch because there is actually decreased blood flow there.

Picture Credit : Google