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Topic 7 ELECTRICITY IN THE HOME Electricity in the Home • Transformers are used to “step up” the voltage for efficient transmission over long distances. • Other transformers “step down” the voltage to the 240/120 V used in homes and factories. • A circuit breaker acts as a switch and safety device that can cut all power coming into the home. • If current exceeds a safe level, a bimetallic strip in the breaker heats up, bends, and opens the circuit. Electricity in the Home • A fuse contains a metallic conductor that melts when excessive current heats it up. This opens the circuit until the fuse is replaced. • A branch circuit supplies power to one or more wall plugs or lights connected in parallel by cables in the house walls. • As additional loads are plugged into a branch circuit, the current flowing through the connecting cables increases. The small resistance of the cable wires causes them to heat up when enough current flows Electricity in the Home • The breakers, plugs, lights, and switches in each branch circuit are connected by two “live” wires – a white insulated wire (usually called the neutral wire) and a black insulated wire (usually called the hot wire). • The third wire, is the ground wire. It reduces hazards by safely channeling back into the ground any current that has “leaked” onto metal components in the electric circuit. Digital Devices • Digital electronic technology – machines that process numerically coded information. • A Binary code is two states (on or off) that represent numbers and letter. • Logic circuits containing many switches can process binary information. • The electronic switches in modern digital devices are transistors, that can be turned on and off by electric signals. • Every digital device you use, is designed around components that contain enormous numbers of transistors. Measuring Electric Power • Power is energy per unit of time. • Electric power describes the amount of electric energy that is converted into other forms of energy every second. • Electric power can also describe the amount of electric energy that is transferred from one place to another in a certain amount of time. energy (joules) power (watts) = time (seconds) -or𝐸 𝑃= 𝑡 Measuring Electric Power • One joule per second is also called a watt (W) • Electric power is notpower watts usually calculated = current amps × voltage (volts) directly by measuring energy and time. -or• It is much more common to measure the voltage 𝑃 =𝐼×𝑉 and current in a circuit and use these quantities to calculate power Example A current of 13.6 A passes through an electric baseboard heater when it is connected to a 110 V wall outlet. What is the power of the heater? On Your Own • Please work on the next four examples in your notes. If you get stuck, raise your hand. Paying for Electric Energy • Most power companies measure electric energy in a customary unit – the kilowatt hour (kWh). • One kilowatt hour is the total energy supplied to a 1000 W load during 1 h of operation. Reading a Power Meter Solving Cost of Power Problems • Determine the # of kW of power used. • Multiply the # of kW by the # of hours to get kWh. • Multiply the # of kWh by the cost/kWh Example A family uses 3000 kWh of electric energy in a two-month period. If the energy costs 11.0 cents per kilowatt hour, what is the electric bill for the period? On Your Own • Please work on the next three examples in your notes. If you get stuck, raise your hand. Electric Devices and Efficiency • If an electric light bulb were perfect, all of the electric energy it took in (input energy) would be converted into light (useful output energy). • No real device, however, is a perfectly efficient energy converter. Some input energy is always converted into waste heat. Calculating Efficiency • You can express efficiency as a percentage using the following mathematical relationship. useful energy output efficiency = × 100 total energy input • To find the efficiency of electric devices, it is often necessary to calculate energy inputs or outputs. To determine the electric energy input of a device, the E formula for power P = can be manipulated to t solve for energy. Efficiency of Common Light Bulbs • Incandescent bulbs are about 5% efficient, which means about 5% of the input electrical energy is converted to waste heat. • Halogen bulbs are filled with high-pressure gas containing traces of iodine – Halogen bulbs are about 15% efficient, – Last two to six times longer than incandescent bulbs • Fluorescent tubes convert about 20% of the total input energy into useful light energy – Last 10 to 13 times longer than incandescent bulbs, and are much more expensive Example A 1000 W electric kettle takes 4.00 min to boil some water. If it takes 1.96 × 105 J (196 000 J) of energy to heat the water, what is the efficiency of the kettle? On Your Own • Please work on the next three examples in your notes. If you get stuck, raise your hand. Home Electric Safety • Do not overload an outlet or a circuit. • Never work on or clean appliances that are still plugged in. • Replace frayed or worn out electric cords. • If the bare wires touch directly (a short circuit), a high current flows between them, causing sparks and often starting a fire. • Use receptacle cover on easily accessible outlet. • Never use appliances close to a sink of bathtub with water in it or when your hands are wet. • Ground fault circuit interrupter, a GFCI, monitors current flowing into and out of a load. If any electricity is diverted out of the circuit wires, the GFCI immediately cuts off the current. Electric Safety Outdoors • Never allow your body or something you are holding to come into contact with live electric wires. • Never use ungrounded or frayed two-prong electric cords outdoors. • Do not operate electric equipment outdoors when it is raining. • Before digging deeply in your yard, make sure that there are no underground utility cables. Locate Marks Topic 8 ELECTRICITY PRODUCTION AND THE ENVIRONMENT Electric Energy from Burning Fuels • Fuel oil, natural gas, and coal, are burned in large thermo-electric generating plants to produce about one quarter of our country’s electricity. Fossil Fuels Affect Land and Air • Open pit mining of surface deposits disturbs soil and vegetation. • Underground mines produce waste materials called “tailings,” which accumulate near the mine. • Water seeping through the tailings becomes acidified and contaminated. • When fossil fuels are burned, contaminants such as visible particles and invisible gases escape form smokestacks. Fossil Fuels Affect Land and Air • Electrostatic precipitators can remove most of the solid particles. • Scrubbers can remove sulfur dioxide. Scrubbers spray a water solution through the waste gases. SO2 and other pollutants react with chemicals in the water and are removed. • Complete burning of pure coal or natural gas would produce carbon dioxide gas. Fossil Fuels Affect Land and Air • Carbon dioxide is a greenhouse gas, which means that it helps delay the escape of heat from Earth’s atmosphere. This could lead to a rise in the average temperatures on Earthglobal warming. • Natural gas burns much cleaner than coal, producing less SO2. Electric Energy from Flowing Rivers • Hydro-electric plants use water pressure to generate electric energy. Hydro-electric power plants appear to be a very clean form of electric energy generation. However, reservoirs, which store behind the dams, flood many hectares of valuable land. Homes, small villages even entire towns may need to be moved, displacing people and industries. • When submerged vegetation decays, microorganisms responsible for this process use up the oxygen supplies in the water sometimes decaying matter produces methane gas. Other forms of life, such as algae, take over and change the ecosystem. Energy From Atomic Reactions • Bombarding uranium atoms with tiny particles called neutrons causes the uranium to split into two smaller atoms. • In the process called nuclear fission, a tremendous amount of energy is released. This is referred to as thermonuclear electric generation. Energy From Atomic Reactions • Fission reactors do not release soot or gases that cause acid rain, nor do they release greenhouse gases. However, used (spent) reactor fuel is highly radioactive for thousands of years. • The reactors themselves also become slightly radioactive so they are difficult and expensive to decommission. Energy From Atomic Reactions • In the Sun and other stars, nuclear fusion joins very small atoms to form a larger atom. Huge amounts of energy are released in this process • Developing technology to create and use continuous controlled fusion as a power source is an extremely difficult and expensive project. Heating the Environment • All thermonuclear and thermo-electric-generating plants release thermal energy into the environment. • Thermal pollution occurs when this warm water is returned directly to the lake or river from which it is taken, increasing the water temperature. Even a change of a few degrees can affect the plant and animal life in the water. • To reduce thermal pollution, large generating plants have cooling ponds or towers where waste water can return to the temperature of the surroundings before it is released. Cogeneration • It makes environmental sense to design electricity-generating stations as cogeneration systems that produce electricity and also supply thermal energy, such as hot water or steam, for industrial or commercial heating. Distribution of Electricity • Power grid transformers built before 1977 often contained insulating chemicals called “polychlorinated biphenlys” or PCBs. PCBs persist, bioconcentrate, and can cause health problems. • High voltage transmission lines themselves, which produce radiation similar to radio waves, may be linked to health problems in people living nearby. Alternative Energy Sources • Energy from the Sun, wind, and tides, which until now has been too expensive or too difficult to harness, is rapidly becoming competitive with conventional energy sources. • Wind-driven electricity generation must be used together with other electric energy sources or storage devices. • Solar-powered-generating systems often include storage batteries to supply electricity at night or in cloudy conditions. Alternative Energy Sources • Ocean tides are another source of energy that can be used to generate electricity. • The hot inner parts of the Earth contain a great deal of thermal energy, called geothermal energy. Ground water flows down from the surface, absorbs thermal energy, and rises again as hot spring and geysers. Steam produced by geothermal activity can be used to rotate