# Physics background

Electric charge is bipolar positive and negative. Like charges repel, while unlike charges attract each other. The separation of charge (voltage) and charge in motion (current) cause electrical effects.

Current is the net flow of positive charge per unit time passing through a cross section, perpendicular to the flow direction, of conductive materials.

Voltage is the electric potential difference. It is the energy expended per unit of charge against a static electric field to move the charge between two points. It is defined as the ratio of the the energy per unit charge.

Electrical energy is energy derived from electric potential energy before it is delivered to the end-use. Once converted from potential energy, electrical energy can always be called another type of energy, like heat, light, etc…

## Basic circuit elements

There are 5 basic circuit elements, which can be categorized into two groups.

### Passive elements

These do not generate power.

• resistors
• inductors
• capacitors

### Active elements

If generating power, then they are going to be:

• voltage sources
• current sources

## Why do we need reference polarity or direction?

Take a look at the following two circuit designs.

Actual polarity of a voltage or actual direction of a current in a circuit may not be known before solving the circuit. Therefore, we have to assign a reference for each variable to help us figure out what the actual polarity and direction would be. The reference can be chosen arbitrarily.

After the current is solved, if a current has a negative value that means that the actual current direction through the circuit element is opposite the chosen reference direction. If it has a positive value then that means the current actually flows the same as the reference direction of the current.

## Up next…

Next you should probably start reading about passive sign convention, which is another important tool you should be aware of during circuit design!

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