Physics - Electricity



Alternate and direct current

They are both electric currents (movements of electrons), but what is the difference?

DC consists of electrons flowing at a constant speed (what speed?) and always on the same direction. They travel through the wire.That is just what people normally imagine and expect of an electric current. DC is normally used in low power systems, the ones that are typically powered by a battery. Examples are cell phones, mp3 players, equipments on board vehicles, satellites, etc.

In the case of AC, the electrons don't travel through the wire, e.g. they oscillate around the same equilibrium position. That sounds a bit weird but it is the true. Electrons only move about 2.1*10-6 m in either direction. Although electrons don't move forward, the electromagnetic waves produced by their oscillations do move forward. And this is the so called electric energy that travels in power lines until it is consumed in homes and factories.

AC used have higher voltages and currents. In America AC has 110 V and 60 Hz. That means that the electrons oscillate 60 times every second!

The first power lines, up to around 1887, used DC (because AC hasn't yet been discovered) . Tesla, who was one of the greatest inventors of the 20th century, invented AC and demonstrated that it is much more efficient to transport electrical energy, as DC caused a lot of losses on the wires themselves, mainly by heat dissipation.

An interesting movie depicts the life of Tesla and the AC revolution, among other oinventions, like the fantastic Tesla coil.

::At what speed electrons travel inside a wire?::

Firstly, we must talk about DC here, because in AC electrons don't travel, only oscillate.

Electrons travel at about 3*10-4 m/s . Probably slower that you used to think... At this speed it takes about 1 hour to travel only 1 mile!

Note: This speed is the drift speed: the speed at which electrons travel forward. It is important to remember that there is also a speed related to the thermal motion, so that electrons are always moving in all directions at high speeds, even when there is no voltage applied and as a result no drift. The speed due to thermal motion iss of the order of 1.6*106 m/s !!!!

This is similar to what occurs in air. Unless there is a brise (drift of air molecules), we tend to think that the air is not moving. But its molecules are always travelling and at the temperature of 20°C their speeds are around 500 m/s. Because this movement is chaotic, equal in all directions, it averages out to zero and we do't fell it.

The cooler it gets , the lower molecules travel in a gas (electrons inside a wire are not a gas but they behave like one).


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