Category Chemistry

As substances get hotter, their molecules move around more rapidly and they may take up more space. A thermometer contains a liquid that expands as it gains heat energy. This causes the level of the liquid to rise in a narrow tube. A scale beside the tube allows the temperature to be read.

The simplest thermometers really are simple! They’re just very thin glass tubes filled with a small amount of silvery liquid (typically mercury—a rather special metal that’s a liquid at ordinary, everyday temperatures). When mercury gets hotter, it expands (increases in size) by an amount that’s directly related to the temperature. So if the temperature increases by 20 degrees, the mercury expands and moves up the scale by twice as much as if the temperature increase is only 10 degrees. All we have to do is mark a scale on the glass and we can easily figure out the temperature.

How do we figure out the scale? Making a Celsius (centigrade) thermometer is easy, because it’s based on the temperatures of ice and boiling water. These are called the two fixed points. We know ice has a temperature close to 0°C while water boils at 100°C. If we dip our thermometer in some ice, we can observe where the mercury level comes to and mark the lowest point on our scale, which will be roughly 0°C. Similarly, if we dip the thermometer in boiling water, we can wait for the mercury to rise up and then make a mark equivalent to 100°C. All we have to do then is divide the scale between these two fixed points into 100 equal steps (“centigrade” means 100 divisions) and, hey presto, we have a working thermometer!

Not all liquid thermometers use mercury. If the line you see in your thermometer is red instead of silver, your thermometer is filled with an alcohol-based liquid (such as ethanol). What’s the difference? Mercury is toxic, although perfectly safe if it’s sealed inside a thermometer. However, if the glass tube of a mercury thermometer happens to break, that potentially exposes you to the poisonous liquid inside it. Alcohol thermometers are generally safer for that reason and they can also be used to measure lower temperatures (because alcohol has a lower freezing point than mercury; it’s about ?114°C or ?170°F for pure ethanol compared to about ?40°C or ?40°F for mercury).

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Heat energy, like other forms of energy, is measured in joules (J). Temperature is measured in degrees Fahrenheit (°F), Celsius (°C) or Kelvin (K). In Fahrenheit, water freezes at 32° and boils at 212°. The Celsius scale is based on the boiling and freezing points of water, so these are 100°C and 0°C respectively. Kelvin units are the same as Celsius degrees but they start from the lowest temperature possible. On this scale, water freezes at 273K.

We have all felt various levels of heat. Our skin is a good detector of heat and we interpret the average molecular motion within an object as a feeling that the object is hot or cold. However our skin does not always give us consistent measurements of heat energy.

For this we need special instruments which can accurately measure temperature, like a thermometer. Thermometers, and other temperature measuring devices, are used to get a quantitative measure of the average motion of the molecules in a substance. They interpret this average molecular motion as a certain number of degrees which we call the temperature.

We have all used thermometers to measure the level of heat but sometimes we need to measure heat in places where you can’t put a thermometer. For example, in space, in molten metals and in hot fires. To make measurements in these situations we need instruments which can measure heat without touching the heat source. These instruments measure the heat radiation emitted by the heat source. Examples of these types of devices are infrared cameras and detectors.

Heat is measured in quantities called joules (pronounced the same as jewels) in the metric system and in British Thermal Units (BTU) in the English system. Heat can also be measured in calories.

Joule’s experiment was ground breaking because he demonstrated that we can heat water without using fire. He put water in a glass with a thermometer to monitor the increase in heat. Then he added a paddle system and turned it vigorously. After a while he realized that the water temperature had increased. Infect he repeated this experiment many times with different systems and always reached the conclusion that 4.19 Joule of work was required to raise the temperature of 1 gram of water by 1 degree Celsius.

A BTU is the amount of heat needed to raise temperature of one pound of water by one degree Fahrenheit.

1 BTU = 1,000 joules

A calorie is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.

1 calorie (cal) = 4.186 Joules

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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.