4.1 Picturing Voltage

What is voltage? Voltage is the amount of power each coulomb of electricity can deliver. If person A has 1 coulomb of charge at 5 volts and person B has 1 coulomb of charge at 10 volts, that means that person B’s coulomb can deliver twice as much power as person A’s.

A good analogy to electronics is the flow of water. When comparing water to electricity, coulombs are a similar unit to liters—coulombs measure the amount of electric charge present just like a liter is the amount of water volume present. Both charge and water move as a flow. In water, we can measure the flow of a current of a stream in liters per second. Likewise, in electronics, we measure the flow of charge through a wire in coulombs per second, which are also called amperes.

Now, I want you to image the end of a hose through which water is flowing. Normally, the water just falls out of the hose, especially if the hose is just sitting on the ground. That hose just sitting on the ground is like a current with 0 volt—each unit of water or charge is just not doing that much.

Let’s pretend we added a spray nozzle to the hose. What happens now? Water shoots out of the nozzle forcefully. We haven’t added any more water—it is actually the same amount of water (i.e., current) flowing. Instead, we increased the pressure of the water, which is just like increasing the voltage on an electric charge. By increasing the pressure, we changed the amount of work that each liter of water is available to perform.

Likewise, when we increase voltage, we change the amount of work that each coulomb of electricity can do.

One way we might measure the pressure of water coming out of a hose is to measure how far up it can shoot out of the hose. By doubling the pressure of the water, we can double how far out of the hose it can shoot. Similarly, with voltages, large enough voltages can actually jump air gaps across circuits. However, to do this, it takes a lot of voltage—about 30,000 volts per inch of gap. If you have been shocked by static electricity, though, this is what is happening! The power of the charge is extreme (thousands of volts), but the amount of charge in those shocks is so small that it doesn’t harm you (about 0.00000001 coulomb).

4.2 Volts Are Relative

While charge and current are fairly concrete ideas, voltage is a much more relative idea. You can actually never measure voltage absolutely. All voltage measurements are actually relative to other voltages. That is, I can’t actually say that my electric charge has exactly 1, 2, 3, or whatever volts. Instead, what I have to do is say that one charge is however many volts more or less than another charge. So let’s take a 9-volt battery. What that means is not that the battery is 9 volts in any absolute sense, but rather that there is a 9 -volt difference between the charge at the positive terminal and the charge at the negative terminal. That is, the pressure with which charge is trying to move from the positive terminal to the negative terminal is 9 volts.

4.3 Relative Voltages and Ground Potential

When we get to actually measuring voltages on a circuit, we will only be measuring voltage differences on the circuit. So, to measure voltage, I can’t just put a probe on one place on the circuit. Instead, I have to put my probe on two different places on the circuit and measure the voltage difference (also called the voltage drop) between those two points.

However, to simplify calculations and discussions, we usually choose some point on the circuit to represent “0 volt.” This gives us a way to standardize voltage measurements on a circuit, since they are all given relative to the same point. In theory, this could be any point on the circuit, but, usually, we choose the negative terminal on the battery to represent 0 volt.

This “zero point” goes by several names, the most popular of which is ground (often abbreviated as GND). It is called the ground because, historically, the physical ground has often been used as a reference voltage for circuits. Using the physical ground as the zero point allows you to also compare voltages between circuits with different power supplies. However, in our circuits, when we refer to the ground, we are referring to the negative terminal on the battery, which we are designating as 0 volt.

Another, lesser-used term for this designated 0-volt reference is the common point. Many multimeters label one of their electrodes as COM, for the common electrode. When analyzing a circuit’s voltage, this electrode would be connected to whatever your 0-volt point is.

This “ground” analogy also makes sense with our water hose analogy. Remember that a voltage is the potential for a charge to do work. What happens to water after it lands on the ground? By the time the water from my hose lands on the ground, it has lost all its energy. It is just sitting there. Sure, it may seep or flow around a bit, but nothing of consequence. All of its ability to do work—to move quickly or to knock something over—has been drained. It is just on the ground. Likewise, when our electric charge is all puttered out, we say that it has reached “ground potential.”

So, even though we could designate any point as being zero, we usually designate the negative terminal of the battery as the zero point, indicating that by the time electricity reaches that point, it has used up all of its potential energy—it now has 0 volt compared to the end destination (i.e., the other battery terminal).

4.4 Resistance

Resistance is how much a circuit or device resists the flow of current. Resistance is measured in ohms and is usually represented by the symbol Ω. Going back to our water hose analogy, resistance is how small the hose is, because a smaller hose will resist the flow of water more than a larger hose will. Think about a 2-liter bottle of pop. The bottle has a wide base, but the opening is small. If I turn the bottle upside down, the small opening limits the amount of liquid that flows out at one time. That small opening is giving resistance to the flow of liquid, making it flow more slowly. If you cut off the small opening, leaving a large opening, the liquid will come out much faster because there is less resistance.

Ohm’s law, which we will use throughout this book, tells us about the relationship between resistance, voltage, and current flow. The equation is very simple. It says

(4.1)

In this equation, V stands for voltage, I stands for current (in amperes, not milliamperes), and R stands for resistance (in ohms). To understand what this equation means, let’s think again about water hoses. The water that comes out of the faucet of your house has essentially a constant current. Therefore, according to the equation, if we add resistance, it will increase our voltage.

We know this to be true from experience. If we have a hose and just point it forward, water usually comes out about a foot or two. Remember, voltage is how much push the water has, which determines how far the water will go when it leaves the hose. However, if my children are on the other side of the yard and I want to hit them with a water spray, what do I do? I put my thumb over the opening. This increases the resistance, and, since the current is relatively constant, the voltage (the force the water will have when it leaves the hose) will increase, and this force will cause the water to spray further.

However, in circuits, we usually don’t have a constant current source. Instead, batteries provide a constant voltage source. A 9-volt battery will provide 9 volts in nearly every condition. Therefore, for electronics work, we usually rearrange the equation a little bit. Using a little bit of algebra, we can solve our equation for either current or resistance, like this:

(4.2)

(4.3)

4.5 Review

4.6 Apply What You Have Learned