Current in Amperes (I) = Potential Energy in Volts (V) / Resistance in Ohms (R). Let’s just call potential energy voltage. This is known as Ohm’s Law and it also means that V = I x R and R = V / I. In resistive circuits, a voltage coming into the circuit can be controlled to power any device of any voltage and current limits using resistors. For instance, a light bulb requires 6 volts and has an operating current of 0.6 amps. To light the bulb with a 9 volt battery, we can use two resistors and Ohm’s Law.
We know that the bulb draws 0.6 amps. Calculating the resistance using the battery voltage, 9 / 0.6 = 15. A 15 ohm resistor will drop 9 volts at 0.6 amps. Knowing this, we can drop the voltage 3 volts by using a 5 ohm resistor and a 10 ohm resistor connected in series. The point where the two resistors connect is 6 volts, because two resistors in series adds resistance (5 + 10 = 15) and 6 is one-third of 9 for voltage, so 5 is one-third of 15 for resistance.
From this simple explanation of an electrical circuit comes a more complex issue of exactly what is going on. We’re simply assuming that the voltage comes from a infinite source, but to explain current it might be better to look at voltage from a limited source. Say that the source is a battery. The battery holds a charge and as the current is drawn from the battery, it starts losing charge. The current is the amount of charge flowing past a point in a given amount of time. So voltage is the potential energy available with no resistance and current is the amount of charge flowing at a given time. With resistance, we have a way of blocking the flow of energy.
If we needed much lower current (which would help to keep the charge in the battery longer), we could use higher resistance. The 1/3 volt drop would still require the connection point to be 1/3 of the total resistance. We could do the opposite to get a higher current. This all sound really simple until we use 1/4 watt resistors for the voltage drop. Uh oh, there’s smoke!
What are watts? That is the unit of power (P) and in Ohm’s Law it’s signified as P = I x V. If 9 volts is multiplied by 0.6 amps, the power is 5.4 watts. Those .25 watt resistors didn’t stand a chance. In this case, 10 watt resistors work fine. Then, there is the battery itself. 0.6 amps from a 9 volt battery will drain it in no time. An engineer would not design this, fortunately. This is a good way to describe how electrical circuits work using the most simple law. The best thing about it is that this law is absolute.
There is much more involved in electronics. This circuit was designed for Direct Current (DC). That is what a battery puts out. If I used Ohm’s Law to design a circuit that handled 120 volts and plugged it into the wall, it would still follow the law, but now we’re working with Alternating current (AC). When Thomas Edison was vying for the nation’s power grid, he was promoting the use of DC to enter each home. On the other hand, Nikola Tesla was promoting the use of AC. Both had applied successful power grids but Tesla’s won out.
I’m not the greatest physicist, but there is no such thing as a perfect conductor. Wires have resistance and longer wires have higher resistance. As Ohm’s Law states, the higher the resistance, the greater the voltage drop at a given current. This is true for both AC and DC, but it turned out that by alternating the current, there is less of a resistive effect in the conducting wires. Remember what happened when the resistors smoked? Resistance, in the wires creates heat. But with AC, there is a new effect called impedance.
Resistance is the opposition to a steady electric current. That’s what direct current faces when travelling through a wire. With alternating current, the resistance of the wire is far less a factor. The current changes direction sixty times per second here in the U.S. (fifty times per second in Europe). Now a far less voltage drop occurs in the wires and much less heat is produced. Of course, AC has its own resistive property called Reactance. Here is an introduction to capacitance and induction.
In DC, a simple insulator can come between two wires and no electrical flow will occur. But, if the direction of current continuously changes, the insulator has no potential effect. One side of the insulator will build a charge, then discharge when the flow reverses. In electronics there are capacitors. In DC, the capacitor can store a charge like a battery, but the discharge is quick. In AC, which is what radio and audio signals are basically, capacitors are used to allow the signal to pass while blocking the DC voltage from the next stage of the circuit. This effect is only relevant to AC power when considering that a two-conductor wire is in itself a capacitor. Under the right circumstances, the wire can be considered a short.
Then there’s reactance. Reactance has no effect in DC but creates resistance in AC. The simplest way to create reactance is to create inductance. Now I may be going too far. Without explaining, an inductor is a coiled wire. In AC, a coiled wire acts as a resistor, but in DC it does not. So coils and capacitors impact what comes from your AC socket. But here is why AC is used to give us power:
Reactance does not create heat; only DC resistance creates heat. The AC coming through the power lines ignores the DC resistance and only impedance has an effect on the signal. Reactance and capacitance effect impedance. Impedance is measured in ohms. when working with impedance, the most conductance of energy occurs when the impedance of a circuit is connected using a cable or wires that have the exact same impedance. That allows the signal (remember that AC, audio, and radio signals use the same properties) to pass without reflection. Any reflection of the signal results in a drop of the signal. Since the signal is the voltage, a properly transmitted AC voltage has virtually no voltage drop through the power grid.
This is why AC is used. It is safer and creates no heat. Getting shocked can be deadly with either method, but heat can start fires. This little tutorial started out with an explanation of simple battery circuitry, but ended up getting really complex. Think about a radio. The simple DC circuit provides the voltage and current to operate the components but the AC reactance, inductance, capacitance and impedance are factors with the reception of the radio signal and conversion to audio. I hope that using our power grid helped in any way to understanding some electronics. Discovering these led to a method of transmitting audio, creating radio, and that could be explained at another time.