### Categorized |General Basics, Power Electronics Basics

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Posted on 21 June 2019

# Basic Principles of Electricity and Physics of Semiconductors

Power electronic converters can be found wherever there is a need to change voltage, current, or frequency of electric power. This implies that the field of power electronics draws some of its roots from the theory of electricity and applied physics. It is therefore inevitable that some basic terms and formulae found in these two fields of study will come up frequently  in our quest to understand, design, and ultimately apply or use semiconductors wherever they are needed. This is a brief, rather simple, albeit essential guide to such terms and their definitions. Sometimes, to turn a phrase, a formula is worth a thousand words. Some of the laws introduced here will therefore be expounded upon using relevant equations.

Electrical properties can be pragmatically defined as a basic characteristic or property of materials just like density, mass, etc. Thales Milet (600 bc), a Greek natural philosopher was the first to discover this when he rubbed a piece of woolen cloth on amber. Little fluffs of wool fluttered against the amber as if an invisible force was drawing them to it. Since the Greek word for amber is ήλέκτρον, which essentially means electron. The charge observed on the amber was thus named electricity.

### Electric Charge Q

Electric charge is a fundamental property of matter. The unit of measurement of charge is the Coulomb (C).

An atom is a basic unit of matter that consists of a dense, positively charged central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons. The electrons of an atom are bound to the nucleus by the electromagnetic force. In the same way, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral. Otherwise, it has a positive charge if there are fewer electrons (electron deficiency) than protons, or negative charge if there are more electrons (electron excess) than protons. The smallest fundamental charged particles of an atom, i. e. protons and electrons, carry electric charge +e and -e, respectively, where e = 1.6 X10-19C.

When charged particles are in the vicinity of one another, the particles experience a force called the electrostatic force. Charges of opposite sign (one positive, one negative) attract, while charges with the same sign repel.

The electrostatic force between two charge particles is described by Coulomb's Law.

### Coulomb’s Law

The magnitude of the force, F, between two electric charges Q1 and Q2 is inversely proportional to the square of the distance r between the two charges. Force is measured in Newtons (N).

### Work, Energy E (in Joules)

The mechanical definition of work is Force x Distance.

Energy is loosely defined as the ability to do work and is measured in Joules (J), having units of Newton-meters (Nm).

### Electric Potential, Voltage

Electric potential, U, at a given point is defined as the work needed to take one unit of electric charge from infinity to that point. Electric voltage is the difference between the electric potential of two given points. In practice, voltage potential is weighed against some reference potential such as “earth” or “ground”. Voltage is measured in Volts (V), havings units of Joules per Coulumb (J/C).

If there is no connection to a source of voltage, the charge is said to be “floating”.

### Electric Current I

Moving electric charges result in what is called current, I, and is measured in Amperes (A). The current through a given point is defined as the amount of charge passing through that point per unit time,so amperes have the units of Coulomb per second (C/s). When two points with different potentials are connected using an electrical conductor, current flows from the positive to the negative potential.

### Power

Power, P, is defined as the amount of work performed per unit time and is measured in Watts (W), with units of joules per second (J/s).

In terms of current and voltage, we thus have: $P=I\cdot V$

### Electrical Resistance (R)

All matter apart from superconductors apply resistance to current through which a drop in voltage occurs (Ohm’s law). Resistance, R, of an object is defined as the votage drop accross the object per unit current through the oject. Resistance is measured in Ohms (Ω), with units of Volts per Amp (V/A).

We may write this as $R=\frac{U}{I}$, or alternatively as $U=I\cdot R$.

### Conversion to heat due to resistance R (power dissipation)

When current flows through a resistive material, energy is lost in the form of heat.

The resulting power loss is refered to as power dissipation, and is given by $P=I \cdot U=I\cdot (I\cdot R)$, or

A great deal of electrical design is aimed at methods of coping with and limiting power loss.

The general information provided above forms much of  the basic foundation for electrical applications.

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