Think back to some time when you were taking a shower and someone started washing clothes or running the dishwasher at the same time. What happened to your shower?

We say that the water pressure dropped off because it was being used in different places at the same time.

©GolfC, Adobe Stock Photos

So we all understand water pressure intuitively, from everyday experience.

Let’s run our water tank experiment again, and add a new wrinkle: pressure gauges on both tanks.

Look at the setup from before, but notice that at the bottom of each tank we have now installed a pressure gauge. It essentially tells us how much all that water in the tank is pressing down on the bottom and sides of the tank, because of gravity.

If a tank has a tremendous amount of water in it, it would show a lot of pressure on the gauge. If it has less water, it has less pressure. And if it has no water, then it has no pressure.

The pressure gauge reads in pounds per square inch, or PSI. You can see that the full tank shows 80 PSI, and the empty tank shows 0 PSI. Watch what happens when we open the valve time, paying close attention to the pressure gauges.

What did you observe?

• When we open the valve, water runs between the tanks just like before—same amount of time, same balanced water levels when complete.
• But notice that the pressure in the full tank drops from 80 PSI to 40 PSI when the water stops flowing.
• On the other hand, the pressure in the empty tank rises from 0 PSI to 40 PSI when the water stops.
• The water stops flowing because there is no difference in the pressure between the two tanks. That pressure difference was what pushed the water from the full tank to the empty tank, until they had the same amount of water, and the same final pressure. They both have the same potential.

We actually introduced this concept at the very beginning of the water tank analogy. When we said that the full tank had higher potential than the empty tank, that was because of the pressure difference between the two tanks. Likewise, electricity in a battery has a pressure associated with it, due to the difference in charge (which creates a difference in potential) between the two half-cells.

There is also a “pressure gauge” for electricity, but it doesn’t measure in pounds per square inch. Instead, it measures electrical “pressure” in volts, another extremely important concept in the study of electricity. An electrical pressure gauge is called a voltmeter, and you’ll be learning to use one in this course. The symbol for volts is a capital “V.”

Let’s rerun our battery experiment, this time with a voltmeter in place. You use a voltmeter by pressing its two contact points, called probes, against the two places you want to measure a difference in potential across. In our case, we want to see the difference in pressure between the two half-cells, so we will hold one lead to the positive terminal on the battery (the top, with the little button on it) and the other lead to the negative terminal (the flat area on the bottom of the battery).  This will tell us how much “pressure” the battery is applying to current that flows through our circuit.  We call this our “supply voltage.”

We can also use the voltmeter to measure how much the electric “pressure” drops when the current passes through some important part of a circuit. Let’s run this experiment again, but we’ll check the voltage drop across the battery, which is the part of our circuit that is doing work for us (giving us light).

Watch the voltmeter closely as we drain the battery down again:

What did you observe?

• When we turn the switch on, current flows just like before, and the bulb lights until we have discharged the battery.
• But watch the voltmeter. It starts at 3.0 volts (pretty typical for an AA or a D-cell battery), and drops to zero when the two half-cells have the same charge level (reach the same potential).
• Like the water analogy, it’s not that there isn’t a charge in the battery overall—it’s because the charge in both half-cells is the same. Both half-cells have reached the same potential.
• Unlike the ammeter, we do NOT put the voltmeter end-to-end with the bulb. Instead, we put it side-by-side with the bulb. We just touch the voltmeter leads to the two points on either side of the place we want to measure a difference in potential. We say that the voltmeter is connected in parallel with the bulb. You’ll learn more about using a voltmeter later in the course.

So now we’ve learned another important aspect of electricity: it has a pressure associated with it, just like water. In the world of electricity, volts are conceptually the same as pounds per square inch are for water pressure.

🌟 KEY CONCEPTS

• Voltage is the force pushing electrons through a circuit. You can think of it as the amount of “pressure” moving an electron through a circuit.
• The unit of measurement for electric “pressure” is called a volt.
• V is the abbreviation for volts.
• The instrument used to measure voltage is called a voltmeter.
• Voltmeters are connected side-by-side (in parallel) with other loads in a circuit.