# Spinning Numbers

Welcome to Spinning Numbers! Learn about electricity and electrical engineering.

### Circuit sandbox simulator

- Circuit sandbox in English
- Circuit sandbox en español
- 电路沙箱 或简体中文 (simplified Chinese)
- 回路サンドボックス 日本語で (Japanese)

Want to help translate Circuit Sandbox into other languages? It’s easy!

### Introduction

Become familiar with the most important electrical quantities: charge, current, and voltage.

### Circuit elements

Circuits are made of resistors, capacitors, inductors, and power sources.

### Resistor circuits

Resistors in series and parallel, the voltage divider, delta-wye resistor networks.

### DC circuit analysis

Kirchhoff’s Laws, node voltage method, mesh and loop current methods.

### Special topics in DC analysis

Linearity and superposition. Source transformation. Thévenin and Norton equivalents.

### Natural and forced response

Natural response is what a circuit does with its own internal stored energy.

### AC circuit analysis

If we limit signals to only sine waves, we get AC analysis based on complex exponentials.

### Special topics in AC analysis

Decibels and RMS.

### Amplifiers

The operational amplifier is the building block of analog electronics.

### Semiconductors

Using a diode in a circuit.

### Electrostatics

Electric force is described by Coulomb’s Law. Electric field. Formal definition of voltage.

### The name

Where does the name Spinning Numbers come from?

#### Printing

Tips on the best way to print articles from Spinning Numbers.

#### Everything

List of everything on Spinning Numbers. Topics, articles, videos, images, and on and on.

These videos and articles began at Khan Academy where I had the honor of being the Electrical Engineering Content Fellow in 2016. The concepts are explained at the level of an undergraduate EE student. Motivated middle school and high school students can get a lot from the course.

I set up this Spinning Numbers web site after completing my KA fellowship. I’ve reviewed and significantly improved all the articles, and added several new articles. A nice new feature is the Circuit Sandbox simulator. You will find simulation models included in most articles, to let you try your hand and bring the concepts to life.

Good luck as you learn about electronics and electrical engineering. Please leave a comment if you enjoy your visit, and ask questions. I’m here to help.

- Willy McAllister

## Questions

Mr. McAllister, My system is showing a voltage drop error Across the first resistor I have a 2.5 Volt drop Across the second resistor I have a one Volt drop Together as you see that’s 3.5 volts From a 3 Volt supply All resistors all nodes are connected correctly The ground Fire clowns seems to be a different style than the one depicted In the Class But it is indeed a ground All voltages 3 bolts Applied All resistances 1K 3K,2K Are correct Please adVISE If you know of any fixes For the system

Robert - I think your circuit is okay. You described a circuit with a 3V source in series with 1k, 3k, and 2k resistors. You are getting mixed up between “node voltage” with “element voltage”. An element voltage is what appears across a single resistor. A node voltage is different—it is measured between a node and ground. Here is the definition of node voltage: https://spinningnumbers.org/a/node-voltage.html#definition-node-voltage.

The current in the circuit is 0.5mA. The

element voltageappears across a resistor. The 1k resistor has an element voltage of v = i R = 0.5mA 1k = 0.5V. One end of the 1k resistor is at 3V with respect to ground (because it is connected to the source). The node at the other end of the 1k resistor is at 2.5V with respect to ground. That’s thenode voltageat that point in the circuit. Simulators work with node voltages.