Basic Electrical

Electric current is a stream of charged particles, like electrons or ions, moving through an electrical conductor or conducting space. Current is measured as the net rate of flow of electric charge through a region. Current is denoted by using the letter “I”, actually this letter I originated from a French phrase “intensité du courant” which means “current intensity”
Unit
The SI unit of electric current is the ampere or amp (symbol: A). The word ampere was named after the famous person André-Marie Ampère, this same person used the letter “I“ for current in his works.
Current is the flow of electric charge across a surface at the rate of one coulomb per second. Ie. 1-ampere current is equal to 1 coulomb of charge flowed per second.
Current & medium
Electric current is a stream of charged particles. The moving particles are called charge carriers. These charge carriers may be different depending on the conductor medium.
In electric circuits or conductors, the charge carriers are often electrons
In semiconductors, they can be electrons or holes.
In an electrolyte the charge carriers are ions
In plasma, an ionized gas, an electric current is formed by both electrons and ions.
Some interesting properties of the current
Electric currents create magnetic fields, which are used in motors, generators, inductors, and transformers.
In ordinary conductors, they cause Joule heating, which creates light in incandescent light bulbs.
Time-varying currents emit electromagnetic waves, which are used in telecommunications to broadcast information.
Instrument for measurement
Current can be measured using an ammeter.
Current can also be measured without breaking the circuit by detecting the magnetic field associated with the current. Devices, at the circuit level, use various techniques to measure current
Current transformer clamp meters for AC only
Hall Effect clamp meter for AC & DC

Alternating and direct current

Alternating currentor AC is an electric current which periodically reverses direction and changes its magnitude continuously with time
The power which we normally receive at our home and office is AC. The AC power circuit which comes to our home and office is usually in a sine wave. But for special applications we can use alternative waveforms, such as triangular or square waves.Audio and radio signals carried on electrical wires are also examples of alternating current.
The major advantage that AC electricity has over DC electricity is that AC voltages can be readily transformed to higher or lower voltage levels, while it is difficult to do that with DC voltages. Since high voltages are more efficient for sending electricity great distances, AC electricity has an advantage over DC.
Direct current or DC refers to a system in which the movement of electric charge in only one direction. Direct current is produced by sources such as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type.
Alternating current can also be converted to direct current through use of a rectifier.