Tag: electric power distribution

What is the hum you hear when walking under large power lines?

The electric currents in those lines are reversing 120 times a second in the United States (60 full cycles of reversal, over and back, each second). That means that the electrostatic forces between the charges they carry and anything nearby reverse 120 time a second and the magnetic forces that they exert on one another when currents flow through them turn on and off as well. You hear all of the motions that are caused by the pulsating electric and magnetic forces.

How can we talk about positive particles flowing through wires when it is really negatively charged electrons?

The fiction of current being carried by positive charges really does work nicely. If a wire is carrying negatively charged electrons to the east, then the east end of the wire is becoming more and more negative and the west end is becoming more and more positive. The same would happen if that wire were carrying positively charged particles to the west. Even though these positively charged particles aren’t really there, we can pretend that they are. By pretending that current is carried by positive particles, we don’t have to worry about the arrival of a positive number of negatively charged electrons lowering the voltage of an object.

What is the purpose of the iron core in a transformer?

The iron core of a transformer stores energy as power is being transferred from the primary circuit to the secondary circuit. This energy is stored as the magnetization of that iron. The transformer needs to store that energy for roughly one half cycle of the alternating current or about 1/120th of a second. The more iron there is in the transformer, the more energy it can store and the more power the transformer can transfer from the primary circuit to the secondary circuit. Without any iron, the energy must be stored directly in empty space, again as a magnetization. But space isn’t as good at storing magnetic energy as iron is so the iron increases the power-handling capacity of a transformer. Without the iron, the transformer must operate at much higher frequencies of alternating current in order to transfer reasonable amounts of power.

How does hydroelectric power work?

Hydroelectric power begins with water descending from an elevated reservoir, such as a lake in the mountains. While it’s in the elevated reservoir, this water has stored energy—in the form of gravitational potential energy. As this water flows downward through a pipe, its gravitational potential energy becomes either kinetic energy or pressure potential energy or both. By the time the water arrives at the hydroelectric power plant, it is either traveling very quickly or has an enormous pressure or both. In the power plant, the water flows past the blades of a huge turbine and does work on those blades. The blades are shaped somewhat like airplane wings and they “fly” through the moving water. Since the blades are attached to a central hub, they cause this hub to rotate and allow it to turn the rotor of a huge electric generator. The rotor of this generator typically contains a giant electromagnet. The electromagnet turns within a collection of stationary wire coils and it induces electric currents in those coils. These electric currents carry power out of the generator to the homes or business that need it.

What makes alternating current alternate?

The pump for alternating current (usually an electrical generator) creates electric fields that reverse their directions 120 times a second (60 full cycles of reversal, over and back, each second). This reversal pushes the current backward and forward through the wires connecting to this power source. The currents direction of flow alternates and so does its voltage.

How does power get from the plant to my house? Where do the voltages go up and down?

The voltage is stepped up at the power plant so that a small current of very high voltage charges (high energy per charge) can carry enormous power across the countryside. When this current arrives at your city, its voltage is stepped down so that a medium current of medium high voltage charges can carry that same enormous power through your city. Finally, near your house, its voltage is again stepped down so that a large current of low voltage charges can carry this power into your house. Naturally, you do not use all of the power from the power plant yourself, so it is distributed among all of the buildings in the city.

When going from 12 volts to 240 volts, is the point that with higher voltage the power transfer proceeds with fewer particles?

Yes. If you use higher voltages, you can transfer the same amount of power with a small current of charged particles. The energy lost in the transmission through wires increases as the square of the amount of current through those wires so reducing that current is very important.

How is AC converted in certain items to DC?

These devices use diodes, which are one-way devices for current. They only allow the current to flow a certain direction and block its flow the other way. With the help of some charge storage devices called capacitors, these diodes can stop the reversals of AC and turn it into DC. Those little black battery eliminators that you use for household electronic devices contain a transformer, a few diodes and a capacitor or two.

A step-up transformer has a secondary coil with many, many turns. As the current in the primary circuit flows back and forth, it creates a reversing electric field around the iron core of the transformer. This electric field pushes charges through the secondary coil so that it travels around and around the core. Each charge goes around many times, picking up more energy with each passage. By the time the charge leaves the transformer, it has lots of energy so its voltage is very high.

When you say that a transformer can change a small current with a high voltage into a large current with a low voltage, where do those extra charges come from?

A transformer involves two completely separate circuits: a primary circuit and a secondary circuit. Charges circulate within each circuit, but do not move from one circuit to the other. If the primary circuit of a transformer has a small current flowing through it and that current experiences a large voltage drop as it flows through the transformer’s primary coil, then the primary circuit current is transferring power to the transformer and that power is equal to the product of the primary circuit current times the voltage drop. The transformer transfers this power to the current flowing in the secondary circuit, which is an entirely separate current. That current may be quite large, in which case each charge only receives a modest amount of energy as it passes through the secondary coil. As a result, the voltage rise across the secondary coil is relatively small. The power the transformer is transferring to the secondary circuit current is equal to the product of the secondary circuit current times the voltage rise.

How is AC current (alternating current) made?

Usually with alternating current generators, which we will discuss next. It can also be made by electronic alternators, such as those found in the uninterruptible power supplies that provide backups for computers.