Why conduction happens

As the distance from the flame increases the temperature decreases by a proportional amount. Heat Thermal Energy and Heat Transfer Conduction Conduction is the transfer of energy from one atom or molecule to another atom or molecule.

Poor conductors or insulators do not possess free moving electrons. The animation below shows heat transfer in a metal by conduction:. Next: Convection. Metal is a good conduction of heat. Conduction occurs when a substance is heated, particles will gain more energy, and vibrate more. These molecules then bump into nearby particles and transfer some of their energy to them.

This then continues and passes the energy from the hot end down to the colder end of the substance. Thermal energy is transferred from hot places to cold places by convection. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. Cooler liquid or gas then takes the place of the warmer areas which have risen higher.

This results in a continous circulation pattern. Water boiling in a pan is a good example of these convection currents.

Another good example of convection is in the atmosphere. The earth's surface is warmed by the sun, the warm air rises and cool air moves in. Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object as is the case with conduction and convection. Heat can be transmitted through empty space by thermal radiation often called infrared radiation.

This is a type electromagnetic radiation. No mass is exchanged and no medium is required in the process of radiation.

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.