Direct Current DC Alternating Current (AC) AC Current
L 27 Electricity and Magnetism [4] Direct Current DC
• Alternating current (AC) vs • a circuit containing a battery is a DC circuit direct current (DC) • in a DC circuit the current always flows in • electric power distribution the same direction • household electricity • household wiring –GFIC’s • the kilowatt-hour (what you pay for) Duracell +
How does the line voltage Alternating Current (AC) change in time? 1 • In an AC circuit the current reverses s 60 direction periodically 200 150 peak • AC is what you get from the power 100 rms companies 50
0
-50
-100
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-200 • Tesla and Edison fought over this, 0 0.02 0.04 0.06 0.08 0.1 and Tesla won! time (seconds)
Why do we use AC ?? AC current (DC seems simpler ??) • The line voltage reverses polarity 60 times a second (60 Hertz) see • AC power is easier to generate • late 1800’s Æ the war of the currents • the current through the bulb reverses direction 60 times a second also • Edison (DC) vs Tesla (Westinghouse) (AC) • Edison opened the first commercial power • for heaters, hair dryers, irons, toasters, plane for producing DC in NY in 1892 waffle makers, the fact that the current • Tesla who was hired by George reverses makes no difference Westinghouse believed that AC was • battery chargers (e.g., for cell phones) superior convert the AC to DC • Tesla was right, but Edison never gave up!
1 Why AC is better than DC The electric generator • When a coil of wire is rotated inside a • DC power is provided at one voltage only magnet, electricity is produced • AC power can be stepped up or down to • http://www.walter-fendt.de/ph14e/generator_e.htm provide any voltage required • this electricity is AC • DC is very expensive to transmit over • the voltage depends on how much wire large distances compared to AC, so many the coil has and how fast it is rotated. plants are required • devices called transformers can make the • DC power plants must be close to users voltage bigger or smaller • AC plants can be far outside cities • transformers only work with AC Í • by 1895 DC was out and AC was in
Wind Power
Coal fired power plant
Hydroelectric power
Electric power generation Transformers and distribution This is a typical step-down transformers used to bring the line voltage down from 5000 V to 240 V before it gets to your home
In your home two voltages are available: 240 V &120 V. The 240 is used for the high • It is more efficient to transmit electrical power appliances like the power (P = IV) at high voltage and low current. clothes dryer, oven, etc. The • The losses along the transmission lines are 120 V is for everything else. reduced compared to transmission at low V.
2 electrical power House wiring
• the power is how much electrical energy used per second = 1 Watt (1000 W = 1 kW) • Power = current x voltage • the appliances required high power, like your electric range or clothes dryer operate at the higher voltage, so less current is used. • we pay for the total energy (not power) used each month - KW-hours (KWH) all circuits are connected in parallel
What everybody needs to know Circuit overload about electricity • if you have too many things plugged into the same circuit, the voltage may drop. hot • you may notice that a lamp plugged into neutral (black) the same outlet as a hair dryer dims a bit (white) when you turn on the hair dryer because a ground (green hair dryer draws a lot of current or bare) • according to Ohm V = I R, a big I can cause enough drop in the voltage to be noticeable!
Man says live wire in bath Electric outlets was to save marriage • The current is supposed to flow from the hot side Thursday, October 28, 2004 Posted: 7:53 AM EDT (1153 GMT) to the neutral, if too much current flows the fuse LA CROSSE, Wisconsin (AP) -- A man who said blows or the circuit breaker trips. he threw a live electrical wire into his wife's bath • the ground is there for protection Æ to provide a hoping a near-death experience would save safe path for current in the event of a short their marriage was convicted of attempted circuit first-degree intentional homicide Wednesday. • on some circuits (kitchens and bathrooms) there William Dahlby said in court he was only trying to is additional protection ÆGFCI Æ ground fault scare his wife the evening of May 9. He told jurors circuit interrupt. If current accidentally flows the wire was hooked to a "ground fault interrupter“ through anything other than the hot or neutral it designed to cut the electricity when the cord interrupts the circuit very quickly encountered water. His wife was not hurt.
3 Paying for electricity $$$ example $$$
• You pay for the total amount of electrical • At a rate of 10 cents per kWh, how much energy that is used does it cost to keep a 100 W light bulb on • the energy is measured in kilowatt-hours for one day? • the kilowatt (kW) = 1000 W is the energy • Solution: First 100 W = 0.1 kW, one full used per unit time day has 24 hours, so • When kW are multiplied by a time unit cost = 0.1 kW x 24 hours x $0.10/kWh (hrs) we get total energy = $0.24 = 24 ¢ Æ for one month that amounts to $7.20
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