Electric Power Electric Power Household voltages are typically 120V - PDF document

Electric Power Distribution 1 Electric Power Distribution 2 Observations about Observations about Electric Power Distribution Electric Power Distribution � Household electricity is alternating current (AC) Household electricity is alternating current (AC) Electric Power Electric Power � Household voltages are typically 120V or 240V Household voltages are typically 120V or 240V Distribution Distribution � Power is distributed at much higher voltages Power is distributed at much higher voltages � Power transformers are common around us Power transformers are common around us � Power substations are there, but harder to find Power substations are there, but harder to find Electric Power Distribution 3 Electric Power Distribution 4 4 Questions about 4 Questions about Question 1 Question 1 Electric Power Distribution Electric Power Distribution � Why isn’t power transmitted at low voltages? Why isn’t power transmitted at low voltages? � Why isn’t power transmitted at low voltages? Why isn’t power transmitted at low voltages? � Why isn’t power delivered at high voltages? Why isn’t power delivered at high voltages? � What is “alternating current” and why use it? What is “alternating current” and why use it? � How does a transformer transfer power? How does a transformer transfer power? Electric Power Distribution 5 Electric Power Distribution 6 Electric Power and a Wire Electric Power and a Wire Large Currents are Wasteful Large Currents are Wasteful � An electric current passing through a wire An electric current passing through a wire � The goal of a power distribution system is to The goal of a power distribution system is to converts electrical power in thermal power converts electrical power in thermal power transmit lots of electric power to a city, transmit lots of electric power to a city, power wasted = current · voltage drop in wire. power wasted = current · voltage drop in wire. power transmitted = current · voltage drop at city power transmitted = current · voltage drop at city, , � Si � Since the wire obeys Ohms law, Si Since the wire obeys Ohms law, th th i i b b Oh Oh l l � while wasting little electric power in the wires, while wasting little electric power in the wires, hil hil ti ti littl littl l l t i t i i th i th i i � voltage drop in wire = resistance · current voltage drop in wire = resistance · current, , power wasted = resistance · current power wasted = resistance · current 2 2 . . � the power that wire wastes is the power that wire wastes is � That energy efficiency can be achieved by using That energy efficiency can be achieved by using power wasted = resistance · current 2 . power wasted = resistance · current . � a small current, a small current, � a huge voltage drop, a huge voltage drop, � Doubling current quadruples wasted power! Doubling current quadruples wasted power! � and low and low- -resistance wires. resistance wires. 1

Electric Power Distribution 7 Electric Power Distribution 8 Question 2 Question 2 High Voltages are Dangerous High Voltages are Dangerous � Why isn’t power delivered at high voltages? Why isn’t power delivered at high voltages? � When large voltage drops are available, When large voltage drops are available, � strong electric fields are present, strong electric fields are present, � charges experience enormous forces, charges experience enormous forces, � nd � and currents tend to flow through unexpected paths. and currents tend to flow through unexpected paths. nd rr nt t nd t fl rr nt t nd t fl thr thr h n h n p p t d p th t d p th � High High- -voltage electrical power in a home is voltage electrical power in a home is � a spark hazard, a spark hazard, � a fire hazard, a fire hazard, � and a shock hazard. and a shock hazard. Electric Power Distribution 9 Electric Power Distribution 10 The Voltage Hierarchy The Voltage Hierarchy Question 3 Question 3 � Large currents are too wasteful for transmission Large currents are too wasteful for transmission � What is “alternating current” and why use it? What is “alternating current” and why use it? � High voltages are too dangerous for delivery High voltages are too dangerous for delivery � So electric power distribution uses a hierarchy: So electric power distribution uses a hierarchy: � high high- -voltage circuits in the countryside voltage circuits in the countryside � medium medium- -voltage circuits in cities voltage circuits in cities � low low- -voltage circuits in neighborhoods and homes voltage circuits in neighborhoods and homes � Transformers transfer power between circuits! Transformers transfer power between circuits! � Electric Power Distribution 11 Electric Power Distribution 12 Alternating Current (AC) Alternating Current (AC) AC and Transformers AC and Transformers � In alternating current, In alternating current, � AC has little effect on simple electric devices AC has little effect on simple electric devices � (e.g., (e.g., lightbulbs lightbulbs, space heaters, toasters) , space heaters, toasters) � the voltages of the power delivery wires alternate the voltages of the power delivery wires alternate � and the resulting currents normally alternate, too. and the resulting currents normally alternate, too. � AC is a nuisance for electronic devices AC is a nuisance for electronic devices (e g comp ters televisions so nd systems) (e g comp ters televisions so nd systems) (e.g., computers, televisions, sound systems) (e.g., computers, televisions, sound systems) � Alternating voltage in the US Al Al Alternating voltage in the US i i l l i i h US h US � AC permits the easy use of transformers, AC permits the easy use of transformers, � completes 60 cycles per second, completes 60 cycles per second, � reversing every 1/120 second. reversing every 1/120 second. � which can move power between circuits: which can move power between circuits: � from a low from a low- -voltage circuit to a high voltage circuit to a high- -voltage circuit voltage circuit � from a high from a high- -voltage circuit to a low voltage circuit to a low- -voltage circuit voltage circuit 2

Electric Power Distribution 13 Electric Power Distribution 14 Question 4 Question 4 Electromagnetism Electromagnetism (Version 2) (Version 2) � How does a transformer transfer power? How does a transformer transfer power? � Magnetic fields are produced by Magnetic fields are produced by � magnetic poles (but free poles don’t seem to exist), magnetic poles (but free poles don’t seem to exist), � moving electric charges, moving electric charges, � and changing electric fields [more later…] � nd h n in and changing electric fields [more later…]. nd h n in l l tri fi ld [m r l t r tri fi ld [m r l t r ] ] . � Electric fields are produced by Electric fields are produced by � electric charges, electric charges, � moving magnetic poles moving magnetic poles, , � and changing magnetic fields and changing magnetic fields. . Electric Power Distribution 15 Electric Power Distribution 16 Electromagnetic Induction Electromagnetic Induction Lenz’s Law Lenz’s Law � Moving poles or changing magnetic fields Moving poles or changing magnetic fields � Lenz’s law predicts the nature of the induced Lenz’s law predicts the nature of the induced � magnetic fields: magnetic fields: � produce electric fields, produce electric fields, � which propel currents through conductors, which propel currents through conductors, � which produce magnetic fields. which produce magnetic fields. p p g g “When a changing magnetic field induces a current in a “When a changing magnetic field induces a current in a g g g g g g conductor, the magnetic field from that current conductor, the magnetic field from that current � Changing magnetic effects Changing magnetic effects induce induce currents currents opposes the change that induced it.” opposes the change that induced it.” � Induced currents produce magnetic fields Induced currents produce magnetic fields Electric Power Distribution 17 Electric Power Distribution 18 Transformer Transformer Current and Voltage Current and Voltage � Alternating current in one circuit can induce an Alternating current in one circuit can induce an � A transformer must obey energy conservation A transformer must obey energy conservation alternating current in a second circuit alternating current in a second circuit � Power arriving in its primary circuit must equal Power arriving in its primary circuit must equal � A transformer A transformer power leaving in its secondary circuit power leaving in its secondary circuit � uses induction uses induction i d i d i i � Since power is the product of Since power is the product of voltage · current Si Si i i h h d d f f voltage · current, l , � to transfer power to transfer power � a transformer can exchanging voltage for current a transformer can exchanging voltage for current between its circuits between its circuits � or current for voltage! or current for voltage! � but doesn’t but doesn’t transfer any charges transfer any charges between its circuits between its circuits 3