Physics of Heat

The Physics of Heat – Welding Resource Guide

Heat Defined
Heat is a form of energy that is transferred by kinetic energy from particles in a substance. A substance can be solid, liquid, or gaseous. According to the kinetic theory, heat is transferred when particles in a substance bounce into one another. Heat is essentially a form of energy, and it is not the amount of energy that is contained in a body. This is the subtle difference between heat and temperature.

How Heat is Created
From a scientific point of view, heat cannot be created. It can only be transferred. Nonetheless, friction between two bodies can generate heat, since friction will draw out the kinetic energy from the particles in a body. This friction will result in the transfer of heat energy. When an object is heated, thermal energy gets transferred from the source of heat to the object that is being heated.

Heat Transfer
Heat will always move from a body with higher temperature to one with lower temperature, and it progresses towards thermal equilibrium. Two objects achieve thermal equilibrium when they reach the same temperature after they are heated or cooled. For example, when a hot piece of metal is placed in a bucket of cold water, it will become cooler and the cold water will get slightly warmer after some time. Eventually, they will have the same temperature, and this same temperature state is called thermal equilibrium.

Heat transfer between objects usually takes place by conduction, convection, or radiation. Conduction is the transfer of heat between two bodies by direct contact or through molecular interaction, whereas convection is the transfer of heat in liquids and gases, which takes place through molecular motion. Radiation transfers heat energy by way of waves or rays.

Conduction will take place when two objects come in close contact with each other. Heat gets transferred from the warmer object to the cooler object by means of molecular collision. The warmer object has faster-moving molecules that will excite the relatively slower-moving molecules of the cooler object. This will in turn cause the excited molecules in the cooler object to gain more thermal energy, and the object will become warmer. Solids are generally better conductors of heat than liquids and gases, with metals being the best conductors.

Convection refers to the transfer of heat through molecular motion. This is the most efficient heat transfer method in liquids and gases. The warmer areas of a fluid flow to the cooler areas. In nature, the work of convection is evidenced in the movements of land and sea breezes or ocean currents.

Conduction and convection require matter in the form of solid, liquid, or gas to transfer heat energy. On the other hand, radiation is a method of thermal transfer that uses waves or rays to transfer heat. One obvious example of heat transfer by radiation is the heat of the sun, which travels millions of miles through empty space to reach the Earth. This type of radiation is called thermal radiation. There is another form of radiation called electromagnetic radiation, wherein electrons in atoms possessing high energy are drawn to low energy levels. This loss of energy is emitted as electromagnetic radiation.

One of the heat experiments that can be easily carried out is the rubbing of hands. When a person rubs his or her hands together, the palms and fingers start to get warmer. The rubbing of hands causes friction, and since friction generates heat, the hands will become warmer.

Another popular heat experiment is the evaporation of salt water. Fill up a large bowl with water and add lots of salt to the water. When the water is stirred, the salt will dissolve. An empty glass is then placed in the middle of the bowl, with its tip above the water level. Next, the bowl is covered completely with saran wrap. Leave a gap between the tip of the glass and the saran wrap. The bowl is then placed under sunlight for a few days. When the saran wrap is removed, salt-free water will accumulate in the glass, and the water level in the bowl will become lower. Heat from the sun causes the water to evaporate, resulting in the formation of salt-free water on the saran wrap. The water then drips from the saran wrap to the bottom of the glass.

Further Resources
Heat has a number of units of measurements. One commonly used unit is the SI Unit for heat, which is called the Joule or (J). Heat is also measured as calories or (Cal). A calorie is defined as the amount of heat that is required to raise the temperature of one gram of water by one degree Celsius. Heat is sometimes measured in British Thermal Units or Btu as well.