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Work, Energy and Power

Hydro power Hydropower The power that is derived from the energy of falling water can be harnessed for useful purposes, such as to rotate turbines which finally generate electricity. This type of generation of electricity from water sources is known as hydro power. Lets learn more about the relation between power and energy with relevance to work, in this topic.

Learning Objectives

After completing the topic, the student will be able to:

  • Define, discuss, understand and correlate the terms work, power and energy.
  • Differentiate between mechanical and non–mechanical energy.
  • Understand different types of mechanical energy, namely kinetic and potential energy.
  • Convert one form of mechanical energy to another.
  • Deduce the importance of the work–energy theorem.
  • Distinguish between conservative and non-conservative forces and apply them to real world problems.
  • Apply the power concept to the real world applications.
Microwave oven magnetron Microwave oven magnetron This cavity magnetron essentially converts electrical energy into microwave energy. It consists of a cathode (hot filament) inside a high-powered vacuum tube (inside the black part at centre) within a magnetic field. The microwaves generated in the chamber are directed to the oven′s food-heating compartment by the waveguide (pink tube at lower left).

Energy comes in many forms and it can be converted from one form into another. Energy can neither be created nor destroyed. This is the law of conservation of energy. In an automobile engine, chemical energy stored in the fuel is converted partially to the energy of the automobile’s motion and partially to thermal energy. In microwave oven, electrical energy is converted to thermal energy of the food being cooked.

Energy is the ability to do work and work done depends on how much force is used to move the object and how far the object moves. The greater the applied force or the longer the distance the object is moved ‐ more work is done. Power is a measure of how quickly work is done: work done divided by the time it takes to do it.

Energy of an object in motion called kinetic energy. A system can transfer energy to another by radiating electromagnetic energy or by transferring kinetic energy to another by physically impacting it. Transfer of thermal energy by heat occurs by both of these mechanisms: heat can be transferred by electromagnetic radiation, or by physical contact in which direct particle-particle impacts transfer kinetic energy.

Potential energy is the energy which may be stored in systems without being present as matter, or as kinetic or electromagnetic energy. A simple example of potential energy is the work needed to lift an object in a gravity field, up to a support. Any form of energy may be transformed into another form. For example, all types of potential energy are converted into kinetic energy when the objects move to different position.

A car spark plug firing A car spark plug firing The spark plug is the trigger that causes the gasoline to burn at a specific time in the car′s internal combustion engine, providing the gas/air mixture the activation energy to start to burn. This is a critical component in the thermodynamic cycle of an internal combustion engine. A dirty or poorly adjusted spark plug will cause an engine to mis-fire, or fail to run.
Matter and energy

When we discussed about motion, we had to describe the idea of motion in terms of displacement, velocity and acceleration. Then we asked, what causes motion? The answer was, "forces cause motion". Newton′s laws of motion give a framework for understanding motion in terms of inertia, force, momentum and impulse. Force is an unseen physical entity that is responsible for all motion. Force can be a contact force or an action at a distance force.

We can proceed with our inquisitiveness and ask next, what gives rise to force? The answer to this is that energy is responsible for force. And what is energy? Well energy is another of those mysterious physical entities that cannot be seen and is difficult to express. The concept most central to all of science and especially Physics, is energy. The combination of energy and matter makes up the whole universe. Matter is substance and energy is the mover of substance.

Matter is something that we can see, smell and feel. It has mass and occupies space. This is not so with energy. It is difficult to define energy, because it is not only an entity but also a process that gives rise to motion. Persons, places and substances have energy, but we usually observe energy only when it is happening – only when it is being transformed. It comes to us in the form of electromagnetic waves from the sun and we feel it as thermal energy (heat). Energy is captured by plants during photosynthesis, energy binds molecules of matter together and energy is in the food materials we eat. Even matter itself is condensed, bottled–up energy.

Energy is the thing that is all pervading and everywhere. It is an important concept of Physics. We will now discuss work and energy and how it is responsible for motion. We will also try to understand how rate of work (energy) or power, is responsible for many day to day applications.

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