Convection

Convection is the transfer of thermal energy by means of currents in a fluid (liquids or gases). Suppose we heat up a local region of air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. Being less dense than the surrounding cooler air, the hot air will subsequently rise due to buoyant forces - this movement of hot air into a cooler region is then said to transfer heat by convection.

The transfer of heat by convection can be seen clearly by a simple example like heating a pot of water over a stove. When the stove is first turned on heat is transferred first by conduction between the element through the bottom of the pot to the water. However, eventually the water starts bubbling - these bubbles are actually local regions of hot water rising to the surface, thereby transferring heat from the hot water at the bottom to the cooler water at the top by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated. These convection currents are illustrated in the following figure.

Consider now two regions separated by a barrier, one at a higher pressure relative to the other, and subsequently remove the barrier, as in the following figure. These convection currents are illustrated in the following figure.

When the barrier is removed, material in the high pressure (high density) area will flow to the low pressure (low density) area. If the low pressure region was originally created by heating of the material, one sees that movement of material in this way is an example of heat flow by convection.

Convection in our daily lives...

1. Creation of breezes over land masses next to large bodies of water.

Water has a larger heat capacity than land, and subsequently holds heat better. It therefore takes longer to change its temperature, either upward or downward. Thus, during the day the air above the water will be cooler than that over the land. This creates a low pressure area over the land, relative to the high pressure area over the water, and subsequently one finds breezes blowing from the water to the land.

On the other hand, during the night water cools off more slowly than the land, and the air above the water is slightly warmer than over the land. This creates a low pressure area over the water relative to the high pressure area over the land, and breezes will blow from the land to the water.
2. Convection currents in the Earth's System

Large convection currents in the aesthenosphere (70 ~ 250 km from surface of the Earth)transfer heat to the surface, where plumes of less dense magma break apart the plates at the spreading centers, creating divergent plate boundaries. As the plates move away from the spreading centers, they cool, and the higher density basalt rocks that make up ocean crust get consumed at the ocean trenches/subduction zones. The crust is recycled back into the aesthenosphere. Thus, convection currents are also part of the eruption of volcanoes and earthquakes.

Applications of Convection

*Fun experiment: the papers can be lifted above the warm radiator, due to rising convective air current

1. Convection Oven

In a convection oven, the air in the oven must circulate freely. Shape of food affects convection cooking. A long thin meat cooks faster than a bulky one of the same weight because more surface is exposed to moving hot air. Size of pan also must be considered. The same quantity of food cooks faster in two small pans than it does in one large pan since air can circulate more freely.

2. Water kettle

The heating coil of a kettle is always placed at the bottom of the kettle. This is to aid transfer of thermal energy in water by convection. When the power is on, the water nearest to the coil would heat up, making it expand and less dense. This heated water would then rise while the cooler water regions in the upper part of the water body would descend to replace this heated water. Thus, a convection current is set up.

3. Hot water systems

Procedure:

1. Water is heated in the boiler by the gas burners. This water expands and becomes less dense; causing it to rise and flow into the upper half of the cylinder.

2. To replace this hot water, cold water would come down from the cistern and into the lower portion of the cylinder, and then to the boiler. This is caused by the pressure difference.

3. The overflow pipe is attached to the cylinder lest the temperature of the water becomes too high and cuases a large expansion of the water.

4. The hot water tap (leads to the overflow pipe) must be lower than the cistern; so as to ensure that the pressure difference between the cistern and tap causes the water to flow out of the tap.

Conduction

From the video, it can be seen that heat transfer, conduction (or heat conduction) is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. It always takes place from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences.

Fourier's law (aka Law of Heat Conduction)

It states that the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area at right angles, to that gradient, through which the heat is flowing.

The differential form of Fourier's Law of thermal conduction shows that the local heat flux (meaning: the amount of energy flow through a particular surface per unit area per unit time), , is proportional to the thermal conductivity, k, times the negative local temperature gradient, .

where (including the SI units)
is the local heat flux, [W·m−2]
is the material's conductivity, [W·m−1·K−1],
is the temperature gradient, [K·m−1].

The thermal conductivity, k, is often treated as a constant, though this is not always true. While the thermal conductivity of a material generally varies with temperature, the variation can be small over a significant range of temperatures for some common materials.

For many simple applications, Fourier's law is used in its one-dimensional form. In the x-direction,

Applications of Conduction

Cooking utensils - thermal energy is required to be transferred quickly through these good conductors of heat (metal) so as to warm up the food fast.

However, on a closer look, it can be seen that these utensils also have handles. These handles are usually made of insulators (bad conductors) of heat so as to prevent so much heat from passing to the handle and thus might cause the user to suffer serious burns.

For example, a spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon. Conduction is most effective in solids-but it can happen in fluids.

*Fun fact: Have you ever noticed that metals tend to feel cold? Believe it or not, they are not colder! They only feel colder because they conduct heat away from your hand. You perceive the heat that is leaving your hand as cold.

Experiment about Conduction

In this experiment, conduction is explored by testing the conductivity of three kitchen utensils made of plastic, wood and metal. This is to determine which utensil heats the quickest. Metal conducts heat at least 500 times better than wood. The relation between heat conductivities for wood and plastic depends on the type of plastic, but the values are similar.

Procedure
1. Press a small piece of warm candle wax from Part I into the handle of each of the three spoons (see diagram). Push the quarters into the wax so that they are attached to the spoons.
2. Fill the beaker with 300 mL water and place the beaker on a hot plate.
3. Place the three spoons in the water so that the quarters come out of the top of the beaker.
4. Turn on the hot plate and allow the water to warm. Observe the quarters and note the order in which they fall from the spoons.

The wax and quarters on the metal spoon should fall off first followed by the wood amd plastic spoon. This shows that thermal energy flows through the material of the sppons without any flow of the material itself. Such a transfer of thermal energy without any movement of the material medium is known as conduction. Different maerials conduct heat at different rates. Since the length of unmelted wax is shortest for metal and longest for wood and plastic, it can be proven that metals are good conductors of heat and wood as well as plastic are insulators.

It works this way because all solids (both metals and non-metals) are made up of tiny particles called atoms and molecules. The difference between metals and non-metals is that metails contain many free electrons which move randomly between the atoms or molecules, while non-metals do not have such free electrons.

Summary