Circuit Bytes · Episode 2 · Circuit Bytes · Basics
Voltage, current and resistance
Every circuit has a push, a flow and a squeeze. The water-pipe picture makes voltage, current and resistance easy to remember.
The water-pipe picture
Picture a water tank feeding a hose. How much water comes out the end depends on three things: how hard the water is pushed, how much of it flows, and how narrow the pipe is. A circuit has the same three things, and each one has its own name and unit.
Voltage: the push
The pressure that pushes water through the hose matches voltage in a circuit. It's measured in volts (V). A battery's voltage tells you how hard it pushes: a 9 V battery pushes harder than a 1.5 V AA cell.
Strictly, voltage is the difference in electrical energy per unit of charge between two points, which is why a meter always measures it across something, between two points.
Current: the flow
How much water passes a point each second matches current: how much electric charge moves past one point each second. It's measured in amperes, or amps (A). One amp is a flow of about 6.24 × 10^18 elementary charges every second.
Resistance: the squeeze
A narrow section of pipe fights the flow. In a circuit, that's resistance, measured in ohms (Ω). Keep the push the same and add more squeeze, and less current flows. In the video, the meter goes from 10 Ω and 0.9 A to 30 Ω and 0.3 A while the battery stays at 9 V. The rule that links those numbers is the next episode: Ohm's law.
Try it yourself
Look at a few batteries and chargers around your home. The label always shows the voltage (1.5 V on an AA, 9 V on a square battery, often 5 V on a USB charger), and chargers usually also list a current in amps, the most they can supply.
Good to know
- The water model is a teaching picture with known limits: a wire isn't "empty" when no current flows, and voltage is energy per unit charge between two points, not pressure at one point.
- One volt is one joule per coulomb (one watt per ampere); one ohm is one volt per ampere (NIST SI).
- One ampere is a flow of about 6.24 × 10^18 elementary charges per second.
- "Squeeze" lumps together the resistance of wires and parts; resistance also depends on the material, length and temperature.
- "Current" here means conventional current; which way it's drawn is left for a later episode.
- Real 9 V and 1.5 V batteries vary a little with their charge level.
Test your knowledge
Quick quiz: Voltage, current and resistance
4 multiple-choice questions. Pick one answer for each, then check how you did.
Sources
- NIST, "Guide to the SI, Chapter 4": the volt (watt per ampere, electric potential difference) and the ohm (volt per ampere)
- NIST, "Definitions of SI Base Units": the ampere, defined from a fixed number of elementary charges per second
- University of Washington, Clean Energy Institute, "Water Model for Electricity": pressure, flow and pipe width as voltage, current and resistance
- Grinnell College, CSC 211, "Voltage, Current, Resistance, and Ohm's Law": the water tank analogy; volts, amperes and ohms
Video transcript
Pressure, flow and squeeze. The water-pipe trick for electricity.
Picture a water tank feeding a hose. Every circuit has the same three things.
Pressure is the push. In a circuit, that's voltage, measured in volts. A 9 V battery pushes harder than a 1.5 V AA.
Flow is how much passes each second. That's current, measured in amps.
Squeeze is a narrow pipe fighting the flow. That's resistance, measured in ohms. More squeeze means less flow for the same push.
Push, flow, squeeze: voltage, current, resistance.
That's your bite. Next: the one simple rule that ties all three together.