Tag: electric power distribution

What is the difference between current and voltage?

Current is the measure of how many charges are flowing through a wire each second. A 1-ampere current involves the movement of 1 Coulomb of charge (6,250,000,000,000,000,000 elementary charges) per second. Voltage is the measure of how much energy each charge has. A 1-volt charge carries 1 Joule of energy per Coulomb of charge. To use water in a pipe as an analogy, current measures the amount of water flowing through the pipe and voltage measures the pressure (or energy per liter) of that water.

What is the difference between single-phase and three-phase electric power?

In single-phase power, current flows to and from a device through a pair of wires. The direction of the current flow changes with time, reversing smoothly 120 times a second in the US or 100 times a second in Europe (60 or 50 full cycles of reversal, over and back, each second respectively). In its simplest form, one of the two wires is called “neutral” and its voltage is always close to 0 volts (meaning that it has essentially no net electric charge on it). The other wire is called “power” and its voltage fluctuates from positive to negative to positive many times a second (meaning that its net electric charge varies from positive to negative to positive). The difference in voltage between “neutral” and “power” propels current through the device.

In three-phase power, current flows to and from a device through a group of three wires. These three wires are often called “X”, “Y”, and “Z”, and each one is a power wire with a voltage that fluctuates from positive to negative to positive many times a second. (A fourth wire, “neutral”, with a voltage of approximately 0 volts, may also be used.) But while the voltages of the three power wires fluctuate up and down the same number of times each second, they do not reach their maximum or minimum voltages at the same time. They reach their peaks one after the next in an equally spaced sequence: first “X”, then “Y”, then “Z”, and then “X” again and so on. Because these three wires or “phases” rarely have the same voltages, currents can and do flow between any pair of them. It is such current flows that power the devices that use three-phase electric power. The natural sequencing of the three phases is particularly useful for devices that perform rhythmic tasks. For example, three-phase electric motors often turn in near synchrony with the rising and falling voltages of the phases.

Another advantage of three-phase electric power is that there is never a time when all three phases are at the same voltage. In single-phase power, whenever the two phases have the same voltage there is temporarily no electric power available. That’s why single-phase electric devices must store energy to carry them over those dry spells. However, in three-phase power, a device can always obtain power from at least one pair of phases.

What is the difference, if any, between appliances with a 2 prong plug and a 3 prong plug?

In the 2 prong system, current travels to the appliance through one prong and leaves through the other prong. The roles of the two prongs interchange every 120th of a second. In the 3-prong system, there is one extra prong and that connects the frame of the appliance to the ground (the earth). This extra connection is a safety feature. If a wire comes loose inside the appliance and touches the frame, the frame can deliver charge and current to you through your hand and you can deliver it to the ground through your feet or your other hand. The earth is very large and a large amount of charge can flow into it without repelling further charge. Moreover most electrical systems are actually connected to the ground at some point. So if current can travel out of the circuit feeding power to the appliance and travel through you and into the ground, it will. You’ll get a shock. The ground connection (the extra prong) allows this extra current to flow to ground so easily that a huge current is drawn out of the power source, causing the fuse or circuit breaker in that power source to break the connection. When that power connection is broken, no power can flow to the appliance at all and you can’t get a shock from it. Plastic appliances often omit the extra prong because they have nothing dangerous to touch on their exteriors.

Can you explain power surges?

Sometimes lightning strikes a power line and deposits a large amount of charge on it. This charge has considerable electrostatic potential energy so its voltage is very large (a large positive voltage if the lightning carried positive charge, a large negative voltage if the lightning carried negative charge). A the charge flows outward along the wires, it raises the local voltages of the wires. This sudden, brief increase in the local voltages is what you mean by a power surge. Many devices (e.g. computers and televisions) can be damaged by such a surge in voltage. Even a light bulb can be damaged because the extra voltage pushes too much current through the filament and can burn it out.

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.