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Tesla imagined impossible technologies more than a hundred years ago. He and Thomas Edison have arguably had more of an impact on modern technology than any other scientists.

Tesla’s biggest contribution may be his innovations in alternating current technology, and the invention of the AC motor.
The adoption of this technology was not easy because Thomas Edison’s direct current systems had been the standard early on. What is AC and DC, and why is one superior to the other?

Atoms have a positively charged nucleus and negatively charged electrons surrounding the nucleus. But electrons in the outermost shell of the atom, called the valence shell, can sometime become free due to external forces. These electrons can move from one atom to another. This is what can cause a movement of charge, which is what electricity is. An electrical current is the flow of free electrons from one atom to another.

The relationship between current, voltage, and resistance is described by Ohm’s law, Voltage = Current x Resistance

Current (amps) is the rate at which the charges flow. It is analogous to the rate of flow of the water in a hose. Voltage (volts) is the force required to make current flow. It is analogous to the water pressure in a hose. Resistance (ohms) is a material’s tendency to resist the flow of charge. This is like diameter of the hose. The smaller the hose, the higher the resistance.

DC is similar to the normal flow of water through the hose that we see. The water flows in one direction. AC is like the water flowing back and forth within the hose 50 or 60 times per second, 50 Hertz or 60 Hertz. This is where the water analogy is not so great, because water doesn’t flow back and forth in a hose.

AC won over DC because of efficiency and power delivery. Power is like the volume of water coming out of the hose.

The formula formula for power is P = I x V. The same amount of power can be transmitted either at high current and low voltage, or low current and high voltage. But one is better than the other.

The longer this cable is, the more resistance there is in the power line. When you pass a current through resistance, you create heat, given by Joule’s equation for electric heat, Heat = I^2 R

Heat is wasted energy, so it is crucial to minimize it. Since I = P/V, if we substitute it back into the heat equation. We see that Heat equals Power squared times resistance R, divided by voltage squared. We want to MAXIMIZE voltage V to minimize heat.

In modern electric power grids, electricity is routinely transmitted for hundreds of kilometers at hundreds of thousands of volts. But voltage can't be this high when it arrives at your home because it would be dangerous. So, it has to be stepped down before it gets to your house. This is done via a transformer which reduces voltage from hundreds of thousands of volts to between 110 to 240 volts.

This stepping up and stepping down of voltage is where alternating current is superior to direct current. Direct current cannot be easily transformed from low voltage to high voltage and visa versa, but alternating current can. And here’s the reason why: When alternating current passes through a coil, it produces a constantly changing magnetic flux, per Maxwell’s equations. If we put a loop or ring of iron through the coil, it can concentrate the changing magnetic flux to within the ring.

Now if we wind another coil around the other side of this ring, we can create electricity and induce voltage within the new coil. The voltage created in the second coil is proportional to the number of loops we place around the iron ring. Using this method we can transform voltage from high to low and vice versa.

But transformers like this require a time-varying voltage to function, so only alternating current works. In Edison and Tesla’s time, there was no easy way to transform voltage with direct current.

Can devices work equally well with DC or AC current? Many devices—like light bulbs—can.

Was DC really a losing concept? No. Most high tech appliances today that are powered batteries, like your laptop, cell phone, and iPad are all powered by direct current.
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Also, in the 20th century, engineers figured out a way to transmit electricity using high voltage direct current, or HVDC. HVDC is even more efficient than high voltage AC for transmitting electricity over very large distances, because smaller cheaper lines can be used to transmit the same amount of power, and there is less induction loss because no changing magnetic field exists with DC, unlike with AC. But the cost of DC transformers is millions of dollars vs. thousands of dollars for AC transformers. So HVDC is only cost effective for very long transmission lines.