Some scientific phenomena play a significant part in our everyday lives, but most of the time we are unaware of their presence. Take the example of coffee, which for many is a favorite morning beverage. Did you ever wonder what gives coffee its refreshing hot and steamy vapors? It is convection, the same scientific process that allows hot air balloons to go up in the sky.
In nature, one process of heat or energy transfer takes place through convection. Convection is movement within a fluid - a liquid or gas, which is driven by differences in temperature. Convection transfers heat energy from hot places to cooler places.
What causes convection?
The particles in liquids and gases move faster when they are heated than they do when they are cold, so the particles take up more volume as the gap between particles widens with the increased movement (the particles themselves stay the same size).
This increase in temperature and volume causes a decrease in the density of the fluid. The lower-density fluid then rises into the cooler, denser areas of fluid. As the lower-density fluid rises and cools, it becomes denser again, and sinks. In this way, convection currents are created which transfer heat from one place to another.
Convection does not generally take place in solids because the more rigid molecular structure does not allow for the flow of particles. In viscous liquids, convection takes place, but at a slower rate than in thinner fluids.
Newton’s law of cooling and the heat transfer coefficient
Before convection was known to lead to the transfer of heat, it was considered as one of the many characteristics of fluids. In 1701, Sir Isaac Newton derived the relationship between convection and heat transfer by observing empirically the convective cooling of hot bodies. He postulated that the rate of heat loss by a body is directly proportional to the excess temperature of the body in relation to its surroundings.
Newton also introduced the convective heat transfer coefficient (h) and derived the law of cooling to quantitatively explain the reason why objects get colder in the air.
According to Newton’s law of cooling, the rate of loss of heat of a body (dQ/dt, where Q = the change in temperature) is directly proportional to the temperature difference (ΔT = (T2 – T1), where T2 is the temperature of the fluid and T1 is the temperature of the surroundings) between the object and its surroundings.
Newton defined the heat transfer coefficient (h) as the rate of heat transfer per unit surface area per unit temperature.
h = Q/AT
Q = h.A.ΔT
Q = Rate of heat transfer
h = convection heat-transfer coefficient
A = Exposed surface area
ΔT = Difference in temperature
During his experiments, Newton assumed the temperature of the surroundings as a constant value, and this is also considered as the biggest limitation of Newton’s law of cooling.
Types of convection
Convection can be classified into two categories, natural convection and forced convection. However, in real world situations, convection may also occur in both forms simultaneously, resulting in mixed convection.
Natural convection is when heat transfer is not generated by an external source. Instead, the fluid motion is caused by buoyancy, the difference in fluid density occurring due to temperature gradients.
Cloud formation is a classic example of natural convection. As the sun heats the Earth’s surface, the air above it heats up and rises. As air continues to rise, it cools down, and Cumulus clouds form. Stronger convection can result in much larger clouds developing as the air rises higher before it is cooled, sometimes producing Cumulonimbus clouds and even thunderstorms.
When the movement in a fluid is induced by external devices such as a pump or a fan, then this process is called forced convection. Generally, when large amounts of heat transfer are required during a process then forced convection is employed.
Forced convection allows one fluid stream to cool or heat another fluid stream, and therefore it has made possible some of the most commonly used inventions such as air conditioners, heating systems, ventilation systems, etc.
Difference between convection, conduction, and radiation
Other than convection, conduction and radiation are two other modes of heat transfer that exist in nature. Each of these mechanisms plays a major part in the energy transfer process that takes place in the atmosphere around us. There are several differences that exist between the three that form the basis on which they are distinguished.
- During convection, the transfer of heat occurs through fluids (liquid or gas) but in the case of conduction, heat transfer takes place through solids, and when it comes to radiation, electromagnetic waves carry out the process of heat transfer.
- The difference in molecular densities plays a role in convection, but conduction is caused primarily by the difference in temperature. In many cases, conduction is followed by convection, for example when you boil water, the water gets heated through conduction, and then the low-density water molecules rise up due to convection. Radiation on the other hand is emitted by all objects that have a temperature greater than 0 Kelvin. The heating of the Earth by the Sun and the heating of a room by an open-hearth fireplace are both examples of heat transfer by radiation.
- The rate of heat transfer is fastest in the case of radiation because light travels faster than any other form of energy, it is slowest in the case of conduction, where heat transfer takes place between solids due to molecular collisions.
- Conduction and convection do not follow the law of reflection and refraction, but radiation follows the same.
- Some common examples of conduction involve warming of muscles by a heating pad, metal utensils getting hot when a hot liquid is poured into them, conduction of electricity that allows us to enjoy television, etc. A refrigerator is an example of convection, whereas the energy that we receive from the sun and from an x-ray machine are examples of radiation.
Convection in our planet
When our planet was first formed, it was made up of hot molten rocks with temperatures exceeding thousands of degrees, and as years passed they gradually cooled down. According to the US National Oceanic and Atmospheric Administration, when the moon was formed, the temperature of the earth could have been around 2,300 Kelvin (about 2027 degrees Celsius).
The Earth has a cool surface temperature now, but it still retains heat from the time of its formation. As the Earth began to take shape about 4.5 billion years ago, iron and nickel quickly separated from other rocks and minerals to form the core of the new planet. The molten material that surrounded the core was the early mantle. Over millions of years, the mantle cooled and solidified.
Although it is mostly solid, convection takes place in the mantle as heat is transfered from the white-hot core to the brittle lithosphere. Because the mantle is heated from below and cooled from above, its overall temperature decreases over long periods of time as mantle convection takes place.
According to one model, a convective system exists between cold and hot boundary layers, also called the thermal boundary layers (TBLs). Thermal convection taking place through the large thermal gradients across the TBLs is the mechanism for heat transfer. Heat energy moves across the TBLs, in and out of the mantle through convection.
Convection currents transfer hot, buoyant magma to the lithosphere (the surface of the Earth) at plate boundaries and hot spots. Convection currents also transfer denser, cooler material from the crust to Earth’s interior through the process of subduction. This process of mantle convection is what causes tectonic plates to move around the Earth's surface.
Thus, the mantle acts as a method of heat transfer within the planet and leads to various important geological actions such as volcanic eruptions, movement in the tectonic plates, rifting, earthquakes, etc.
The theory of convection within the earth’s mantle was originally proposed by Arthur Holmes, a British geologist who championed and further developed Alfred Wegener's theory of continental drift. Holmes proposed that the mantle moved because it contained convection cells that dissipated radioactive heat and moved the crust at the surface. He also contributed to oceanographic research of the 1950s, which introduced the phenomenon of seafloor spreading.
The mechanism of mantle convection led to earth’s temperature decreasing over the course of billions of years, and become favorable for various chemical activities that ultimately led to life on the planet.
As warm-blooded animals, we employ various techniques in our everyday life to regulate the external conditions as per our internal body requirements. Convection helps us in many ways to achieve the same, it enables us to turn the fluids around us cooler or hotter as per our comfort and there are numerous examples to prove this.
- A refrigerator works using the process of convection. The refrigerant gas is circulated in copper lines through the refrigerator and freezer compartments. The lines and the gas absorb the heat in the refrigerator and freezer and the gas circuated back to the compressor. As the gas is compressed, it discharges the heat it absorbed inside of the food compartments into the room.
- The hot air popper used to prepare popcorn is based on the principle of convection. It is equipped with a fan, heating element, and vent. When you put the popcorn kernels inside the popper and turn it on, the heating element heats the air, the fan then directs this onto the popcorn. When this air comes in contact with the kernels, your favorite movie time snack gets ready.
- Tea, coffee, soup, and many other hot beverages that you drink to either refresh or comfort yourself can not be prepared without convection. Apart from this, the convection ovens that you use to cook your favorite cookies also utilize the same principle of energy transfer that is employed by the hot air popper.
- Radiators that keep your home warmer during winters set up a convection current inside your living space. The hot air they produce rises, displacing the cooler air, which sinks and is in turn warmed by the radiator.
- Convection can also play a role in the melting of ice. As warm air blows over the surface of the ice, the warmth is tranferred to the ice through convection and the ice begins to melt.
The principle of convection is also evident in hot air balloons, rainfall, air-cooled engines, sea breeze, land breeze, thunderstorm, etc. It is an important phenomenon affecting our environment, weather, and lifestyle.