11.2 Electric power distribution

problem with cores

-Although magnetizable cores make small efficient transformers practical, they also introduce a few problems. 1) the crew must magnetize and demagnetize easily to keep up with the energy investment and withdrawal processes. If they lag behind, they'll waste power as thermal power. Sadly, perfect magnetic softness is unobtainable and all cores waste a least a little power through delay in their magnetization. b/c these cores are subject to the same electric fields that push currents around in the coils, they shouldn't conduct electricty. If they do, they'll develop useless internal currents known as eddy currents and thereby waste power heating themselves up. Since most soft magnetic materials are electrical conductors, transformer cores are frequently dived up tiny insulated particles or sheets to that little or no current can flow through them.

power in ohmic devices

-a calculation: power consumption=voltage drop x current voltage drop=resistance x current power consumption = resistance x current^2 -impact of the calculation: wires waster power as heat, doubling current quadruples waster power.

AC electric generators

-a transformer "converts" electric power into electric power-it extracts electric power from a primary circuit and delivers electric power to a secondary circuit. However, electric power and mechanical power are physically equivalent, and mechanical power can substitute for electric power. What if we replace on the electric circuits in a transformer with a mechanical system? -if we replace the transformer's primary circuit with a mechanical system, we obtain a generator. A generator is a device that extract mechanical power from machinery and delivers electric power to a circuit. (A) shows a simple generator, one that looks like the transformer. Both devices have a secondary wrapped around a magnetizable core. However, in place of the transformer's primary coil, the generator has a spinning magnet or rotor . -As the generator's magnetic rotor spins, it produces a sinusoidally alternating magnetic field in the coil. This alternating magnetic field, produces an alternating electrical field and induces an alternating emf in the coil. That emf propel an AC through the circuit that delivers electric power to the lamp. --the current in the generator's coil has consequences for its root. That current magnetizes the coil so that the coil interacts with the rotor and extracts mechanical power from it! To keep the rotor spinning, the machinery must continue to supply mechanical power to the generator. —> generator is converting mechanical power into electric power.

Transformers

-advantage people saw in AC was that its power could be transformed -it could be passed via electromagnetic action from one circuit to another by a device called a trasnformer- uses two important connections between electricity and magnetism to convey power from one AC circuit to another. 1. moving electric charge creates a magnetic fields-> allows electricity to produce magnetism 2) magnetic fields that change with time create electric fields--> allows magnetism to produce electricity. -whether you wave a permanent magnet back and forth, or switch an electromagnet on and off, you are changing a magnetic field with time and producing an electric field. -rather than returning electric power to where it stared, the transformer moves the power from the current in one coil of wire through a magnetic field to the current in a second coil.

transformer

-alternating current in one circuit induces an alternating current in a second circuit -transfers power between the two circuits -doesnt transfer charge between the two circuits

root means square voltage

-an outlet's nominal AC voltage is defined to be equal to the DC voltage that would cause the same average power consumption in an ohmic device. Thus, 120 V AC power delivers the same average power to a toaster as 120 V DC power. -

alternating current

-makes it easy to transfer power from one AC circuit to another so that different parts of the AC power-distribution system can operate at different voltages with different currents. -the wires that carry the power long distances are part of a high-voltage, low current circuit and therefore waste little power -an AC is one that periodically reverses direction. EX: when you plug your lamp into an AC electrical outlet and switch it on, the current that flows through the lamp's filament reverses its direction of travel many times each second. -power company propels AC through the lamp's filament by subjecting it to an alternating voltage drop, a voltage drop that periodically reverses direction. -current in an ohmic filament flows from a higher voltage to a lower--> power company subjects your lamp's filament to an alnterating voltage drop and it options an AC.

step up transformer

-more turns in secondary circuit-large voltage rise -a smaller current at high voltage flows in the secondary circuit

cont...

-msot transofmers have unequal coils and therefore different emus in their coils. Since the secondary coil's induced emjf is proportional to the number of turns, it acts as a source of AC power with a voltage equal to the voltage applied to its primary coil times the ration of secondary turns to primary turns. -- > secondary voltage = primary voltage x secondary turns/primary turn.An isolation transformer is simply the special case in which the turn numbers are equal and their ration is 1.

REal trasnformers

-not flawless: in reality the wires used in those devices have electrical resonates and waste power in proportion to the squares of the currents they carry. To minimize this wasted power, real inductors and transfomrers are designed to minimize their resistances. To the extent it is practical, they employ thick wires made of highly conducting metals and those wires are kept as short as possible -unfortunately, indicators and transformer built only from wires can't develop the strong magnetic field they need to store large amount to energy unless they use long, many-turn coils. To avoid long coils, many inductors and all transformers wrap their coils are magnetizable cores. These cores responds magnetically to the AC around them, boosting the resulting magnetic fields and making it easier to store large amount of energy. Aided by those magnetizable materials-cored inductors and transformers work well event with short, few turns.

current and voltage

-power arriving int he primary circuit must equal power leaving the secondary circuit -power is the product of voltage x current -a transformer can change the voltage while keeping power unchanged!

power transmission

-power delivered to a city is: power delivered=current x voltage -power wasted in transmission wires: power wasted =resistance x current ^2 -for efficient power transmission: use low-resistance wires (thick, short copper) use low current and high voltage

back emf

-the coil's self induction and back emf allow it to handle AC and alternating voltages with grace. You can plug the two ends of a properly designed coil into an AC electrical outlet without trouble—the coil will rehymically store energy and return it. -unlike an ordinary wire, which can't safely receive current from the outlet at one voltage and return it to the outlet at a different voltage, the coil can use its back emf to ride the outlet's alternating voltages. Pushed forward and backward by the induced emf, current can enter this coil at one voltage and leave it at a different voltage. -the coil''s back emf leases has just the right voltage so that current passing through the coil's hot end at the hot voltage passes through the coil's neutral end at the neutral voltage (0V). EX: when hot is +170 V, the back emf is -170 V, when hot is -50 V, the back emf is +50 V.

currents produce ,magnetic field

-the direction of the current determines the direction of the poles -effect of core: creates a larger magnetic field.

Lenz's law

-the opposition to change is universal in magnetic induction. When a changing magnetic field induces a current in a conductor, the magnetic field from that current opposes the change that induced in. -> the effects of magnetic induction opposes the changes that produce them. -self directed magnetic induction or sell induction leads our coil to oppose its own changes in current. A wire coil's natural opposition to current change make sit quite useful in electrical equipment and electrons, where is called an inductor.

step down transformer

-the transformer in the lamp is called a step-down transformer because it has fewer secondary turns that primary turns and provides a secondary voltage that is less than the primary voltage. -if the ration of secondary turns to primary turns is only .1, the secondary coil with act as a source of 12 V AC power. --the primary coil has 10 times as many turns as the secondary coil. The secondary voltage is .1 times the primary voltage and the secondary current is 10 times the primary current. If the 12-V bulb that you install in the lamp has been designed to consume 24 W of power, a current of 2 A will flow through the secondary circuit. To provide this power, the transformer's primary coil will carry .2 ampere of current supplied at 120 V AC. In all, 24 watts of power are flowing from the transformer's primary circuit to its secondary circuit.

distrubting electric power via DC problem

-there's no easy way to transfer power from one DC circuit to another. B/c the generator and the light bulbs must be part of the same circuit, safety requires that the entire circuit use low voltages and large currents. DC power distribution wastes much of its power in the wires connecting everything together. -

power delivered

-to keep power losses in the long transmission wires as low as possible, deliver power at very high voltages and low currents.

AC and a coil of wire

-what happens when you send an AC through a single coil of wire? -> b/c currents are magnetic, the coil becomes and electromagnet. However since the current passing through it reverses periodically, so does its magnetic field. Also b/c a magnetic field that changes with time produces an electric field, the coil's alternating magnetic field produces an alternating electric field. -this electric field has a remarkable effect-pushes on the very AC that produces it. -as the coil's current increases, the induced electric field pushes that current backward and thereby opposes its increase (b). As the coil's current decreases the induced electric field pushes that current forward and thereby opposes its decrease (d). -no matter how the coil's current changes, the induced electric field always opposes that change!

Changing voltages

-why does a lamp need a transformer? why not just connect the bulb directly to the power outlet to form a single circuit with the power company? -in the lamp, the transformer's job is to provide the bulb with low-voltage AC power. Like th bulb in a flashlight, the lamp bulb is designed to operate on small voltages. This low voltage bulb derives its heating power from a large current a small voltage drop, so its filament has a small electrical resistance and is which, short and study. -A high voltage bulb derives its heating power from a small current and a large voltage drop, so its filament needs a large electrical resistance and must be thin, long and fragile. -the shorter, low voltage filament is also a more concentrated light source, ideal for a desk lamp. To provide this low voltage AC power, the transformer's secondary coil is wound difrent from its primary coil .

cores and flux lines

-winding both coils around a ring-shaped magnetic core makes it easy for the flux lines to pass through both coils b/c those flux lines are drawn into the core's soft magnetic material and follow it as if in a pipe. Although the flux lines leaving a coil must still return to it eventually, most of them complete that trip by way of the core-a journey that then take them through the other coil. With nearly all the flux lines channeled by the core through both coils, power can flow easily from one coil to the other. -A core thus provides a transformer with great flexibility: its coils can be practically anywhere as long as they encircle that core. However, cores aren't quiet perfect pipes for flux; they leak slightly. Therefore, the most efficient transfomers have coils that are would nearly or on top of one another.

EX: if you send DC through the primary coil of a transformer, no power will be transferred to the secondary circuit. Explain.

When DC flows through the transformer's primary coil, it creates a constant magnetic field around the iron core. Since that field doesn't change, it doesn't create any electric fields and doesn't induce current in the transformer's secondary coil. The current through the primary coil must change so that the magnetic field in the coils will change and current will be induced in the secondary coil. Transferring power from one circuit to another is so useful that there are many DC powered devices that switch their power on and off to mimic AC so that they can use transformers.

DC (direct current

batteries produce DC

induction

changing magnets produce changing currents in metals

EX: An MR diagnostic unit fills about .1 m^3 of space with a 4-T magnetic field. How much energy is congaed in that field?

field contains about 640,000J -since 1 T is equivalent to 1 N/A x m, egive us the energy of a 4-T field occupying .1 m^3 as: energy=(4 N/A x m)^2 x .1 m^3 / 2 x (4pi x 10^-7 N/A^2) =640,000 Nxm=640,000J

two coils together: A transformer

in a single coil, energy that's transfer from the current to the magnetic field must eventually rent to the current. It has nowhere else to go. But is there are two coils and two currents energy transferred from one current to the magnetic field can move to the second current! that possibility is the basis for a transformer—> a transformer consists of two separate coils that share the same electromagnetic enevirnoment. Some or all that energy invested in the magnetic by current in the first coil can be withdrawn from the magnet field by current in the second coil. Although the 2 current never touch and don't exchange a single charge, power can move from one current to another with ease.

motors

it we replace the transformer's secondary circuit with a mechanical system we obtain a motor. A motor is a device that extracts electric power from a circuit and delivers mechanical power to machinery. (b) shows a simple motor, one that again resembles a transformer. Like the transformer, the motor has a primary coil wrapped around a magnetizable core. In place of the transformer's secondary coil, however, the motor has a spinning magnetic rotor. As an AC flows through the motor's circuit and coil, it produces a sinusoidally alternative magnetic field in the coil. That magnetic field interacts with the magnetic rotor and delivers mechanical power to it. -the rotor's rotation affects the current in the motor's coil. As the magnetic rotor spins, it induces an alternating emf in the motor's coil and that emf extract power from the AC. To keep the rotor spinning, the circuit must continue to supply electric power to the motor—> Overall the motor is converting electric power into mechanical power. -the images are almost mirror images—> thats b/c generators and motors are wonderfully similar devices. In fact, a single device can often act as either a generator or a motor. If you supply electric power to its circuit, its rotor will spin and provide mechanical power. If you supply mechanical power to its rotor, current will flow through its circuit and provide electric power.

EX: if a power utility were able to increase the voltage of its transmission line from 500,000 to 1,000,000 V, how would that affect the power lost to heat in the wires?

it would reduce the amount of heat produced by only 25% of the previous value. At 1,000,000 V, the transmission line would be able to carry the same power as a 500,000 V transmission line with only half the current. Since the power wasted by the transmission line itself is proportional to the square of the current, halving the current would reduce the power waste to 25%

EX: Large power transformers have cooling fins and often fans to flow are across them. Why does a transformer need this cooling?

its magnetic core convert some of the electric power into thermal power. Unless that thermal power is eliminated, the transformer will overheat. Transformers aren't perfectly energy efficient, they convert a small fraction of power they handle into thermal power. Their magnetic cores contribute to that inefficient b/c their limited magnetic softness and electric conductivity cause them to heat up.

AC outlet

offers three connections: hot, neutral and ground. -the absolute voltage of neural remains near 0V, while the abolsute voltage of hot alternates above and below 0. -Ground which is an optional safety connection remans near 0V absoltue. -one end of lamp's filament is connected to hot and other to natural. Since current always flows through the filament from higher voltage to lower voltage, it flows from hot to neutral when hot has a positive voltage and from neutral to hot when hot has a negative voltage. -In normal AC electric power, the hot voltage varies sinusoidally. During each reversal, the current in the filament gradually slows to a stop before gathering strength in the opposite direction. -fortuanetly, these reversal have little effect on household devices. They consume power b/c of their electrical resistnace and don't care which way current passes through them. -some and and most electronic devices are sensitive to the direction of current flow and must handle the reversals carefully, and the power available from and ordinary AC outlet rises and falls with each voltage reversal and is momentarily zero at the reversal itself. The lamp actually flickers slightly because of these power fluctuations, and devices that can't tolerate and instant without power must store energy to avoid shutting down during reversals.

core

provide another crucial benefit to a transformer, it guides the transformers's magnetic flux lines so that nearly all of them pass through both coils, even when those coils are somewhat seperated in space. Sharing their flux lines in that manner gives the coils a common electromagnetic environment and permits them to exchange electric power easily. -making 2 separate coils share their flux lines isn't easy. Since a coil has no net magnetic pole, each flux line that emerges from it must ultimately return to it. Without a core, most flux lines leaving a coil return to it almost directly and remain nearbly thoughout their whole trip. Those unadventurous flux lines are unlikely to pass through a second, separate coil. A scoreless transformer works well only when its two coils are wound so closely together that they can't help but share the same flux lines.

ohm's law

the currents passing through most wires and other devices experience voltage drops -in an "ohmic device" the voltage drop is proportional to the current: voltage drop=resistance x current where resistance is constant for the divide

EX: if you drop a strong magnet onto a nonmagnetic but highly conducting surface, the magnet will descend remarkably slowly. what's delaying the magnet's fall?

the falling magnet is inducing currents and magnetism in the surface. in accordance with Lenz's law, that induced magnetism opposes the change that produces it; it acts to slow the magnet's descent. A strong magnet induces such powerful magnetic opposition in a good conductor that moving the magnet is difficult. This effect is most evident with a superconductor, a material that conducts electricity perfelcty and can sustain induced currents forwever. A super conductor can slow falling magnet to a stop and hold it suspended indefinitely

EX: in the days of vinyl record, a phonograph reproduced sound by sliding a diamond stylus through a record' undulating groove. A magnet attached to that stylus moved up and down with each undulation and produced a current in a nearby coil of wire. Why did the magnet's motion affect the coil?

the moving magnet produced an electric field, which pushed mobile charges through that wire coil. The tiny vibrating magnet affected the coil's current via magnetic induction.

Ex: your portable lava lamp operates on 120-V AC power, but you're visiting a country with 240-V AC power. You plug a travel adapter into the 240 V AC outlet and its trasnformer provides your lamp with the 120-V AC power it expects. Compare the number of turns in the transformer's two coils.

the transformer's secondary coil has half as many turns as its primary coil. To step down the voltage, a transformer must have fewer turns in its secondary coil than in its primary coil. Fewer turns leads to a smaller emf in the secondary coil and a smaller output voltage for the transformer.

EX: sticking your finger into an electrical outlet isn't a good idea, but is there a moment when you could do it without getting a shock?

yes, you could do it at the moment the voltages are reversing. While the ground and neutral wires of the electrical outlet are normally without charge and therefore relatively safe, the hot wire is usually charged and dangerous. That hot wire's voltage alternates rapidly between high positive voltage and high negative voltage. Only when its passing through 0V can you touch it without risking a shock. -edison was opposed to AC, viewed it as dangerous and its fluctuating voltages and moments without power make it unattractive.

AC power distribution

-basic conflcts of power transmission: to minimize resistive heating in the power lines connecting a power plant with a distant city, electric power should travel through those lines as small currents at very high voltages. To be practical and safe, electrical power should be delved to homes as large currents at modest voltages. -no simple way to meet both simultaneously with DC, transformers make it easy to satisfy them both with AC. We can use a step up transformer to produce the very high voltage current suitable for cross country transmission and a step down transformer to produce the low voltage current that's appropriate for delivering to communities. --at the power plant, the generator pushes a huge AC current through the primary circuit of a step-up transformer at a supply voltage of about 5000 V. The current flowing through the secondary circuit is only about 1/100 the current in the primary circuit, but the voltage supplied by the secondary coil is much higher, typically about 500,00 V. -this transformer's secondary circuit is very long, extending all the way to the city where the power is to be used. Since the curren in this circuit is modest, the power wasted in heating the wires is within tolerable limits. -once it arrives in the city, this very high voltage current passes through the primary coil of a step down transformer. The voltage providing by the secondary coil of this transformer is only about 1/100 the voltage supplied to its primary coil, but the current is flowing through the secondary circuit is about 100 times the current in its primary circuit. -now the voltage is reasonable for use in a city. Before entering homes, this voltage is reduced still further by other transformers. The final step-down transformers can frequently be seen as oil-drum size metal cans hanging form utility poles. Current enters the buildings at between 110 and 240 V, depending on the local standards. Although 240 V electricity wastes less power in home wiring, it more dangerous than 110 V power.

direct current

-each of the Edison Electric Light company's generators acted like a mechanical battery, producing DC that always left the generator through one wire and returned through another. -Edison placed his generators in central locations and conducted the current to and from the homes he served through copper wires. The farther a building was from the generator, the thicker the copper wire had to be---> wire impede the flow of current and making them thicker allows them to carry current more easily.

step down transfromer

-fewer turns in secondary circuit -smaller voltage rise -a large current at low voltage flows in the secondary circuit

step up transformer

-have more secondary turns than primary turns and that provide secondary voltages that are greater than their primary voltages. the transformer that powers a neon sign typically has 100 times as many turns in its secondary coil as its primary coil. When its primary coil is supplied by 120 V AC power, its secondary coil provides the 12,000 V AC power needed to illuminate the neon tube. -even when primary and seconardy voltages are different,t a transformer manages to conserve energy. As each additional secondary coil turn increases the secondary coil's voltage, it also increase the rate at which the secondary current withdraws energy from the transformer's magnetic field. -If you scale up the number of turns in the secondary coil to increases its voltage, you must scale down the current flowing through that coil to leave the amount of energy that it withdraws from the magnetic field unchanged. As a result: the secondary current is equal to the primary current times the reaction of primary turns to secondary turns: secondary current=primary current x primary turns/secondary turns.

voltage hierarchy

-high voltage is dangerous -high current is wasteful -using the following hierarchy: low voltage circuits in neighbor hoods (120V) -medium voltage circuits in citiies (12,000 V) -high voltage circuits across the countryside (500,000V) -uses transformers to transfer power.

alternating current (AC)

-household/power plant

magnetic induction and transformer

-if there are mobile electric charges around to respond to that electric field, they'll accelerate and you'll have created or altered an electric current and possibly have done work too. -this process whereby a time-changing magnetic field initiates or influences and electric current is magnetic indication. --transformer combines these two connections in sequence--electricity produces magnetism produces electricity.

difference in changing voltage

-in any transfomrer, the secondary coil experiences an induced emf that depends on its number of turns, the number of times its wires encircles the core. The more loops the secondary current makes around the core, the more work the transformer's electric field does on that current and the large the induced emf. Since the amount of work does is proportional to the number of turns, so is the secondary coil's induced emf. -actual induced me in a specific transformer: suppose that we have a simple transformer in which the 2 coils, primary and secondary are identical-equal amount of turns. That primary coil of that transformer is plugged into a 120 V AC outlet.—> since the transformer's primary coil acts as an inductors, it back emf mirror the AC voltage applied and it is therefore 120 AC. However, b/c the two coils are identical and share the same electromagnetic environment, that same induced emf appears in the secondary coil, 120 V Ac. If we connect the secondary coil to an appropriate bulb to form a complete circuit, the secondary coil will act as a source of 120 V AC electric power and light up the bulb. -known as an isolation transformer: when you plug its primary coil into an AC outlet, its secondary coil acts as a source of AC power at the outlet's voltage. Although its secondary coil merely mimics the power outlet, an isolation transformer provides an important measure of electrical safety. Since its primary and secondary circuits are electrically isolate, charge can't move between those circuits and cause trouble.—> ex: when lightening strikes the power company's wire, the resulting burst of charge on the primary circuit can't pass to any appliances that are part of the seconardy circuit.

induced emf

-magnetic induction does more than just push currents around; it can also transfer energy. ITs induced electric field does work on any charge that moves with its push and does negative work on any charge that moves opposite its push. -when induction does work on a charge that goes through a coil, that charge experiences a rise in voltage. The overall voltage rise, from the coil's start to finish is called the induced emf. B/c the coil's current and induced electric field alternate, so does the induced emf-it sings between negative and positive voltages. -Energy alternately leaves the current and returns. -where does the energy reside when its not in the current? -> missing energy is in the coil's magnetic field! Mangnetic fields contain energy. The amount of energy in a uniform magnetic field is half the square of the field strength times the volume of the field dived by the permeability of free space--> energy= magnetic field^2 x volume / 2 x permeability of free space. -the coil is playing with the AC's energy, strong it briefly in the magnetic field and then retuning it to the current. The coil stores energy while the magnitude of the current increase-the field strengthens and the current loses voltages. --> the coil returns energy while the magnitude of the current decreases-the field weakens and the current gains voltage. B/c the coil's self induced emf is responsbile for the bouncing this energy back to the current is is called a back emf.

Wires and DC

-wire thickness is important because they have electrical resistance. -In accordance with Ohm's law the voltage drop through a wire is equal to its electrical resistance times the current passing through it. -In the case of a wire conducting current from a generating plant to a home, primary concern is how much power the wire wastes as thermal power. ---> determined this waster power by combining Ohm's law with the equation for power consumed by a device--> power consumed=voltage drop x current =(current x electrical resistance) x current -current^2 x electrical resistnace. -wire's wasted power is proportional to the square of the current passing through it. -The more current edison tried to deliver over a wire, the more power it lost. Doubling the current in the wire quadrupled the power it wasted. ---> tried to combat this loss by lowering electrical resistance of the wires-> used copper b/c only silver is a better conductor of current, used thick wires to increase the number of moving charged, kept wires short so they didn't have much chance to waste power. -avoid waste by delivering smaller currents at higher voltage. Although less current flowed through each home, the voltage drop was larger so the power delivered was unchanged. -high voltages are dangerous--> creat sparks as current jumps through air.

three steps in transformers

1: changing electric current produces changing magnetic field 2: changing magnetic field travels through core to secondary circuit 3: changing magnetic field in core produces changing current in secondary circuit.

EX: of 2 coil transformer

Ex: lamp: consists of a two-coil transformer and a halogen lightbulb. One coil of the transformer ,the primary coil, is plugged directly into an electrical outlet and completes a circuit with the power company. The power company pushes an AC through this primary cicuit. The other coil of the transformer, the secondary coil, is connected to the lightbulb and completes another circuit-the seconadary circuit. To ensure that the two coils share the same electromagnetic environment, both are wound around a sign shaped magnetizable core. -by itself the primary coil acts as an inductor, alternaterly storing energy in its magnetic field and then retuning that energy to the primary current by way of its back emf. -since this back emf mirror the supply voltage, we will suppose its 120 volts Ac, the back is the same.—> however, because the secondary coil shares the primary coil's electromagnetic environment, the secondary coil also experimences an induced emf and a voltage difference appears between its 2 ends. Since the secondary coil forms a circuit with the lam's filament, this voltage difference imposes a voltage drop on the filament and propels a current through it. That current alternates b/c the emf alternates. In short, power is moving via electromagnetic action from an AC in the primary circuit to an AC in the secondary circuit and lighting up bulb. -if transformer is providing power to current in its secondary circuit, it must be removing the same amount of power from current in its primary circuit. —> using magnetic indiction to do that. This time the induction is reversed-the current in the secondary circuit is inducing and emf in the primary coil and that emf is removing power from the primary current! This removal happens: the emf induced in the primary coil increases the primary current whenever its investing energy in the transformer's magnetic field and decreases that current whenever its withdrawing energy from the field. With more investment than withdrawal, the primary current is leaving energy begin in the magnetic field and the secondary current is carrying that energy away. -the power transfer process responds automatically to any changes in the secondary circuit's power consumption. For example, if you replace the desk lamp's build with one that consumes more power, more current will flow through the secondary current. If you remove the bulb, the secondary current will vanish, and the primary current's energy investment and withdrawal will balance perfect. although it implies that a transformer with nothing attached to its secondary coil consumes no power, you'll save energy by unplugging transformers that aren't providing any power.

Ex: when you pedal a high-tech exercise bike, you are probably spinning the rotor of an electric generator. That generator supplies power to a heating filament with an adjustable electrical resistance. How should the bike alter that electrical resistance to make pedaling more difficult?

It should reduce the heater's resistance. By lowering the heater's resistance, the bike increases the current flowing through the circuit. That increased current carries more power from the generator to the heater, so the generator extracts more mechanical works from the bike.