Voltage

The amount of potential energy between two points (let point a and point b) on a circuit is called voltage. One point has more charge than another. This difference in charge between the two points is called voltage.

Voltage (or potential difference) is the energy required to move a unit charge through an element, measured in volts (V).

Mathematically,   v_ab = dw/dq


where,
 w is energy in joules (J)
 q is charge in coulombs (C).

Unit
The SI unit of quantity of voltage is the volt (V).
1 volt = 1 joule/coulomb = 1 newton meter/coulomb
 


Water Analogy
When describing charge, voltage, current, and resistance, a common analogy is a water tank. In this analogy, charge is represented by the water amount, voltage is represented by the water pressure, and current is represented by the water flow. So for this analogy, remember:

    Water = Charge
    Pressure = Voltage
    Flow = Current

Consider a water tank at a certain height above the ground. At the bottom of this tank there is a hose.

Current-Water Analogy

More water, more pressure and more flow.

Current-Water Analogy

Current

Electric current is rate of flow of negatively-charged particles, called electrons, through a predetermined cross-sectional area in a conductor.

Electron flow in a circuit

Mathematically,    i = dq/dt

The charge transferred between time t0 and t is obtained by integrating both sides of equation. We obtain,     

  



Calculating current

The SI unit of quantity of electric current is the ampere (A).
1 ampere = 1 coulomb/second

 
 
 or,   I=Q/t

where:

I is the current in amperes (amps), A

Q is the charge in coulombs, C

t is the time in seconds, s



Direct Current (DC)
 
An electrical current can flow in either of two directions.  If it flows in only one direction, it is called direct current.  

Direct Current

Alternating Current (AC)

A current which alternates in direction or polarity is called an alternating current.
 

Alternating Current

Note: Direct current is represented by I and alternating current is represented by i.

Charge

In physics, charge, also known as electric charge, electrical charge, or electrostatic charge. Electric charge is the physical property of matter/substance which are made up of atoms.

An atom is built with a combination of three distinct particles: electrons, protons, and neutrons. Atom has a center nucleus, where the protons and neutrons are packed together. Surrounding the nucleus are a group of orbiting electrons. The electrons on the outer orbit are valence electrons.
With enough outside force, a valence electron can escape orbit of the atom and become free. Free electron is floating in a space between atoms.

Carbon Atom Model

Charge is a property of matter–just like mass, volume, or density. There are two types of electric charges: positive and negative. Charge carriers are atomic particles–specifically electrons (negative charge) and protons (positive charge).

Unit

The SI unit of quantity of electric charge is the coulomb (C).
1 C charge is equal to 6.242×10¹⁸ electrons.
The charge of an electron is approximately  −1.602×10⁻¹⁹ C .
The charge of an proton is approximately   +1.602×10⁻¹⁹ C.