What is the direct transfer of heat from one object to another?

By the end of this section, you will be able to:

• Describe how the body regulates temperature
• Explain the significance of the metabolic rate

The body tightly regulates the body temperature through a process called thermoregulation, in which the body can maintain its temperature within certain boundaries, even when the surrounding temperature is very different. The core temperature of the body remains steady at around 36.5–37.5 °C (or 97.7–99.5 °F). In the process of ATP production by cells throughout the body, approximately 60 percent of the energy produced is in the form of heat used to maintain body temperature. Thermoregulation is an example of negative feedback.

The hypothalamus in the brain is the master switch that works as a thermostat to regulate the body’s core temperature. If the temperature is too high, the hypothalamus can initiate several processes to lower it. These include increasing the circulation of the blood to the surface of the body to allow for the dissipation of heat through the skin and initiation of sweating to allow evaporation of water on the skin to cool its surface. Conversely, if the temperature falls below the set core temperature, the hypothalamus can initiate shivering to generate heat. The body uses more energy and generates more heat. In addition, thyroid hormone will stimulate more energy use and heat production by cells throughout the body. An environment is said to be thermoneutral when the body does not expend or release energy to maintain its core temperature. For a naked human, this is an ambient air temperature of around 84 °F. If the temperature is higher, for example, when wearing clothes, the body compensates with cooling mechanisms. The body loses heat through the mechanisms of heat exchange.

Figure 1. Click for a larger image. The hypothalamus controls thermoregulation.

Mechanisms of Heat Exchange

When the environment is not thermoneutral, the body uses four mechanisms of heat exchange to maintain homeostasis: conduction, convection, radiation, and evaporation. Each of these mechanisms relies on the property of heat to flow from a higher concentration to a lower concentration; therefore, each of the mechanisms of heat exchange varies in rate according to the temperature and conditions of the environment.

• Conduction is the transfer of heat by two objects that are in direct contact with one another. It occurs when the skin comes in contact with a cold or warm object. For example, when holding a glass of ice water, the heat from your skin will warm the glass and in turn melt the ice. Alternatively, on a cold day, you might warm up by wrapping your cold hands around a hot mug of coffee. Only about 3 percent of the body’s heat is lost through conduction.
• Convection is the transfer of heat to the air surrounding the skin. The warmed air rises away from the body and is replaced by cooler air that is subsequently heated. Convection can also occur in water. When the water temperature is lower than the body’s temperature, the body loses heat by warming the water closest to the skin, which moves away to be replaced by cooler water. The convection currents created by the temperature changes continue to draw heat away from the body more quickly than the body can replace it, resulting in hyperthermia. About 15 percent of the body’s heat is lost through convection.
• Radiation is the transfer of heat via infrared waves. This occurs between any two objects when their temperatures differ. A radiator can warm a room via radiant heat. On a sunny day, the radiation from the sun warms the skin. The same principle works from the body to the environment. About 60 percent of the heat lost by the body is lost through radiation.
• Evaporation is the transfer of heat by the evaporation of water. Because it takes a great deal of energy for a water molecule to change from a liquid to a gas, evaporating water (in the form of sweat) takes with it a great deal of energy from the skin. However, the rate at which evaporation occurs depends on relative humidity—more sweat evaporates in lower humidity environments. Sweating is the primary means of cooling the body during exercise, whereas at rest, about 20 percent of the heat lost by the body occurs through evaporation.

Metabolic Rate

The metabolic rate is the amount of energy consumed minus the amount of energy expended by the body. The basal metabolic rate (BMR) describes the amount of daily energy expended by humans at rest, in a neutrally temperate environment, while in the postabsorptive state. It measures how much energy the body needs for normal, basic, daily activity. About 70 percent of all daily energy expenditure comes from the basic functions of the organs in the body. Another 20 percent comes from physical activity, and the remaining 10 percent is necessary for body thermoregulation or temperature control. This rate will be higher if a person is more active or has more lean body mass. As you age, the BMR generally decreases as the percentage of less lean muscle mass decreases.

Chapter Review

Some of the energy from the food that is ingested is used to maintain the core temperature of the body. Most of the energy derived from the food is released as heat. The core temperature is kept around 36.5–37.5 °C (97.7–99.5 °F). This is tightly regulated by the hypothalamus in the brain, which senses changes in the core temperature and operates like a thermostat to increase sweating or shivering, or inducing other mechanisms to return the temperature to its normal range. The body can also gain or lose heat through mechanisms of heat exchange. Conduction transfers heat from one object to another through physical contact. Convection transfers heat to air or water. Radiation transfers heat via infrared radiation. Evaporation transfers heat as water changes state from a liquid to a gas.

Self Check

Answer the question(s) below to see how well you understand the topics covered in the previous section.

1. How does vasoconstriction help increase the core temperature of the body?
2. How can the ingestion of food increase the body temperature?

Glossary

basal metabolic rate (BMR): amount of energy expended by the body at rest

conduction: transfer of heat through physical contact

convection: transfer of heat between the skin and air or water

evaporation: transfer of heat that occurs when water changes from a liquid to a gas

metabolic rate: amount of energy consumed minus the amount of energy expended by the body

radiation: transfer of heat via infrared waves

thermoneutral: external temperature at which the body does not expend any energy for thermoregulation, about 84 °F

thermoregulation: process of regulating the temperature of the body

Jul 18, 2022 | Turito Team

Heat transfer is a quite common phenomenon that takes place in most situations. All matter which is made of molecules and atoms is capable of undergoing heat transfer. The motion of atoms varies frequently.

Transfer of heat occurs when there is the movement of atoms and molecules. It is also responsible for creating thermal energy. Every matter contains this energy. The heat energy of the particle will be more when the movement of atoms and molecules is more. 

This article will talk about the transfer of heat in general while closely examining its subtopics.

Heat Transfer – Definition 

It is nothing but the transfer of heat from one medium to another. It is defined as the heat movement throughout the body due to irregular temperatures between the system and its surroundings. 

The temperature difference is the main reason heat transfer occurs from one point to the other.

Types of Heat Transfer

The heat is capable of travelling from one place to another in various ways possible. The following are the three different types :

• Conduction
• Convection
• Radiation

These are also called modes of heat transfer. It has been a common fact that can occur from the higher system to the lower system. In simple words, it starts from a hotter region to a colder region. After it finally reaches the colder region (lower system), the whole system will possess almost the same temperature everywhere if the medium is an even object like a steel rod or a square plate.

Conduction Heat Transfer

Conduction generally means the process of transfer of energy or heat from one particle to another while being in direct contact. Likewise, conduction is the transmission of heat from one particle to another. Here, both the particles are obliged to be in contact for this process to occur.

Furthermore, thermal energy transfers from a higher kinetic energy region to a lower one. Here, the particles that travel at high-speed clash with the ones that are slower. Doing so will enhance the speed and kinetic energy of the slower-moving particles. It involves physical contact between each particle.

In addition, high-speed particles have higher temperatures while slow-speed particles have lower temperatures. Therefore, that is how heat is transferred in conduction. The other name for conduction is heat conduction or thermal conduction.

Conduction Equation

The following is the expression for calculating the conduction rate:

Q = [K A (Thot – TCold)] / d

where,

K indicates the object’s thermal conductivity

Q means heat transfer per unit time

A indicates the heat transfer area of the region

d is the body’s thickness

That  means a hot region’s temperature

TCold means a cold region’s temperature

When it comes to conduction, metal objects are capable of conducting heat better than other objects. 

Examples of Conduction

Below mentioned are a few examples of conduction:

• Conduction of heat through beach sand: During summertime, the beach’s sand can be very hot. It is because the conduction of heat takes place through the sand. Also, sand is a very good heat conductor.
• Transfer of heat from hand to ice cube: Imagine placing an ice cube on your hand. What will happen? Due to the presence of heat in the body, the ice cube will start to melt.
• Ironing of clothes: Here, heat from the iron box gets transferred to the clothes and helps them regain their original and neat shape. It is a very good example of conduction heat transfer.

Convection Heat Transfer

In convection heat transfer, the movement of fluid molecules takes place. This flow starts from a higher temperature area and ends at a lower temperature region.

Convection Equation

The volume of the liquid is subjected to increase if its temperature rises. This increase in volume will be the same factor as the temperature. This effect is called displacement. 

The following is the expression to calculate the convection rate:

Q = hc A (Ts – Tf)

where,

Q is the transfer of heat per unit volume

A indicates the area of heat transfer

hc is the coefficient used in convective heat transfer

Ts is the temperature of the surface

Tf is the temperature of the fluid

Examples of Convection

To understand the concept in detail, let us take a look at the following real-time examples:

• In warm-blooded animals, the circulation of blood occurs with the help of convection. Doing so will regulate their body temperature.
• Warm water around the equator will move towards the poles, where cold water is present. Likewise, cold water in the poles will make its way toward warm water on the equator.
• While boiling the water, denser molecules move towards the bottom, whereas thinner molecules move to the top of the water surface. As a result, a circulation motion is formed, and the molecules get heated.

Radiation Heat Transfer

Radiant heat transfer can be seen in various forms in our lives. We call radiant heat thermal radiation. Electromagnetic wave emission is the main reason for the generation of thermal radiation. These waves are capable of carrying the energy away from the body that is performing emission.

Moreover, radiation occurs through a transparent medium or vacuum, which can be liquid or solid. Molecules in matter move randomly, and as a result, thermal radiation is formed. There are two phenomena responsible for the emission of EM radiation. They are the movement of charged protons and electrons. 

Furthermore, the thermocouple is an instrument used to measure the radiation heat transfer. This device’s main purpose is to measure the temperature. However, while measuring the temperature of heat transfer, some errors might take place. These errors can be calibrated for better accuracy and usage in such cases.

Radiation Equation

There will be a reduction in the emitted spectra radiation if the temperature increases. As a result, the emitted radiation’s wavelengths are shorter. We can use the Stefan-Boltzmann law to calculate thermal radiation:

P = e σ A (Tr – Tc)4

where = 5.67×10-8 Wm-2  K-4

P denotes the net power of the radiation

A represents the area of radiation

Tr is the temperature of the radiator

Tc indicates the temperature of the surroundings

e indicates emissivity 

σ is the Stefan constant 

Examples of Radiation

Take a look at the below-mentioned examples to understand radiation heat transfer better:

• An ideal example of radiation is the emission of Ultraviolet rays from the sun.
• Another good example would be the emission of heat radiation from the microwave oven.
• Apart from that, the release of alpha particles, especially during the decay of Thorium 234 and Uranium 238, is another good example of radiation heat transfer.

Conduction vs Convection vs Radiation

The following is the differentiation table that distinguishes conduction, convection and radiation:

Conclusion

All in all, heat transfer is a very important phenomenon in thermal studies. It acts as a base for all the thermal subjects such as thermodynamics, thermal engineering, etc. 

From this blog, we have comprehensively understood some heat transfer concepts such as types, conduction, convection, radiation and all their equations and examples.

Frequently Asked Questions

1. What is the fourth type of heat transfer?

A. The fourth type of heat transfer is evaporative cooling. It is a technique used in atomic physics. Its other name is adiabatic cooling. This technique works due to the principle of water evaporation, where air can be cooled down to a comfortable temperature. It generally is a cooling and ventilation technique and uses water as its refrigerant.

2. What are the three types of heat transfer?

A. Heat transfer is of three major types. They are as follows:

• Conduction
• Convection
• Radiation

Conduction heat transfer takes place in solids or fluids. On the other hand, convection occurs within the fluids. Lastly, radiation heat transfer takes place through EM waves.

3. What are the types of convection heat transfer?

A. The types of convection heat transfer are as follows:

• Air – free
• Air – forced
• Boiling water
• Liquids – forced