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. The addition of wall insulation prevents airflow, so heat loss or gain is decreased. Fur, fiber and fiberglass also take advantage of the low conductivity of air by trapping it in spaces too small to support convection.
In animals, fur and feathers are lightweight and thus ideal for their protection. Some interesting phenomena happen when convection is accompanied by a phase change. It allows us to cool off by sweating, even if the temperature of the surrounding air exceeds body temperature. Heat from the skin is required in order for sweat to evaporate from the skin, but without air flow the air becomes saturated and evaporation stops.
Air flow caused by convection replaces the saturated air by dry air and thus evaporation continues. Another important example of the combination of phase change and convection occurs when water evaporates from the ocean. Heat is removed from the ocean when water evaporates. If the water vapor condenses in liquid droplets as clouds form, heat is released in the atmosphere this heat release is latent heat.
Thus, an overall transfer of heat from the ocean to the atmosphere occurs. This process is the driving power behind thunderheads—great cumulus clouds that rise as much as Water vapor carried in by convection condenses, releasing tremendous amounts of energy, and this energy allows air to become more buoyant warmer than its surroundings and rise.
As the air continues to rise, more condensation occurs, which in turn drives the cloud even higher. Such a mechanism is called positive feedback, since the process reinforces and accelerates itself. These systems sometimes produce violent storms with lightning and hail, and constitute the mechanism that drives hurricanes. Cumulus Clouds : Cumulus clouds are caused by water vapor that rises because of convection.
The rise of clouds is driven by a positive feedback mechanism. You can feel heat transfer from a fire or the Sun. Yet the space between Earth and the Sun is largely empty, without any possibility of heat transfer by convection or conduction. Similarly, you can tell that an oven is hot without touching it or looking inside—it just warms you as you walk by. In these examples, heat is transferred by radiation. The hot body emits electromagnetic waves that are absorbed by our skin, and no medium is required for them to propagate.
We use different names for electromagnetic waves of different wavelengths: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Radiation from a Fire : Most of the heat transfer from this fire to the observers is through infrared radiation. The visible light, although dramatic, transfers relatively little thermal energy. Convection transfers energy away from the observers as hot air rises, while conduction is negligibly slow here.
Skin is very sensitive to infrared radiation so that you can sense the presence of a fire without looking at it directly. The energy of electromagnetic radiation depends on its wavelength color and varies over a wide range; a smaller wavelength or higher frequency corresponds to a higher energy.
We can write this as:. Heat will flow from a hotter object to a cooler object. When you heat up water, the water molecules start moving around faster and faster. They bounce off each other and move farther apart. When you put the two together with the hot water on the bottom, the hot water rises to the top, mixing with the cold water along the way and creating purple water. When a warm object comes into contact with a cold object, the faster-moving particles of the warm object bump into the slower- moving particles of the cold object.
As a result, energy is transferred. This causes particles of the cold object to speed up and the particles of the warm object to slow down.
Research suggests that the temperature of the water that people drink can affect levels of sweating and rehydration. There is nothing physical or material moving from the hot water to the cold water; only energy is transferred from the hot water to the cold water. Other than the loss of energy, there is nothing else escaping from the hot water and other than a gain of energy, there is nothing entering the cold water.
Understanding planetary habitability is partly an extrapolation of the conditions on Earth, as this is the only planet known to support life. Begin typing your search term above and press enter to search.
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