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Chapter 2: Problem 78

What is heat generation? Give some examples.

Short Answer

Expert verified
Answer: Heat generation refers to the process where energy is transformed into thermal energy, causing a temperature increase in objects or substances. Some examples of heat generation processes include combustion, nuclear reactions, electrical resistance, and friction. Combustion produces heat through the burning of fuels, nuclear reactions release heat during fission and fusion, electrical resistance generates heat when current passes through a conductor, and friction generates heat when objects rub against each other.

Step by step solution

01

Define Heat Generation

Heat generation refers to the process where energy is transformed into thermal energy, which causes a temperature increase in objects or substances. The heat can be produced from various sources, such as chemical reactions, nuclear reactions, electrical resistance, and friction.
02

Provide Examples of Heat Generation

Here are some examples of heat generation processes: Example 1: Combustion Combustion is a chemical reaction that generates heat through the burning of fuels such as coal, wood, or natural gas. In this process, the fuel reacts with oxygen, producing water, carbon dioxide, and heat energy. \[ C_xH_y + \frac{x + y}{4}O_2 \longrightarrow xCO_2 + \frac{y}{2}H_2O + \text{Heat} \] Example 2: Nuclear reactions Nuclear reactions, such as fission and fusion, release enormous amounts of heat energy. In nuclear fission, the nucleus of an atom splits into smaller nuclei, releasing energy in the form of radiation and heat. In nuclear fusion, two or more atomic nuclei combine to form a single, heavier nucleus, also releasing energy as heat. Example 3: Electrical resistance Heat generation due to electrical resistance occurs when electrical current is passed through a conductor with finite resistance, such as a wire. According to Joule's law, the heat generated is proportional to the square of the current (I), the resistance (R), and the time (t) for which the current flows. \[ \text{Heat} = I^2 R t \] Example 4: Friction Friction is a force that opposes the motion between two surfaces in contact. When objects rub against each other, some of the mechanical work they do is transformed into heat. The heat generation from friction can be found using the equation: \[ \text{Heat} = \mu N d \] where \(\mu\) is the friction coefficient, \(N\) is the normal force, and \(d\) is the distance moved. By understanding these examples of heat generation processes, it becomes clear that heat generation is a vast concept with multiple applications in our daily lives, from warming our homes to powering our cars and generating electricity.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Thermal Energy Production
Thermal energy production is a fundamental concept in physics and engineering, representing the transformation of energy into heat. This heat is the result of various natural phenomena and human-designed processes. In our everyday life, we rely on thermal energy for comfort and industry.

For instance, during winter, heating systems convert electrical energy or burn fossil fuels to keep our homes warm. Similarly, industrial furnaces use thermal energy to melt metals, while power plants convert thermal energy from combustion into electricity. As part of responsible learning, it's important to recognize that the thermal energy production is not just about warmth, but plays a critical role in technological advancements and even survival.

Combustion Chemical Reactions
Combustion reactions are vital chemical processes that release heat and light. Most commonly, they involve a fuel, like gasoline or wood, and an oxidizer, such as atmospheric oxygen.

When a combustible material ignites, its molecules react rapidly with oxygen, breaking bonds and forming new compounds like carbon dioxide and water. This exothermic reaction releases a substantial amount of energy. To visualize it, think of a campfire or a lit gas stove where chemical energy stored in fuel is transformed into energy that we can see (light) and feel (heat).

Nuclear Reactions

Fission and Fusion

Within the nucleus of an atom, there lies an incredible amount of energy. Nuclear reactions harness this energy in two primary ways: fission and fusion.

Fission is the splitting of a heavy nucleus into lighter nuclei, releasing energy, while fusion combines light nuclei to form a heavier nucleus and also releases energy. The sun is a natural example of fusion, where it powers our solar system with light and heat. Meanwhile, fission occurs in nuclear reactors, providing a sizeable portion of the world's electricity.

The study of these reactions not only expands our knowledge in physics but also offers insights into potential energy sources for the future.

Electrical Resistance
Electrical resistance is a measure of the opposition to the flow of electric current through a material. The heat generated by this resistance can be both a useful tool and a challenge to manage.

In devices like toasters or electric ovens, resistance is intentionally used to generate heat for cooking. However, in electronic components, unwanted resistance can lead to overheating, which must be carefully controlled. Electrical engineers must understand resistance to design efficient electrical systems, where minimizing heat loss is often crucial.

Friction
Friction is a fundamental force encountered in our daily interactions with the physical world. It arises at the interface between two surfaces in contact and opposes motion.

While typically associated with the idea of slowing down or wearing out materials, friction also plays a crucial role in heat generation. It converts kinetic energy into thermal energy. Consider how rubbing your hands together on a cold day makes them feel warmer - that's the effect of friction at work. Understanding the principles of friction not only helps in designing more efficient machines but also in predicting and managing wear and heat generation in mechanical systems.

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Most popular questions from this chapter

The heat conduction equation in a medium is given in its simplest form as $$ \frac{1}{r} \frac{d}{d r}\left(r k \frac{d T}{d r}\right)+\dot{e}_{\text {gen }}=0 $$ Select the wrong statement below. (a) The medium is of cylindrical shape. (b) The thermal conductivity of the medium is constant. (c) Heat transfer through the medium is steady. (d) There is heat generation within the medium. (e) Heat conduction through the medium is one-dimensional.

A large steel plate having a thickness of \(L=4\) in, thermal conductivity of \(k=7.2 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft} \cdot{ }^{\circ} \mathrm{F}\), and an emissivity of \(\varepsilon=0.7\) is lying on the ground. The exposed surface of the plate at \(x=L\) is known to exchange heat by convection with the ambient air at \(T_{\infty}=90^{\circ} \mathrm{F}\) with an average heat transfer coefficient of \(h=12 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft}^{2} \cdot{ }^{\circ} \mathrm{F}\) as well as by radiation with the open sky with an equivalent sky temperature of \(T_{\text {sky }}=480 \mathrm{R}\). Also, the temperature of the upper surface of the plate is measured to be \(80^{\circ} \mathrm{F}\). Assuming steady onedimensional heat transfer, \((a)\) express the differential equation and the boundary conditions for heat conduction through the plate, \((b)\) obtain a relation for the variation of temperature in the plate by solving the differential equation, and \((c)\) determine the value of the lower surface temperature of the plate at \(x=0\).

Liquid ethanol is a flammable fluid that has a flashpoint at \(16.6^{\circ} \mathrm{C}\). At temperatures above the flashpoint, ethanol can release vapors that form explosive mixtures, which could ignite when source of ignition is present. In a chemical plant, liquid ethanol is being transported in a pipe \((k=15 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\) with an inside diameter of \(3 \mathrm{~cm}\) and a wall thickness of \(3 \mathrm{~mm}\). The pipe passes through areas where occasional presence of ignition source can occur, and the pipe's outer surface is subjected to a heat flux of \(1 \mathrm{~kW} / \mathrm{m}^{2}\). The ethanol flowing in the pipe has an average temperature of \(10^{\circ} \mathrm{C}\) with an average convection heat transfer coefficient of \(50 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Your task as an engineer is to ensure that the ethanol is transported safely and prevent fire hazard. Determine the variation of temperature in the pipe wall and the temperatures of the inner and outer surfaces of the pipe. Are both surface temperatures safely below the flashpoint of liquid ethanol?

What kind of differential equations can be solved by direct integration?

Consider a spherical container of inner radius \(r_{1}\), outer radius \(r_{2}\), and thermal conductivity \(k\). Express the boundary condition on the inner surface of the container for steady onedimensional conduction for the following cases: \((a)\) specified temperature of \(50^{\circ} \mathrm{C},(b)\) specified heat flux of \(45 \mathrm{~W} / \mathrm{m}^{2}\) toward the center, (c) convection to a medium at \(T_{\infty}\) with a heat transfer coefficient of \(h\).
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