Let’s approach electricity at its basics…

We all agree that every object, person, everything is made of atoms. They are the most fundamental bits of everything also, behind the idea of electric current. But let’s not go into too much details.

Anatomy of the Hydrogen Atom

Above is the structure of a Hydrogen atom. This is the simplest of all with one proton in the middle and an electron around. Other atoms are similar with a differing number of protons and electrons. Protons are positively, electrons are negatively charged, so as a whole the atom itself is electrically neutral.

But.…… atoms can lose a loosely attached electron as well as can gain extras. That is to say, this will make the atom positively or negatively charged and it will be called an ‘ion

What is the Electromotive Force (EMF) ?

A conductor – like a piece of wire, is also made of atoms. In such materials, an external force easily knocks off the electrons. Therefore, we call this force the Electromotive Force.

To picture this, imagine you connect this hypothetical wire to the positive and negative side of a battery. Don’t do this though, you need some kind of a resistive element like a light bulb in between, but we’ll talk about this later.

So, the Electro Motive Force is generated when you connect the positive and negative sides of the battery with the wire. What happens?

  • Electrons get knocked off
  • They are negatively charged thus,
  • Electrons are attracted and start to flow towards the +ve (positive) terminal of the battery.

This is the flow of electric current.

Flow of electric current in a battery

Let’s take another example: a phone charger.

It says 5V 2A. Here it is what it means:

  • 5V – The transformer inside the charger converts the wall electricity (230V) to 5V.
    • This 5V is the EMF (electro motive force)
    • Measured in volts (V)
  • 2A – is the current the charger can supply at 5V. It is the flow rate of electrons or in other words how many electrons pass per second.
    • Electric current is measured in Amperes (A)

Electric Current flows from + to

There is a catch here.

We know that electrons travel from the negative towards the positive terminal of the battery… yet the flow of electric current is considered to be in the opposite direction: from minus to plus.

They got it wrong in the past and it kind of stuck. When electrons are knocked off, they leave ‘holes’ behind that are positively charged. So, if electrons go from minus to plus, holes travel from plus to minus.

In EEE we consider this by convention as the current flow.

To Sum it Up

Okay, now that we know a bit about current flow let’s sum up some facts:

  • Current is the flow of electrical charge
    • Electrons are negatively charged
    • Holes are positively charged
  • We measure current by how much electron passes in 1 second
    • 1A = 6 million million million electrons passing through a given cross section of a wire in 1 sec.
    • 1 coulomb = 6 million million million electrons thus,
Current and Time relationship
  • the unit of current is ampere (A)
    • the short form is ‘amps’
    • Current flow is induced by an EMF (electro motive force)
    • Measured in volts (V)
    • Once wire is connected across battery terminals, EMF acts as quickly as the speed of light starting to move the electrons straight away.
    • Electrons however only move approx. 1mm/sec.
  • The current flow is opposite the electron flow – in other words, the flow of holes.

AC/DC

To this point, we talked about a type of current called: Direct Current or DC. It flows to one direction (from the positive terminal to the negative).

AC or Alternating Current is a different story. Picture this as when the EMF constantly changes direction (polarity). What do electrons do? They try to follow the +ve terminal’s direction, thus they go left and right and left and right.

What we have in the wall is AC. Your phone works on DC. The phone charger transforms the voltage from AC to DC.

optional reading: Success in Electronics book by Tom Duncan
NEXT TOPIC: Resistance