14.2 DIFFERENT KINDS OF ENERGY

In this chapter we discussseven forms of energy: mechanical energy (that is the energy associated with the mass of an object), electrical energy, sound energy, light energy, heat energy, chemical energy and nuclear energy. Forms of energy that involve motionare classified as kinetic energy and forms of energy that do not involve motion are classified as potential energy.

Kinetic energy is the energy of moving things. ‘Kinetic’ comes from the Greek word‘kinesis’meaningmotion. The term kineticenergyis used mainly forthe mechanical energy ofamoving object. Howeverelectricity, sound, light and heat all involve motion,so they are often regarded as forms of kinetic energy too.

Mechanical kinetic energy is the energy of anythingthat moves and has mass. In the photos above, the racing cars have a lot of kinetic energy and so do the women athletes. All the photos in the previous module show examples of kinetic energy. The children and the singer are moving, so is the water in the waterfall and so is the train. Kinetic energy is associated with all kinds of motion, including rotation. The photo on the right shows the flywheel at a pumping station. This flywheel has a mass of more than 30 tonnes and when it is turning fast it has so much kinetic energy that it can drive the pumps on its own for quite a long time.

Electrical energy is a stream of electrons flowing through a conductor (see Chapter 4 and Module 13.5).Electricity is a convenient form of energy because it can be delivered to homes and factories through wires, and it can easily be converted into other kinds of energy. The picture below shows electric wires delivering electricity to peoples’ homes and for an electric train.

Sound energy is a pulse of vibrations travelling through the air (see Modules 7.13 and 14). The source of the sound vibrates and nudges the molecules in the air surrounding it. You can easily feel the vibrations from a loudspeaker like the one shown on the right. A pulse of backwards-and-forwards vibrations of the air molecules carries the sound energy through the air at a speed of about 330 metres per second.

Light energy is an example of electromagnetic radiation. Electromagnetic radiation includes radio and TV waves, heat (infra-red) rays, light, X-rays andγ-rays. These rays all carry energy, but unlike sound waves(which must becarried by a medium such as air or water),they require no medium. They travel throughspace at aspeed of 3 x109 metres per second. In this chapter we

willfocus mainly on light which provides energy for life on Earth through photosynthesis.

Heat energy is part of the electromagnetic radiation that we receive from the sun. We also get heat energy from burning fuels and in other ways. As you learnt when you studied the kinetic theory (see Modules 9.2 and 3), heat energy is associated with the random motions of atoms and molecules. We can think of this as internal kinetic energy. When a solid is heated, the particles do not move from place to place, they jiggle about at random, faster and faster, nudging one another and conducting the heat all through the solid. When a fluid (a liquid or a gas) is heated, the particles move around all over the place, faster and faster in all directions at random. Convection currents quickly spread the heat all through the fluid (see Module 7.3).Heat is a low grade form of energy because a lot of it is always wasted as internal kinetic energy and therefore cannot do useful work.

Potential energy is stored energy, or energy-in-waiting. ‘Potential’ comes from the Latin word ‘potentia’ meaning power or capability. The term potential energyis used mainly forthe stored mechanical energy of astationary objectwhich isbeing acted on by a force. However, chemical and nuclear energy are alsoforms of stored energy.

Mechanical potential energy is the stored energy of a stationary objectthat isbeing acted on by a force; an object that is poised, ready to move. In the photo on the right, a large rockis balanced high up on a cliff. Itis being pulled down by the force of gravity but is supported by the rocks underneath it. If this rock slips, or if someone pushes it, it will crash down and damageanything that it hits.Any object above ground level has potential energy of this sort. In the photo on the left, the arrow is being pushed forward by the tension in the bowstring. When the archer lets go of thebowstring, it will shoot the arrow through the air towards its target.Any stationary object that is being pushed or pulled by a piece of elastic, or a spring,has potential energy of this sort.

Chemical energyis the potential energy that is stored in substances that are readyto undergo chemical reactions. A good example is an electric cell or battery. When the terminals are connected and the circuit is switched on, chemical reactions in the cell will send a stream of electrons through the circuit to do their work. Other examples of chemical potential energy are explosives and fuels, including food which is the fuel for our bodies.

Nuclear energy is the potential energy that is stored in the nuclei of atoms. This energy is released slowly during radio-active decay and fast during nuclear fission and nuclear fusion. Nuclear fission is the source of energy in nuclear power stations and atom bombs, and nuclear fusion is the source of energy of the stars including our sun.

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  • 1. List seven forms of energy and classify each of them as kinetic energy or potential energy.
  • 2. Why is electricity a convenient form of energy?
  • 3. Explain how the potential energy of the rock and the arrow above can be described as energy-in-waiting. Describe two chemical examples of energy-in-waiting.

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