Temperature and Heat

Background

Thermal Imaging

Hot objects give out rays of energy called infra-red radiation. These form one part of the electromagnetic spectrum, which includes visible light. The wavelength of the infra-red radiation depends on the body that emits it: the hotter the object, the shorter the wavelength. Thermal imaging uses a detector which can identify the wavelength of infra-red radiation. This information is sent to a computer which produces an artificially coloured image on a screen. ‘Forward Looking Infra-Red’ detects radiation in front of a police helicopter to track suspects. The technique can be used with a satellite to survey crops, helping farmers to identify the better areas of their fields. From a helicopter it is possible to survey the heat being lost from a building in order to make decisions about insulation. Thermal imaging is increasingly used in medicine. Changes in the distribution of blood help doctors to diagnose breaks, tumours and varicose veins. Simple infra-red detectors pick up the remote-control signal sent to a television, and detect the heat from a burglar’s body to trigger an alarm.

Sensing Temperature

The skin has a number of sensory cells including temperature sensors. These sensors send information to the hypothalamus in the brain. The hypothalamus controls a number of mechanisms which keep our bodies within a comfortable temperature range. If we get too hot, we sweat; if we get too cold, we shiver. However, the information that skin sensors pass on is not an absolute measure of temperature. The sensors at the skin surface compare the temperature of anything we touch with the temperature below the surface of the skin. This makes us feel cool or warm depending on our internal temperature. Our ability to sense temperature can be affected by our health, our clothing and the weather.

Inventing a Thermometer

The first thermometers relied on the expansion and contraction of gases. The larger the volume of gas used, the greater the expansion and the more accurate the measurement. Big thermometers were more accurate, but not very convenient for a doctor visiting a patient. A good liquid thermometer can be smaller because some liquids expand a great deal for a small increase in temperature. The expansion has to be regular so that the scale can be regular. Alcohol has replaced mercury in school thermometers because mercury is poisonous.

Fahrenheit

Early thermometers, or thermoscopes, had a range of designs and many different scales. Santorio Santorio (1561–1636) was a Croatian doctor who may have been the first to give his thermoscope a scale. In 1724 Daniel Gabriel Fahrenheit, a German physicist, invented the temperature scale that bears his name. He began with two fixed points: the temperature of the human body and that of a mixture of ice, water and salt. His scale had 96 degrees. Fahrenheit had already invented a spirit thermometer and the first mercury thermometer in 1714.

Celsius and Kelvin

In 1742 the Swedish astronomer Andreas Celsius produced a temperature scale, now widely used, also by fixing two points, the melting point of ice and the boiling point of water. Celsius made the boiling point zero and the melting point of ice 100. It was his thermometer maker, Daniel Ekstrom, who turned the scale around. In order to make his measurements reliable, Celsius repeated them in various places, including Paris, and wrote down a detailed method for setting the scale. This method stated the pressure at which the scale had to be marked, because Celsius knew that the boiling point varied with pressure.

The Kelvin scale is the standard scale of temperature used by physicists. Zero on this scale is the point where molecules have no kinetic (movement) energy (approximately –273^oC), and temperature differences are then measured in degrees Celsius. The boiling point of water is 373 K; the flame of a Bunsen burner is about 1,200 K; and the outer part of the sun is about 5,600 K.

Different thermometers are needed to measure different ranges of temperature. Some thermometers measure a broad range of temperatures, others a narrow one. Modern electronic thermometers rely on thermistors. These devices change their electrical resistance according to the temperature. They allow very accurate temperature measurements over a wide range.

Temperature and Heat

The particles that make up all states of matter move. Temperature is a measure of the average kinetic energy of the particles of a substance. The amount of heat in an object is the total kinetic energy of all the particles. This distinction has some interesting consequences: an iceberg will have more heat energy than a cup of tea despite having a lower temperature.

The kinetic energy of particles can be passed from one particle to another, just like snooker balls colliding. When two objects touch, the one with the higher temperature will lose heat to the one with the lower temperature. The transfer of heat will cease when both objects have the same temperature. A ‘cold box’ uses this idea. Cold packs are frozen so that they will absorb heat from the food and drink in the box. Eventually both the food and the cold packs will reach the same temperature, which will be cooler than the original temperature of the food.