What value remains the same in Circuit A?
Around Circuit A, what values decrease in the direction of the arrows? Check all that apply.
The difference between the voltage going into and coming out of a Town is always how many volts?
The difference between the power going into and coming out of a Town is always how many watts?
Those 1,152 watts of power you just computed above across each Town represents the useful amount of energy
supplied by the power plant, that can be used to cool homes, light streets, run the dishwasher, etc.
The total power generated by the power plant is shown in the top left box of each circuit. So
the power plant in Circuit A generates 10,464 watts.
Compare the power generated by Power Plant A and the useful power used by Town A:
The Town uses more power than the Power Plant generates
The Power Plant generates just about the same amount of power as the Town uses
The Power Plant generates a little more energy than the Town uses
The Power Plant generates much more power than the Town uses.
Where does all the missing power go?
It just magically disappears
The wires that carry the electricity to the Town heat up, and that wastes the electricity
The energy is recycled by the Power Plant and used again
So, now we see mostly how a power grid works. Electrical power is generated in
the Power Plant, and sent along wires to the towns where it will be used. Along
the way, power is lost in the wires as heat and other excess radiation.
That means the goal of a Power Grid Engineer is to reduce the energy lost
in the power lines as much as possible.
Let's compare the three circuits below to see which does the best job
of reducing power lost.
Complete this table:
Which circuit wastes the least power?
Yes, Circuit C loses only 182 watts out of the 1334 watts it generates.
In other words, only 14% of the energy is wasted.
Circuit A wastes a whopping 89% !
What are the differences between Circuit A and Circuit C? Check all that apply.
Town C is closer to the power plant, so the wires are shorter.
Town C uses less power than Town A
The wires between the power plant and Town C go through two electrical substations
Great, yes, shorter wires and substations must explain the difference.
Let's examine the shorter wires. Circuit B and Circuit C are almost identical, except that
the distance between the Circuit B substations is twice the same distance in Circuit C.
So, if we can investigate the differences in those circuits, we can see how
longer or shorter wires affect the power loss.
Let's compute the power lost in each wire. Complete the table below:
Compare the power losses computed above, and check all that apply:
Most of the power lost is in the long stretch between the two substations
The wires between the substations in City B lose about twice the power as the same wires in City C
The voltage drops in the wires between the substations are about the same in the two Circuits
The current in the wires between the substations are about the same in the two Circuits
YES! So, the shorter the power line, the less electricity it wastes.
Now, let's see what the substations are doing, because Circuit B still wastes less power than Circuit A, and their wires are exactly the same length. Compare the voltages entering (293 volts) and leaving (2032 volts) the first substation in Circuit B.
The voltage leaving the substation is about 5 times the voltage entering
The voltage leaving the substation is about 6 times the voltage entering.
The voltage leaving the substation is about 7 times the voltage entering.
The voltage leaving the substation is about 8 times the voltage entering.
Ok, so the voltage increases a lot inside the first substation. What about the current?
Compare the current entering (4.8 amps) and leaving (0.68 amps) the first substation in Circuit B.
The current leaving the substation is about 5 times less than the voltage entering
The current leaving the substation is about 6 times less than the voltage entering
The current leaving the substation is about 7 times less than the voltage entering
The current leaving the substation is about 8 times less than the voltage entering
Lastly, what about the power? How does the power change as the electricity passes through the first substation?
The power drops a lot in the substation
The power mostly stays the same in the substation, maybe dropping just a bit
The power increases a lot in the substation
It turns out that an electrical substation is mostly just a big
A transformer is an electrical device that can easily increase or decrease the voltage
in a wire, while leaving the power mostly unchanged.
If it doubles the voltage, it cuts the current in half.
So, Circuit B might lose less power than Circuit A because
the first substation drastically increases the voltage and drastically decreases
the current in those long wires.
Can we understand why? The reason these wires lose energy is because they have
Resistance. Resistance is complicated, but everything
(except maybe a Superconductor) has some Resistance, and it drains energy
from your circuit.
A scientist named Jeorg Ohm figured out the relationship between Voltage, Current,
and Resistance, so there's a rule called
Ohm's Law that says:
Voltage = Resistance x Current
A little algebra rewrites that as:
We measure Resistance in Ohms, in honor of Jeorg Ohm.
Resistance = Voltage ÷ Current
Use the equation above to compute the resistance in the wires.
Let's analyze our results above. Compare the resistance of the wires in Circuit B to the resistance of the single long wire in Circuit A:
Circuit A has a lot less resistance
Both circuits have about the same resistance.
Circuit B has a lot less resistance.
So, the wires in the two circuits are pretty much the same. So, why does Circuit A lose so much more power? Let's do a bit more algebra, and combine the two equations that we know:
If we re-arrange them a bit, we see that:
Voltage = Resistance x Current
Power = Voltage x Current
That means that the Power lost in a wire depends on the resistance of the wire but also, more importantly, on the Current in the wire. If we double the current in the wire, we quadruple the power lost!
Power = Resistance x Current x Current
Now, finally, we understand why Circuit A loses so much more power than Circuit B.
Circuit B loses less energy in the long wires between the substations because the current in those wires is much less than the current in the wires in Circuit A.
One final question: if you are designing a power grid, how would you best minimize power losses in your circuits? Check all that apply.
String long wires straight from your Power Plant to the town, to minimize Resistance in your circuit
Add substations to your circuits to get the current as low as possible in your wires, since lower current means less power wasted to resistance
Keep your Power Plants as close as possible to the towns they are serving, since the power loss in a wire increases as the wire gets longer.
Do your results help explain why power is transferred for long distances
via huge towers instead of regular telephone poles?
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