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Chapter 5: Problem 20

(a) What is heat? (b) Under what conditions is heat transferred from one object to another?

Short Answer

Expert verified
(a) Heat is a form of energy transferred between two objects due to temperature differences, and it is often expressed in units of Joules (J). (b) Heat transfer occurs when there is a temperature difference between two objects, with heat spontaneously flowing from the hotter object to the colder object. This process continues until their temperatures equalize. The three main methods of heat transfer are conduction (direct contact), convection (fluid motion), and radiation (electromagnetic waves).

Step by step solution

01

Part (a): Defining Heat

Heat is a form of energy that is transferred between two objects due to temperature differences between them. It is a measure of the thermal energy that an object has, and it is often expressed in units of Joules (J).
02

Part (b): Conditions for Heat Transfer

Heat transfer happens when there is a temperature difference between two objects or systems. When two objects have a difference in temperature, heat will flow spontaneously from the object with a higher temperature (hot) to the object with a lower temperature (cold). This process will continue until the temperatures of the two objects equalize. There are three main methods of heat transfer: 1. Conduction: This occurs between two objects that are in direct contact with each other. The heat is transferred through the material by the motion of particles, which transmit their kinetic energy to adjacent particles in the colder regions. 2. Convection: This method occurs within fluids (liquids and gases) when the warmer fluid rises and cooler fluid sinks due to differences in their densities. As the fluid flows, the heat is carried along and it gets transferred throughout the fluid. 3. Radiation: In this method, heat is transferred as electromagnetic waves, typically in the form of infrared radiation. This does not require any medium (like air or a solid) to transfer the heat, allowing it to occur even in a vacuum, such as space.

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Key Concepts

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

Conduction
Conduction is a method of heat transfer that happens when two objects are in direct contact. This process mainly involves the transfer of kinetic energy among particles. When you place a cold pan on a hot stove, heat moves from the stove to the pan through conduction. The particles in the stove vibrate more energetically due to their higher temperature. As they collide with the particles in the pan, they transfer energy. This increase in energy causes the particles in the pan to vibrate faster, which raises the pan's temperature.

Key aspects to remember about conduction include:
  • It requires physical contact between objects.
  • Good conductors of heat, like metals, transfer heat efficiently.
  • Poor conductors, or insulators, like wood or rubber, resist the flow of thermal energy.
Understanding conduction helps us realize why we use certain materials for cooking utensils or as insulating layers in buildings.
Convection
Convection occurs within fluids, which includes both liquids and gases, and it involves the movement of the fluid itself. Imagine heating a pot of water. As the water at the bottom of the pot gets hot, it becomes less dense and rises. When this happens, cooler water from the top moves down to replace it. This creates a circular motion known as a convection current.

Some important points regarding convection are:
  • It depends on fluid motion, differing from conduction's reliance on particle interaction.
  • Convection is the reason why room heaters placed on the floor can warm an entire room by circulating the air.
  • In nature, convection currents are responsible for ocean currents, weather patterns, and even the movement of lava in volcanoes.
By understanding convection, we can better design systems to distribute heat evenly in various applications, ranging from industrial equipment to home heating solutions.
Radiation
Radiation differs from conduction and convection as it doesn't require a medium to transfer heat. Heat transfer by radiation occurs through electromagnetic waves. The most common form is infrared radiation. A classic example of radiation is the heat you feel from the sun, which travels through the vacuum of space to reach Earth.

Consider these features of radiation:
  • It can occur in a vacuum, making it essential for space heat transfer.
  • All objects emit some level of radiant energy, which increases with their temperature.
  • The efficiency of radiation depends on the surface characteristics of the material, such as color and texture, with darker surfaces typically absorbing more heat.
Radiation is a crucial concept when designing thermal insulation for buildings, protective clothing for firefighters, and spacecraft shielding. Understanding radiation can help us manage heat in environments where conduction and convection are not effective options.

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Most popular questions from this chapter

When a 9.55-g sample of solid sodium hydroxide dissolves in \(100.0 \mathrm{~g}\) of water in a coffee-cup calorimeter (Figure 5.17), the temperature rises from \(23.6^{\circ} \mathrm{C}\) to \(47.4^{\circ} \mathrm{C}\). Calculate \(\Delta H\) (in \(\mathrm{kJ} / \mathrm{mol} \mathrm{NaOH}\) ) for the solution process $$ \mathrm{NaOH}(s) \stackrel{-\cdots}{\mathrm{Na}}^{+}(a q)+\mathrm{OH}^{-}(a q) $$ Assume that the specific heat of the solution is the same as that of pure water.

(a) What is meant by the term standard conditions, with reference to enthalpy changes? (b) What is meant by the term enthalpy of formation? (c) What is meant by the term standard enthalpy of formation?

(a) Calculate the kinetic energy in joules of a 45-g golf ball moving at \(61 \mathrm{~m} / \mathrm{s}\). (b) Convert this energy to calories. (c) What happens to this energy when the ball lands in a sand trap?

Suppose an Olympic diver who weighs \(52.0 \mathrm{~kg}\) executes a straight dive from a \(10-\mathrm{m}\) platform. At the apex of the dive, the diver is \(10.8 \mathrm{~m}\) above the surface of the water. (a) What is the potential energy of the diver at the apex of the dive, relative to the surface of the water? (b) Assuming that all the potential energy of the diver is converted into kinetic energy at the surface of the water, at what speed in \(\mathrm{m} / \mathrm{s}\) will the diver enter the water? (c) Does the diver do work on entering the water? Explain.

Calculate \(\Delta E\), and determine whether the process is endothermic or exothermic for the following cases: (a) A system absorbs \(105 \mathrm{~kJ}\) of heat from its surroundings while doing \(29 \mathrm{~kJ}\) of work on the surroundings; (b) \(q=1.50 \mathrm{~kJ}\) and \(w=-657 \mathrm{~J} ;\) (c) the system releases \(57.5 \mathrm{~kJ}\) of heat while doing \(22.5 \mathrm{~kJ}\) of work on the surroundings.
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