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Chapter 21 – Electricity and Magnetism at Work • All magnetism is created by a moving charge • Electrons moving through a wire (electricity) created a magnetic field. • A magnet can make the wire move by repelling or attracting the electricity’s magnetic field. N e- S _ + N N S eS • The commutator in an electric motor changes the direction of the electric current every ½ turn to keep the coil turning (see pg. 678) • The brushes are the contact point to the commutator. • The many coils around an iron core is called an armature. • Galvanometer – used to measure small currents. It uses the same principle as an electric motor. • Electric motor – uses current to turn an axle • Electric motor – converts electrical energy into mechanical energy • Mechanical energy – is the energy of an object due to its movement or position. • Work energy is mechanical energy Generators • Turning a coil of wire through a magnetic field causes electrons in the wire to move • Turning the shaft of an electric motor will make electrons move through the wire • This is generating electricity • Electromagnetic induction – the process of generating an electric current from the motion of a conductor through a field. • Generators convert mechanical energy into electrical energy • Alternating current – as the coil is turned, the electrons pulse back and forth as they are attracted to one magnetic pole and then the other. • How can electrical contact be maintained to a turning shaft (by slip rings) see page 684 + brushes Generating Electricity On a Big Scale (electrical power plants) • Need a major power source such as water, wind, coal or nuclear • Wind turns blade on a windmill and water turns a turbine • Coal and nuclear make steam which turns the turbine – (see energy sources pg. 688) • Renewable Resource – one that is replaceable by nature (wind, water, wood, solar) • Non-renewable energy source – one that is not replaced quickly (coal, petroleum, uranium) Coal Mining Uranium Sample Energy and Power • Power - how fast energy is being supplied or consumed P = V (I) I=P/V watts volts amps 1000 watts = 1 kilowatt Energy = power (time) Energy = kilowatts (hours) How much energy does a 300 watt light use if it is on for 10 hours? E = P (time) Energy = .3 kilowatts (10 hours) Energy = 3 kilowatt hours Transporting electricity – It is first brought to a very high voltage to reduce line loss, then it is brought to 120 volts for home use. Step-up transformer – increases the voltage Step-down transformer – decreases the voltage Electrochemical Cells and Batteries • Metals tend to give up electrons • Non-metals and positive ions attract electrons • All chemical reactions are competitions for electrons • In a battery the metal that provides electrons is kept apart from the materials that accept electrons • The electrons must travel through a wire to get from one to the other. • A metal and a chemical that attracts electrons make a cell • The voltage of the cell is the difference in attraction between the metal and the other compound • The voltage of a cell is small (about 1.5 or 2 volts). • Cells are added in series to make a battery of higher voltage • A 12 volt car battery is made from six two volt cells. pg 691 practice problems 1 & 2 (Read War of the Currents on page 694) page 696 questions 1,2,4 page 701 questions 1-4