Heat and Temperature

Heat energy is most intense in substances whose molecules are moving rapidly in a very disorderly way. Such a substance will give up some of its heat to another substance whose molecules are less agitated. When this happens, the heat is said to flow from one substance to another (or from one body to another). The energy transfer is indicated by a change in temperature.

Temperature, therefore, is not the same thing as heatalthough the two words are often used interchangeably. Temperature can be defined as the degree of intensity of hotness or coldness. Hotness and coldness, however, are comparative terms. A flame, for example, is hot when compared with ice but cold when compared with the sun. This definition of temperature, therefore, is vague and unscientific, although it does convey the correct impression that temperature is a measure of relative intensity rather than of quantity.

A more specific definition is: temperature is the ability of one body to give up heat energy to another body. A hot body becomes cooler, and a cold body becomes warmer, as long as heat is flowing from one to the other. The hot body has a greater ability to give up heat and therefore has a higher temperature. After a time the two bodies may reach a condition of heat equilibrium, or balance of heat intensity. Then, heat flow ceases. At the point of equilibrium both bodies can be said to be at the same temperature.

Measurement of Temperature

Temperature is measured by means of instruments called thermometers. Several temperature scales have been devised for relating the hotness and coldness of bodies to fixed temperatures, such as the freezing point and boiling point of water. On most temperature scales, the unit of temperature is called a degree. The Kelvin scale is an exception; its unit of temperature is the kelvin.

The Fahrenheit, Celsius (or centigrade), and Reaumur scales are used in the range of temperatures important for human comfort, laboratory experiments, and industrial processes.

The Rankine scale and the Kelvin scale are based on the concept of absolute zero; all temperature readings on these scales are positive numbers. The Kelvin scale is widely used in scientific work. The Rankine scale is used primarily by British and American engineers.

Absolute Zero

Experiments have shown that every 1 C. increase or decrease in temperature causes the pressure exerted by a gas to increase or decrease at the constant rate of 1/273.15 of its pressure at 0 C. This means that at -273.15 C. an ideal (theoretical) gas would exert no pressure at all. Since experiments with real gases have shown a clear relation between pressure and temperature, zero pressure would indicate that the ideal gas had lost all its ability to give up heat. Its molecules would be absolutely motionless. This is impossiblemolecules are always agitated, to some extentand therefore the absolute zero of temperature remains a theoretical concept. The concept is, however, a useful one, for it gives a base point to which all temperature measurements may be referred, in positive numbers.

The idea that absolute zero can never be reached is sometimes considered important enough to be called the third law of thermodynamics. Scientists have succeeded in cooling substances to within a small fraction of a degree above absolute zero. The study of the behavior of substances at very low temperatures is called cryogenics.

High Temperatures

Absolute zero is the lower limit for temperature, but there is no upper limit. The hottest substances known are ionized gases in certain stars, with temperatures of a billion degrees or more.

Measurement of Heat

The heat released or absorbed in a physical or chemical process can be measured with an instrument called a calorimeter. Commonly used units for measuring heat are the calorie and the British thermal unit, or Btu. Heat is also measured in such other units as the joule (the unit of energy in the SI, or metric system).

The existence ofelectricity, the phenomenon associated with stationary or moving electric charges, has been known since the Greeks discovered that amber, rubbed with fur, attracted light objects such as feathers. Ben Franklin proved the electrical nature of lightning (the famous key experiment) and also established the conventional use of negative and positive types of charges.

It was also known that certain materials blocked electric charge, called insulators, such as glass or cork. Other materials transfered electric charge with ease, called conductors, such as metal.

By the 18th century, physicist Charles Coulomb defined the quantity of electricity later known as a coulomb, and determined the force law between electric charges, known as Coulomb's law. Coulomb's law is similar to the law of gravity in that the electrical force is inversely proportional to the distance of the charges squared, and proportional to the product of the charges.

By the end of the 18th century, we had determined that electric charge could be stored in a conducting body if it is insulated from its surroundings. The first of these devices was the Leyden jar. consisted of a glass vial, partly filled with sheets of metal foil, the top of which was closed by a cork pierced with a wire or nail. To charge the jar, the exposed end of the wire is brought in contact with a friction device.

Modern atomic theory explains this as the ability of atoms to either lose or gain an outer electron and thus exhibit a net positive charge or gain a net negative charge (since the electron is negative). Today we know that the basic quantity of electric charge is the electron, and one coulomb is about 6.24x1018electrons.

The battery was invented in the 19th century, and electric current and static electricity were shown to be manifestations of the same phenomenon, i.e. current is the motion of electric charge. Once a laboratory curiosity, electricity becomes the focus of industrial concerns when it is shown that electrical power can be transmitted efficiently from place to place and with the invention of the incandescent lamp.

The discovery of Coulomb's law, and the behavior or motion of charged particles near other charged particles led to the development of the electric field concept.

A field can be considered a type of energy in space, or energy with position. A field is usually visualized as a set of lines surrounding the body, however these lines do not exist, they are strictly a mathematical construct to describe motion. Fields are used in electricity, magnetism, gravity and almost all aspects of modern physics.

An electric field is the region around an electric charge in which an electric force is exerted on another charge. Instead of considering the electric force as a direct interaction of two electric charges at a distance from each other, one charge is considered the source of an electric field that extends outward into the surrounding space, and the force exerted on a second charge in this space is considered as a direct interaction between the electric field and the second charge.

Magnetism is the phenomenon associated with the motion of electric charges, although the study of magnets was very confused before the 19th century because of the existence of ferromagnets, substances such as iron bar magnets which maintain a magnetic field where no obvious electric current is present (see below). Basic magnetism is the existence of magnetic fields which deflect moving charges or other magnets. Similar to electric force in strength and direction, magnetic objects are said to have `poles' (north and south, instead of positive and negative charge). However, magnetic objects are always found in pairs, there do not exist isolated poles in Nature.

Although conceived of as distinct phenomena until the 19th century,electricityandmagnetismare now known to be components of the unified theory ofelectromagnetism.

A connection between electricity and magnetism had long been suspected, and in 1820 the Danish physicist Hans Christian Orsted showed that an electric current flowing in a wire produces its own magnetic field. Andre-Marie Ampere of France immediately repeated Orsted's experiments and within weeks was able to express the magnetic forces between current-carrying conductors in a simple and elegant mathematical form. He also demonstrated that a current flowing in a loop of wire produces a magnetic dipole indistinguishable at a distance from that produced by a small permanent magnet; this led Ampere to suggest that magnetism is caused by currents circulating on a molecular scale, an idea remarkably near the modern understanding.

Faraday, in the early 1800's, showed that a changing electric field produces a magnetic field, and that vice-versus, a changing magnetic field produces an electric current. An electromagnet is an iron core which enhances the magnetic field generated by a current flowing through a coil, was invented by William Sturgeon in England during the mid-1820s. It later became a vital component of both motors and generators.

The unification of electric and magnetic phenomena in a complete mathematical theory was the achievement of the Scottish physicistMaxwell(1850's). In a set of four elegant equations, Maxwell formalized the relationship between electric and magnetic fields. In addition, he showed that a linear magnetic and electric field can be self-reinforcing and must move at a particular velocity, the speed of light. Thus, he concluded that light is energy carried in the form of opposite but supporting electric and magnetic fields in the shape of waves, i.e. self-propagating electromagnetic waves.