Voltage, Current and Resistance

This is episode one of a beginner's guide to electronics. Today, we're covering the three ideas that everything else is built on. Voltage. Current. And resistance.

You don't need any background. You don't need any tools. All you need is a willingness to go slowly, and to hear a few things more than once. The repetition is on purpose. These ideas are the foundation, and they're worth hearing until they feel obvious.

Here's the plan. First, we'll build one simple picture in your mind, a picture made of water. Then we'll use that picture to understand voltage, then current, then resistance, one at a time. After that, we'll connect all three with one short equation called Ohm's law. And at the end, we'll go back over everything one more time.

Let's start with the picture.

Imagine a large water tank, sitting at the top of a hill. A hose runs from the bottom of the tank, down the hill, to a garden at the bottom.

When you open the valve, water flows down the hose. That's the whole picture. A tank, a hose, a hill, and moving water.

Hold on to this picture. Every idea today is a part of it.

In electronics, something moves, just like the water. It's called electric charge. You can't see it, but you can think of it as the water of electronics. A battery is like the tank on the hill. Wires are like the hose. And the moving charge flows through those wires, the same way water flows through the hose.

Now, there are three things you can say about that water. How hard it's being pushed. How much of it is moving. And how much the hose is holding it back. Those three things are voltage, current, and resistance. Let's take them one at a time.

First, voltage. Voltage is the push.

Let's go back to the tank on the hill. Why does the water flow down the hose? Because it's being pushed. Gravity pulls the water down, and the height of the hill decides how hard it pushes. A tall hill means a hard push. A tiny hill means a gentle push.

Voltage works the same way. Voltage is how hard electric charge is being pushed. Voltage is the push.

Voltage is measured in a unit called the volt. So when someone says a five volt battery, they mean a battery that pushes with a strength of five volts. More volts means more push. Fewer volts means less push.

Let's put some real numbers on this, so it doesn't stay abstract. A typical AA battery pushes with about one and a half volts. That's a gentle push. The power from a USB port, like on a phone charger or a computer, is five volts. That's a bit stronger. A car battery is about twelve volts. Stronger again. And the electricity from a wall outlet is far stronger than any of those. We'll come back to that in the safety note, because a wall outlet is a different category, and it can seriously hurt you.

Now here's a detail that really matters. Voltage is always a difference between two points. Think about the hill. The height only matters because there's a top and a bottom. Water at the top is pushed toward the bottom. If there were no lower place to go, there would be no push at all.

A battery works the same way. It has two ends. One end is like the top of the hill, and the other end is like the bottom. The voltage is the difference between them. A one and a half volt battery has a one and a half volt difference between its two ends.

This is why we say voltage is across something. Across a battery. Across a part in a circuit. Voltage is measured across two points. Remember that word, across. Voltage is across.

Here's a related idea you'll hear all the time. Ground. Ground is simply the point in a circuit that we agree to call zero. It's like sea level. When we say a mountain is two thousand meters tall, we mean two thousand meters above sea level. Sea level is the agreed zero. Ground is the sea level of electronics. Everything else is measured as a height above it.

So, to recap voltage. Voltage is the push. It's measured in volts. It's always a difference between two points. And we say it's across something. Voltage is the push. Voltage is across.

Next, current. Current is the flow.

Back to the picture. The water in the hose is moving. The question now is, how much water is moving? Is it a slow trickle, or a roaring rush? That amount of moving water, each second, is the current.

In electronics, current is how much electric charge flows past a point each second. Current is the flow. Say it again. Current is the flow.

Current is measured in a unit called the amp, which is short for ampere. A bigger flow means more amps. A smaller flow means fewer amps.

Most small electronic parts use very small currents. So you'll hear a smaller unit all the time. The milliamp. The word milli means one thousandth. So a milliamp is one thousandth of an amp. A thousand milliamps make one amp. Remember, milli is the small prefix, and it shows up constantly.

Some real numbers. A small indicator light, like the little red or green light on a gadget, uses around ten to twenty milliamps. That's a trickle. A phone charger can deliver a few amps. That's a much bigger flow. And a car's starter motor, when you turn the key, pulls hundreds of amps for a moment. That's a roaring river.

Notice what we're comparing. Voltage and current are not the same thing. Voltage is the push, and current is the flow. You can have a strong push with a tiny flow, like a tall hill with a very thin hose. And you can have a big flow with a modest push, like a wide hose on a gentle slope. They are related, and we'll see exactly how in a few minutes. But they are two separate ideas.

Here's a way to keep them apart. Voltage is across. Current is through. Voltage is measured across two points, like across the ends of a battery. Current is the flow through a wire, or through a part. Across and through. Across is voltage. Through is current.

Recap on current. Current is the flow. It's measured in amps, and small currents in milliamps. Milli means one thousandth. And current flows through things. Current is the flow. Current is through.

Now the third idea. Resistance. Resistance is what holds the flow back.

Return to the hose. Suppose it's a wide, smooth hose. Water gushes through easily. Now suppose the hose is thin, or kinked, or half blocked. The same push now delivers a much smaller flow. The hose is resisting the water.

Resistance in electronics is the same thing. It's how much something holds back the flow of current. Resistance is what holds the flow back. Say it again. Resistance is what holds the flow back.

Resistance is measured in a unit called the ohm. A higher number of ohms means more resistance, so less current gets through. A lower number of ohms means less resistance, so more current gets through.

Electronics has parts that are made on purpose to add resistance. They're called resistors. A resistor is like a deliberately narrow section in the hose. It doesn't make water, and it doesn't destroy water. It just limits how fast the water can flow. When we build circuits, we'll use resistors constantly to control the current.

You'll often see big numbers of ohms, so there's a shorthand. A kilo-ohm is a thousand ohms. The word kilo means a thousand. So a ten kilo-ohm resistor is ten thousand ohms. Kilo is the big prefix, and milli is the small prefix. Kilo means a thousand times bigger. Milli means a thousand times smaller.

Even a plain wire has a tiny bit of resistance, though it's so small that we usually ignore it. Some materials resist a lot, and some resist almost nothing. That's why wires are made of metal, which resists very little, while the plastic coating around them resists so much that no current gets through.

Recap on resistance. Resistance is what holds the flow back. It's measured in ohms. A resistor is a part that adds resistance on purpose. More ohms, less flow. Fewer ohms, more flow. Resistance is what holds the flow back.

Let's put all three in one picture, using the hill. A tall hill gives a hard push. That's high voltage. A wide hose lets a lot of water move. That's low resistance, and it gives a big flow, which is high current. A thin hose lets only a little water move. That's high resistance, and it gives a small flow, which is low current.

The push, the flow, and what holds it back. Voltage, current, and resistance.

These three are not independent. They follow a rule, and the rule is short enough to remember.

The rule is called Ohm's law. Here it is in words. Voltage equals current times resistance.

Let's say that again. Voltage equals current times resistance.

Written down, it looks like this. The letter V, equals, the letter I, times, the letter R. V is voltage. R is resistance. And the letter I is current. It's an odd choice, but the I comes from the word intensity, and the letter stuck. V equals I times R.

You can also rearrange it. If you know the voltage and the resistance, you can find the current. Current equals voltage divided by resistance. That's the version you'll use most. And if you know the voltage and the current, you can find the resistance. Resistance equals voltage divided by current.

Let's try an example slowly, with easy numbers. Suppose the push is ten volts. And the resistance is one thousand ohms. What's the current?

Current equals voltage divided by resistance. Ten divided by one thousand. That's one hundredth of an amp. And one hundredth of an amp is ten milliamps. So ten volts pushing through one thousand ohms gives ten milliamps of current.

Now let's change only the resistance. Same ten volts. But now the resistance is two thousand ohms. Twice as much. Ten divided by two thousand is five milliamps. Twice the resistance, half the current. It's just like making the hose twice as narrow. The same push now moves half the water.

Now go the other way. Same ten volts, but the resistance drops to five hundred ohms. Half as much. Ten divided by five hundred is twenty milliamps. Half the resistance, twice the current. A wider hose, more flow.

Notice the pattern. When the resistance goes up, the current goes down. When the resistance goes down, the current goes up. And if you raise the voltage, the current goes up too. A harder push moves more water.

Let's do one more example, using the other form of the equation. Suppose a device has a current of two milliamps going through a resistance of five thousand ohms. What's the voltage across it? Voltage equals current times resistance. Two milliamps is two thousandths of an amp. Two thousandths of an amp, times five thousand ohms, is ten volts. So there are ten volts across it.

Why does this matter? Because almost every beginner circuit depends on it. In the next episode, we'll light up an LED, which is a small light. And an LED has a quirk. It doesn't limit its own current. If you connect it straight to a battery, it pulls far more current than it can survive, and it burns out. The fix is a resistor, and Ohm's law tells us exactly which resistor to choose. Hold on to this idea. Limit the flow, so the part survives. You'll see it again and again.

One honest note. The water picture is a tool for thinking, not the real physics. It works beautifully for voltage, current, and resistance. It starts to break down when we reach other parts, like capacitors, and when we reach alternating current. We'll refine the picture as we go, and whenever it stops working, we'll say so.

And one safety note, because this series is for beginners. Everything we build will be low voltage, like the five volts from a USB port, or a small battery. Wall outlets, and anything plugged into one, can seriously hurt you. Don't experiment with them. And don't open anything that plugs into the wall until you truly know what you're doing.

Let's go through everything one more time, slowly.

Voltage is the push. It's measured in volts. It's always a difference between two points, and it's across something.

Current is the flow. It's measured in amps, and small flows in milliamps, where milli means one thousandth. It flows through something.

Resistance is what holds the flow back. It's measured in ohms, and big values in kilo-ohms, where kilo means a thousand. A resistor is a part that adds resistance on purpose.

And Ohm's law ties them together. Voltage equals current times resistance. More resistance, less current. Less resistance, more current. More voltage, more current.

Here's something to try before the next episode. Find a phone charger, or a battery, or any small device. Look for the numbers printed on it. You'll usually find a voltage, in volts, and a current, in amps or milliamps. Read them out loud. Then, in your own words, say what each one means, using the tank, the hose, and the hill. If you can explain it with the water picture, you've got it.

Next episode, we'll pick up a resistor, an LED, and a breadboard, and build the very first circuit. We'll use Ohm's law to choose the right resistor. Thanks for listening.

This episode was written with AI assistance and narrated with a synthetic voice.