Conduction is the most significant means of heat transfer within a solid or between solid objects in thermal contact. Conduction is greater in solids because the network of relatively close fixed spatial relationships between atoms helps to transfer energy between them by vibration. Fluids and gases are less conductive than solids. This is due to the large distance between atoms in a fluid or especially a gas: fewer collisions between atoms means less conduction.
Microscopic Illustration of Conduction : The molecules in two bodies at different temperatures have different average kinetic energies.
Collisions occurring at the contact surface tend to transfer energy from high-temperature regions to low-temperature regions. In this illustration, a molecule in the lower temperature region right side has low energy before collision, but its energy increases after colliding with the contact surface. In contrast, a molecule in the higher temperature region left side has high energy before collision, but its energy decreases after colliding with the contact surface.
The average kinetic energy of a molecule in the hot body is higher than in the colder body. If two molecules collide, an energy transfer from the hot to the cold molecule occurs see the above figure.
The cumulative effect from all collisions results in a net flux of heat from the hot body to the colder body. Therefore, you will get a more severe burn from boiling water than from hot tap water. Conversely, if the temperatures are the same, the net heat transfer rate falls to zero, and equilibrium is achieved.
Owing to the fact that the number of collisions increases with increasing area, heat conduction depends on the cross-sectional area. If you touch a cold wall with your palm, your hand cools faster than if you just touch it with your fingertip.
In addition to temperature and cross-sectional area, another factor affecting conduction is the thickness of the material through which the heat transfers. Heat transfer from the left side to the right side is accomplished by a series of molecular collisions. The thicker the material, the more time it takes to transfer the same amount of heat.
If you get cold during the night, you may retrieve a thicker blanket to keep warm. Effect of Thickness on Heat Conduction : Heat conduction occurs through any material, represented here by a rectangular bar. Lastly, the heat transfer rate depends on the material properties described by the coefficient of thermal conductivity.
All four factors are included in a simple equation that was deduced from and is confirmed by experiments. A fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current. Most houses are not airtight: air goes in and out around doors and windows, through cracks and crevices, following wiring to switches and outlets, and so on.
The air in a typical house is completely replaced in less than an hour. Suppose that a moderately-sized house has inside dimensions Calculate the heat transfer per unit time in watts needed to warm the incoming cold air by Newly constructed homes are designed for a turnover time of 2 hours or more, rather than 30 minutes for the house of this example.
Weather stripping, caulking, and improved window seals are commonly employed. More extreme measures are sometimes taken in very cold or hot climates to achieve a tight standard of more than 6 hours for one air turnover.
Still longer turnover times are unhealthy, because a minimum amount of fresh air is necessary to supply oxygen for breathing and to dilute household pollutants. Convection illustrated in is the concerted, collective movement of ensembles of molecules within fluids e.
Convection of mass cannot take place in solids, since neither bulk current flows nor significant diffusion can occur in solids. Convection is driven by large-scale flow of matter. In the case of Earth, the atmospheric circulation is caused by the flow of hot air from the tropics to the poles, and the flow of cold air from the poles toward the tropics.
An example of convection is a car engine kept cool by the flow of water in the cooling system, with the water pump maintaining a flow of cool water to the pistons. While convection is usually more complicated than conduction, we can describe convection and perform some straightforward, realistic calculations of its effects. Natural convection is driven by buoyant forces: hot air rises because density decreases as temperature increases. This principle applies equally with any fluid. For example, the pot of water on the stove in is kept warm in this manner; ocean currents and large-scale atmospheric circulation transfer energy from one part of the globe to another.
Convection in a Pot of Water : Convection plays an important role in heat transfer inside this pot of water. Once conducted to the inside, heat transfer to other parts of the pot is mostly by convection. The hotter water expands, decreases in density, and rises to transfer heat to other regions of the water, while colder water sinks to the bottom. This process keeps repeating. Although air can transfer heat rapidly by convection, it is a poor conductor and thus a good insulator.
The amount of available space for airflow determines whether air acts as an insulator or conductor. The space between the inside and outside walls of a house, for example, is about 9 cm 3. Equally as interesting as the effects of heat transfer on a system are the methods by which this occurs. Whenever there is a temperature difference, heat transfer occurs. Heat transfer may occur rapidly, such as through a cooking pan, or slowly, such as through the walls of a picnic ice chest.
We can control rates of heat transfer by choosing materials such as thick wool clothing for the winter , controlling air movement such as the use of weather stripping around doors , or by choice of color such as a white roof to reflect summer sunlight.
So many processes involve heat transfer, so that it is hard to imagine a situation where no heat transfer occurs. Yet every process involving heat transfer takes place by only three methods:. The more kinetic energy a material has, the higher its internal temperature will be. Figure 1: Mechanisms of heat transfer diagram. Matter with high kinetic energy will also have a high thermal conductivity.
Thermal conductivity describes how efficiently a material can pass heat through it. It is defined by the rate of energy flow per unit area when compared to a temperature gradient. Thermal conductivity explains why walking bare foot on a cold tile floor feels much cooler than walking on carpet even though both are at room temperature.
Tile and rocks have a higher thermal conductivity than carpet and fabrics so they can transfer heat away from a foot at a much quicker rate making the tile appear cool to the touch. Metals are an example of a material with a high thermal conductivity that can quickly transfer heat. The internal structure of a metal molecule contains free electrons that can move freely through the bulk of the material.
These free electrons collide rapidly with other particles causing the internal structure of a metal to vibrate faster and heat up quicker. These rapid vibrations promote energy and heat flow throughout the metal. Metals such as copper, aluminum and silver are frequently used to make thermal appliances and tools. Copper pipes are wires are extremely popular to use within a home to transfer energy and heat quickly from one area to another.
Aluminum has extremely similar thermal properties to copper and is often used as a cost-effective replacement to perform the same functions. Silver is one of the most widely used metals for thermal applications. The demand for silver continues to grow as it is becoming a crucial component in the production of solar panels. Other highly thermally conductive materials such as diamonds also have many practical applications.
Diamond powder is often used in electronics to transfer heat away from sensitive areas to protect them from overheating. Figure 2: Standard solar panels that are frequently manufacture with silver.
Non-metal materials rely on phonons to transfer heat along a gradient from a cold area to a warm area. Plastics, foams and wood are all examples of materials with poor thermal conductivity values.
These materials are known as insulators and can restrict the flow of heat. Insulators have numerous extremely useful applications that can protect energy from being lost to the environment. Foam is an extremely useful home and building insulation material.
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