Work, Energy and Power.

Work, Energy and Power. Work - Definition Of Work Work is the transfer of energy. If you put energy into an object, then you do work on that object. If an object is standing still, and you get it moving, then you have put energy into that object. If you lift a suitcase to place it in its compartment, we say that you are doing work. Work is done when a force moves against an opposing force. The opposing force is often gravity or friction. You can only do work if you have some energy. If the suitcase falls you will feel this energy! Energy is the capacity for doing work. Both the Work Done and the Energy used to do the work are measured in Joules (J). The amount of work is calculated by multiplying the force times the displacement. That formula looks like this: Work Done = Force x distance moved (In the direction of the force) or W = F x s The unit of Work is the Nm or Joule (J). The work done depends on the size of the force and distance. No work is done when there is no motion. T 2017 - Unit 3 Work,Energy & Power Page 1

Power Work has nothing to do with the amount of time that a force acts to cause a displacement. Sometimes, the work is done very quickly and other times the work is done rather slowly. For example, two pupils can take different times to climb a stair. The two people might do the same amount of work. However the pupil who does the work in considerably less time has a greater power rating than the slower one. Power is the rate at which work is done. It is the work/time ratio. The standard metric unit of power is the Watt. A Watt is equivalent to a Joule/second. (J/s) Energy Everything happens because of energy. Energy is a quantity that is often understood as the ability to perform work. Without energy we could not live or move. We use energy to keep alive, for entertainment and transportation. Work Done = Energy transferred T 2017 - Unit 3 Work,Energy & Power Page 2

DIFFERENT FORMS OF ENERGY Energy comes in many different forms. We cannot create nor destroy energy. The only thing which is possible is for energy to change from one form into another. This is known as the Principle of Conservation of Energy which states that: Energy can neither be created nor destroyed. It can only be changed from one form to another. When a light bulb is switched on, electrical energy is used by the bulb and this form of energy is converted to light ( and some heat). Changes from one form of energy to another can be shown by energy flow diagrams. Note that the following forms of energy are all measured in Joules. This flow diagram represents the energy changes in a mobile phone. T 2017 - Unit 3 Work,Energy & Power Page 3

1. Kinetic Energy (KE) This is the energy an object has because of its motion. For example, a moving train, a moving ship and a moving lorry all have Kinetic Energy. The K.E. of a moving object can be calculated using the following formula. KE = ½ x mass x velocity 2 or KE = ½ m v 2 K.E. is the Kinetic Energy in Joules (J), m is the mass of the object in kg, v is the velocity/speed of the object in m/s. If an object is not moving its Kinetic Energy is zero. 2. Potential Energy (Stored Energy) Potential Energy [measured in J], is the energy stored in an object. There are 3 kinds of Potential Energy or PE: A) CHEMICAL PE Fuels (wood, petrol, etc.), food and batteries are all examples of chemical energy. They have energy stored within them. Fuels release energy when they are burnt to give heat and light energy. Batteries have stored energy which is converted to electrical energy. T 2017 - Unit 3 Work,Energy & Power Page 4

B) ELASTIC PE A stretched elastic band or spring has stored energy called elastic PE. This energy is released as motion when the stretching force is removed. C) GRAVITATIONAL PE Potential energy is energy that is stored within a system. The further away the body is from the surface of the earth, the larger would be its potential energy. Potential Energy (P.E.) The gravitational potential energy can be calculated using the following equation. PE = mass x gravity x height or PE = m g h Where m is the mass of the object in kg, g is the acceleration due to gravity (on earth g =10 N/kg), h is the height of the object above the earth surface in metres (m). The following represents the PE / KE changes for a rollercoaster. T 2017 - Unit 3 Work,Energy & Power Page 5

3. Heat Energy (Thermal Energy) Heat energy is the internal energy in substances. It depends on the vibration and movement of the atoms and molecules within substances. 4. Electrical Energy 5. Nuclear Energy Electrical energy is the movement of electrical charges. Everything is made of tiny particles called atoms. Atoms are made of even smaller particles called electrons, protons, and neutrons. Applying a force can make some of the electrons move. Electrical charges moving through a wire is called electricity. Lightning is another example of electrical energy. Nuclear Energy is energy stored in the nucleus of an atom the energy that holds the nucleus together. The energy can be released when the nuclei are combined or split apart. Nuclear power plants split the nuclei of uranium atoms in a process called fission. The sun combines the nuclei of hydrogen atoms in a process called fusion. 6. Light Energy Light energy forms part of the electromagnetic spectrum. Light is one type of energy that we can see with our eyes. Some of the energy coming from the Sun is in the form of Light rays. T 2017 - Unit 3 Work,Energy & Power Page 6

7. Sound Energy This is the movement of energy through a medium by means of waves. Sound is produced when a force causes an object or substance to vibrate the energy is transferred through the substance in a wave. Falling objects A football of mass 0.5kg is dropped from a height of 4m. P.E. = 20J (mgh = 0.5x10x4 = 20J) K.E. = 0J (ball is not moving) 4m P.E. = 10J (mgh = 0.5x10x2 = 10J) K.E. = 10J (total energy must remain 20J) 2m P.E. = 0J (ball is on the ground) K.E. = 20J (total energy must remain 20J) P.E. at the top = K.E. at the bottom Ke = ½ mv 2 = 20 J v 2 =2 x Ke /m = 2 x20 / 0.5 v 2 = 80 v = 80 = 8.94 m/s we note that neglecting air-resistance, any two masses dropped from the same height will reach the ground at the same time. T 2017 - Unit 3 Work,Energy & Power Page 7

Efficiency A machine would be 100% efficient if all the input energy would be changed to required useful output energy. However no machine is 100 % efficient since some of the input energy is lost as heat in the transformation process. For example a car is only 25 % efficient since only 25 % of the input energy is converted to kinetic energy while the rest is lost as heat and sound from the engine. An energy saving light bulb emits 2880 J of light out of the total 3000 J of electrical energy it consumes. Calculate the efficiency of the bulb. Efficiency = Output Energy / Input Energy * 100% = 2880 / 3000 * 100% = 96 % That means that the bulb gives 96 J of light for every 100 J of electrical energy it consumes. When a machine transfers energy from one form into another, some of it is used up (useful energy) while some of it is wasted (wasted energy) as heat or sound. This affects the efficiency of the machine. If a machine is 100% efficient, then Energy input = Energy output Renewable and non-renewable sources of energy Non-renewable sources of energy are sources of energy that can be used only once. Renewable sources of energy are sources of energy that can be used more than once. T 2017 - Unit 3 Work,Energy & Power Page 8

Renewable sources of Energy These consist of energy sources generated from natural resources. These energy sources can be used more than once and we have unlimited reserves of these resources. Wind power is the conversion of wind energy into a useful form, such as electricity, using wind turbines. Wave power is the transport of energy by ocean surface waves, and the capture of that energy to do useful work. Tidal power, sometimes called tidal energy, is a form of hydropower that converts the energy of tides into electricity or other useful forms of power. Tides are more predictable than wind energy and solar power. T 2017 - Unit 3 Work,Energy & Power Page 9

Solar energy consists of the light and heat rays that come from the Sun. Only a small fraction of the available solar energy is used because most of it is radiated into the space around the Earth. Solar power technologies provide electrical generation by means of heat engines or photovoltaic s. Solar applications includes space heating and cooling through solar architecture, potable water via distillation and disinfection, day lighting, hot water, thermal energy for cooking, and high temperature process heat for industrial purposes Hydroelectricity is electricity generated by hydropower, i.e., the production of power through use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO2) than fossil fuel powered energy plants. Geothermal power is power extracted from heat stored in the earth. This geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. It is used to generate electricity and for direct uses such as wintertime heating. Biomass, as a renewable energy source, refers to living and recently dead biological material that can be used as Fuel. In this context, biomass refers to plant matter grown to generate electricity, example corn or produce for example garbage such as dead trees and branches, yard clippings and wood chips.it also includes energy obtained from landfill gases and alcohol fuels. Biofuel is defined as solid, liquid or gaseous fuel obtained from relatively recently lifeless biological material. Also, various plants and plant derived materials are used for biofuel manufacturing. T 2017 - Unit 3 Work,Energy & Power Page 10

Non-Renewable Sources of Energy These refer to Energy sources that can be used only once. Fossil Fuel Oil & Natural Gas Crude oil is usually found in underground areas called reservoirs. Scientists and engineers explore a chosen area by studying rock samples from the earth. Measurements are taken, and, if the site seems promising, drilling begins. Fossil Fuel Coal is composed primarily of carbon along with variable quantities of other elements, chiefly sulfur, hydrogen, oxygen and nitrogen. Coal, a fossil fuel, is the largest source of energy for the generation of electricity worldwide, as well as one of the largest worldwide source of carbon dioxide emissions. Coal is extracted from the ground by mining, either underground or in open pits. Nuclear Fuel is any type of nuclear elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor. The most common fissile nuclear fuels are 235U (Uranium) and 239Pu (Plutonium), and the actions of mining, refining, purifying, using, and ultimately disposing of these elements together make up the nuclear fuel cycle, which is important for its relevance to nuclear power generation and nuclear weapons. As the world's population increases and there is likely to be demand for more electrical power. Every form of energy generation has advantages and disadvantages as shown in the table below. T 2017 - Unit 3 Work,Energy & Power Page 11

Source - Coal Inexpensive Easy to recover Requires expensive air pollution controls (e.g. mercury, sulfur dioxide) Significant contributor to acid rain and global warming Requires extensive transportation system Source - Nuclear Easy to recover Energy generation is the most concentrated source. Fuel is inexpensive. Waste is more compact than any source.easy to transport as new fuel Requires larger capital cost because of emergency, radioactive waste and storage systems. Potential nuclear proliferation. Requires resolution of the longterm high level waste storage issue in most countries No greenhouse or acid rain effects Source - Gas / Oil Good distribution system for current use levels. Easy to obtain (sometimes). Better as space heating energy source. Very expensive for energy generation. Could be major contributor to global warming. Very limited availability. T 2017 - Unit 3 Work,Energy & Power Page 12

Renewable energy sources Source - Wind Wind is free if available. Good source for water pumping demands of farms. Generation and maintenance costs have decreased significantly. Limited to windy areas. Limited to small generator size; need many towers. Highly climate dependent - wind can damage equipment during windstorms or not turn during still summer days. Source - Hydro-electric May affect endangered birds; however tower design can reduce impact. Very inexpensive once dam is built. Produce no waste. Very limited source since depends on water elevation. Many dams available are already built. Dam collapse usually leads to loss of life. Dams have affected fish (e.g. salmon runs). Environmental damage for areas flooded (backed up) and downstream. T 2017 - Unit 3 Work,Energy & Power Page 13

Source - Solar Sunlight is free when available. Costs are dropping. Limited to sunny areas. Demand can be highest when least available, e.g. winter solar heating) Does require special materials for mirrors/panels that can affect environment. Current technology requires large amounts of land for small amounts of energy generation. Source - Biomass Fuel can have low cost. Inefficient if small plants are used. Industry in its infancy. Could be significant contributor to global warming because fuel has low heat content. Source - Fusion Higher energy output per unit mass than fission. After ~40 years of expensive research,commercially available plants not expected for at least 35 years. Low radiation levels associated with process than fissionbased reactors. T 2017 - Unit 3 Work,Energy & Power Page 14

Source - Refuse Based Fuel Fuel can have low cost Low sulfur dioxide emissions Inefficient if small plants are used. Could be significant contributor to global warming because fuel has low heat content. Flyash can contain metals as cadmium and lead.contain dioxins and furans in air and ash releases Source - HYDROGEN Hydrogen and tritium could be used as fuel source. Combines easily with oxygen to produce water and energy. Takes more energy to produce hydrogen then energy that could be recovered. Very costly to produce. Throughout the world, we need every energy source we can get - including nuclear. As one can see from the table above, all energy sources have BOTH advantages AND disadvantages. Even with conservation efforts, energy demand has been and will continue to increase.in using each and every one of these forms of energy production, we need to make sure we conserve as much as we can so we leave sources for future generations. Energy suppliers need to ensure that they do not contribute to short and long-term environmental problems. Governments need to ensure energy is generated safely to that neither people nor the environment are harmed. T 2017 - Unit 3 Work,Energy & Power Page 15