4.1 Energy And Energy Transformations - Mremrich

4.1 energy: the capacity to do work Energy and Energy Transformations Without light and other radiant energies that come to us from the Sun, life on Earth would not exist. With these energies, plants can grow and the oceans and atmosphere can maintain temperature ranges that support life. Although energy is difficult to define comprehensively, a simple definition is that energy is the capacity to do work. The word energy comes from the Greek prefix en, which means “in,” and ergos, a Greek word which means “work.” Thus, when you think of energy, think of what work is involved. For example, energy must be supplied to a car’s engine in order for the engine to do work in moving the car. In this case, the energy may come from burning gasoline. Forms of Energy heat: the transfer of energy from a warmer body or region to a cooler one It took many years for scientists to develop a system of classifying the various forms of energy. Table 1 lists nine forms of energy and describes them briefly, and Figure 1 shows examples of these forms. Notice in Table 1 and Figure 1 that eight of the energy forms listed involve particles (atoms, molecules, or objects). The other form is radiant energy. Visible light and other types of radiant energy belong to the electromagnetic (e.m.) spectrum. Components of the electromagnetic spectrum have characteristics of waves, such as wavelength, frequency, and energy, and they all can travel in a vacuum at the speed of light, 3.00 108 m/s. (Refer to section 9.1 to review the e.m. spectrum.) Notice also that heat is not listed as a “form” of energy. Heat is the transfer of energy from a warmer body or region to a cooler one. Various methods of heat transfer are described later. In this chapter, we focus on gravitational potential energy, kinetic energy, and thermal energy, which will be defined formally in the appropriate sections. Later in the text, you will study more about the other forms listed. Table 1 Several Forms of Energy Form of energy Description Example thermal The atoms and molecules of a substance possess thermal energy. The more rapid the motion of the atoms and molecules, the greater the total thermal energy. Figure 1(a) electrical This form of energy is possessed by charged particles. The charges can transfer energy as they move through an electric circuit. Figure 1(a) radiant Radiant energy travels by means of waves without requiring particles of matter. Figure 1(b) nuclear potential The nucleus of every atom has stored energy. This energy can be released by nuclear reactions such as nuclear fission and nuclear fusion. Figure 1(b) gravitational potential A raised object has stored energy due to its position above some reference level. Figure 1(c) kinetic Every moving object has energy of motion, or kinetic energy. Figure 1(c) elastic potential This potential energy is stored in objects that are stretched or compressed. Figure 1(c) sound This form of energy, produced by vibrations, travels by waves through a material to the receiver. Figure 1(d) chemical potential Atoms join together in various combinations to form many different kinds of molecules, involving various amounts of energy. In chemical reactions, new molecules are formed and energy is released or absorbed. Figure 1(d) 124 Chapter 4

4.1 (a) (b) (d) (c) Figure 1 Examples of forms of energy (a) Electrical energy delivered to the stove heats the water in the pot. Thermal energy in the boiling water transfers to the pasta to cook it. (b) The Sun emits radiant energies, such as infrared radiation, visible light, and ultraviolet radiation. The Sun’s energy comes from nuclear fusion reactions in its core. (c) At the highest position above the trampoline, this athlete has the greatest amount of gravitational potential energy. The energy changes to kinetic energy as her downward velocity increases. The kinetic energy then changes into elastic potential energy in the trampoline to help her bounce back up. (d) Chemical potential energy is released when fireworks explode. Some of that energy is changed into sound energy. What other forms of energy are involved in this example? Practice Understanding Concepts 1. Give three examples of energy use available today that your grandparents would not have had when they were your age. 2. Name at least one form of energy associated with each object in italics. (a) A bonfire roasts a marshmallow. (b) A baseball smashes a window. (c) A solar collector heats water for a swimming pool. (d) A stretched rubber band is used to launch a rolled-up T-shirt toward the audience during intermission at a hockey game. (e) The siren of an ambulance warns of an emergency. Energy Transformations The forms of energy listed in Table 1 are able to change from one to another; such a change is called an energy transformation. For example, in a microwave oven, electrical energy transforms into radiant energy (microwaves), which is then transformed into thermal energy in the food being cooked. Undoubtedly you can give many other examples of energy transformations. We can summarize these changes using an energy transformation equation. For the microwave oven example described above, the equation is energy transformation: the change from one form of energy to another electrical energy radiant energy thermal energy Energy, Work, Heat, and Power 125

Practice Understanding Concepts 3. Write the energy transformation equation for each example below. (a) Fireworks explode. (b) An arrow is shot off a bow and flies through the air. (c) A paved driveway feels hot on a clear, sunny day. (d) A camper raises an axe to chop a chunk of wood. (e) A lawn mower with a gasoline engine cuts a lawn. 4. Make up an example of an energy transformation involving the creation of your favourite sounds. Then, write the corresponding energy transformation equation. SUMMARY Energy and Energy Transformations Energy is the capacity to do work. Energy exists in many forms, such as thermal energy and kinetic energy. In an energy transformation, energy changes from one form into another. The transformation can be described using an equation with arrows. Section 4.1 Questions Understanding Concepts 1. Show how energy is transformed for each situation by using an energy transformation equation. (a) A hotdog is being cooked at an outdoor concession stand. (b) A truck is accelerating along a level highway. (c) A child jumps on a trampoline. (d) A tree is knocked over by a strong wind. (e) An incandescent light bulb is switched on. work: the energy transferred to an object by an applied force over a measured distance 2. Provide an example (not yet given in this text) of a situation that involves the following energy transformations. (a) electrical energy thermal energy (b) kinetic energy sound energy (c) chemical potential energy thermal energy (d) electrical energy kinetic energy 4.2 Figure 1 The work done in pulling the carts, which are locked together, depends on the force applied to the carts and the distance the carts move. 126 Chapter 4 Work The term work has a specific meaning in physics. Work is the energy transferred to an object by an applied force over a measured distance. For example, work is done when a crane lifts a steel beam for a new building, when a truck’s engine makes the truck accelerate, when an archer bends a bow as the arrow is pulled back, and when the bow releases the arrow. However, when holding a heavy box on your shoulder, you may feel pain and even break into a sweat, but you are not doing any work on the box because you are not moving it. To determine what factors affect the amount of work done in moving an object, consider a situation in which an employee at a grocery store pulls a long string of empty carts at a constant velocity with a horizontal force (Figure 1).

4.2 By fixing the distance that the carts are moved and doubling the applied force required to pull the carts (by adding twice as many carts), the amount of work done is doubled. Similarly, with a constant applied force, if the distance the carts are pulled is doubled, then again the work done is doubled. Thus, the work done by the applied force is directly proportional to the magnitude of the force and directly proportional to the magnitude of the displacement (distance) over which the force acts. Using the symbols W for work, F for the magnitude of the applied force, and d for the magnitude of the displacement, the relationships among these variables are W F and W d Combining these proportionalities, joule: (J) the SI unit for work W F d W kF d where k is the proportionality constant Choosing the value of k to be 1, we obtain the equation for work: W F d Notice that work is a scalar quantity; it has magnitude but no direction. Therefore, vector notations for F and d are omitted. The equation W F d has important limitations. It applies only when the applied force and the displacement are in the same direction. (In more complex situations where two-dimensional motion is analyzed, the equation that is used is W F d (cos v), where v is the angle between the applied force and the displacement.) Since force is measured in newtons and displacement is measured in metres, work is measured in newton metres (N m). The newton metre is called the joule (J) in honour of James Prescott Joule, an English physicist who studied heat and electrical energy (Figure 2). Since the joule is a derived SI unit, it can be expressed in terms of metres, kilograms, and seconds. (Recall that the newton, the unit of force, can also be expressed in these base units: 1 N 1 kg (m/s2).) You will gain a lot of practice with the joule, kilojoule, megajoule, etc., for the rest of this chapter and in other parts of this course. Sample Problem 1 An airport terminal employee is pushing a line of carts at a constant velocity with a horizontal force of magnitude 95 N. How much work is done in pushing the carts 16 m in the direction of the applied force? Express the answer in kilojoules. Solution F 95 N d 16 m W ? W F d (95 N)(16 m) W 1.5 103 J Figure 2 The joule is named after James Prescott Joule (1818 – 1889), owner of a Manchester brewery, who showed that heat was not a substance but, instead, the transfer of energy. He found that thermal energy produced by stirring water or mercury is proportional to the amount of energy transferred in the stirring. DID YOU KNOW ? Joules and Calories Although the joule is the SI unit of energy and work, we still hear of the heat calorie (cal), a former unit of heat, and the food calorie (Cal), a former unit of food energy. These units are related in the following ways: 1.000 Cal 1.000 103 cal 1.000 kcal 1.000 cal 4.184 J 1.000 Cal 4.184 kJ Thus, a piece of apple pie with 395.0 Cal contains 1.652 106 J, or 1.652 MJ, of chemical potential energy. The work done in pushing the carts is 1.5 kJ. Energy, Work, Heat, and Power 127