## What Is Electricity?

### Voltage

- Symbol: V (
**U**in some parts of Europe,**E**is sometimes used in the USA) - Unit: Volt (V)

Voltage is the difference in electric potential energy between two points.

In our water analogy, voltage is like the amout of pressure that has the potential to push the water through a water pipe.

### Current

- Symbol: I
- Unit: Ampere/Amp (A)

Current is the flow of electric charge.

In our water analogy, current would be the water current or water flow. The amount of water that flows in a water pipe.

### Resistance

- Symbol: R
- Unit: Ohm (Ω)

Resistance is the measure of a material’s ability to oppose the flow of electricity.

In our water analogy, resistance is like a narrowing or clog in the pipe that makes it harder for the water to flow through.

### Ohm’s Law

- V = I * R
- I = V / R
- R = V / I

### Important Note: Conventional Flow and Electron Flow

Due to the fact that electricity was discovered before we understood how atoms work, it was first believed that electrical current is something that flows from the positive side to the negative side in a circuit. In reality, electricity is the movement of electrons from one atom to another and it actually happens the other way: from negative to positive.

This has led to two different ways to talk about which way electricity flows. The conventional flow and the electron flow. Most symbols on schematics make more sense, if we talk about electricity according to the conventional flow, also, electrical engineers and text books still mainly use that system.

We are also going to use the conventional flow, since it is a little bit more intuitive to think of it that way based on the water analogy: the current flows from the positive to the negative. The math still works the same no matter which you use. Additionally, we are not going to be working with anything where it would actually be important to understand the actual physics of electrons flowing.

## Other Key Concepts in Electronics

### Power

- Symbol: P
- Unit: Watt (W)

How to calculate power:

- P = V * I
- watts = volts * amps

### AC/DC

#### Direct Current (DC)

- Direction of the current is constant
- Many small electronic devices in your daily life use DC
- We will mainly use fairly low DC voltages in the class

#### Alternating Current (AC)

- Reverses direction periodically (50Hz in Finland)
- Used for mains voltage (230V AC in Finland)
- Used for transmitting power over long distance (very high voltage, but low current)

Sparkfun has made a nice poster about the differences. You can download and print it from here.

### Series and Parallel

More information:

You have most likely heard of electronic components being connected in series or in parallel. It is important to understand how different components behave when connected in either way.

#### Resistors

If you take three resistors and connect them in series, the total resistance of the circuit is the sum of all resistors. If the resistors are connected in parallel, the equivalent resistance is lower than every individual resistor.

In series, there is only one path for the current to flow and each resistor adds another hurdle for the electrons. When you connect resistors in parallel, you are creating more paths for the current to flow and overall resistance gets lower.

##### Calculating the Total Resistance of Resistors in Series

R_total = R1 + R2 + R3 +…+ RN

##### Calculating the Total Resistance of Resistors in Parallel

1 / R_total = 1 / R1 +1 / R2 + 1 / R3 +…+ 1 / RN

Use this handy tool to calculate the total resistance of resistors connected in parallel.

#### Capacitors

Capacitors in parallel and series **work exactly the opposite way to resistors**: when you connect capacitors **in series, the total capacitance decreases**; when you connect capacitors **in parallel**, the total capacitance is the s**um of all individual capacitances**.

##### Calculating the Total Capacitance of Capacitors in Series

C_total = 1 / C1 +1 / C2 + 1 / C3 +…+ 1 / CN

##### Calculating the Total Capacitance of Capacitors in Parallel

C_total = C1 + C2 + C3 +…+ CN

Images from Sparkfun. Creative Commons: CC BY-SA 4.0