Basic Electrical Units and their Definitions

 What is an Electrical Unit ?

An electrical unit is any unit of measurement that is used to describe a property found in electric circuits.

For Example

1. An Ampere : An Ampere which is used to measure current.

2. A Volt : A Volt which is used to measure Voltage.

3. A Coulomb : A Coulomb which is used to measure Electric Charge.

4. A Farad : A Farad is a standard unit used to measure Capacitance.

Electric units provide an absolute measurement of the state of a particular circuit at any one time, which is essential for building and maintaining electrical circuits.

Basic Electrical Unit with Definitions

1. AC ( Alternating Current )

Alternating Current (AC) is a type of electrical current, in which the direction of the flow of electrons switches back and forth at regular intervals or cycles. Current flowing in power lines and normal household electricity that comes from a wall outlet is alternating current.

The rate of change of direction determines the frequency, measured in Hertz (cycles per second).

2. DC ( Direct Current )

The electrons flow in one direction only. Current flow is from negative to positive, although it is often more convenient to think of it as from positive to negative. This is sometimes referred to as “conventional” current as opposed to electron flow.

3. Frequency ( Hertz )

In physics, the number of waves that pass a fixed point in unit time; also, the number of cycles or vibrations undergone during one unit of time by a body in periodic motion.

The accepted audio range is from 20 Hz to 20,000 Hz. The number of times the signal completes a complete cycle in one second is the frequency.

4. Current ( Ampere )

One ampere of current represents one coulomb of electrical charge, i.e. 6.24×1018 charge carriers, moving in one second. 

Ampere is defined as the unit of electric current that is equal to the flow of one Coulomb per second.

In other words, “an ampere is the amount of current produced by the force of one volt acting through a resistance of one ohm”.

The magnitude of the current is determined by the available voltage, and the resistance (or impedance) of the load and the power source.

Current can be AC or DC, positive or negative, depending upon the reference.

For electronics, current may also be measured in mA (milliamps) – 1,000 mA is 1A. Nanoamps (nA) are also used in some cases.

The relationship between ampere and coulomb is represented as follows:

Ampere = 1 Coulomb / Second

At any given point in an area experiencing current, the Ampere value will increase proportionately if the charge on particles moving through it increases.

5. Voltage ( Volt )

The Volt (often denoted by the symbol 'V') is a unit of electric potential difference (commonly abbreviated to voltage), electromotive force (commonly abbreviated to EMF), and electric potential.

According to the International System of Units (SI), one volt is equal to the difference in electric potentials between two given points in a wire carrying an electric current of magnitude 1 ampere and dissipating one watt of power between those two points.

It can also be defined as the potential difference that exists between two points and imparts one Joule of energy to each Coulomb of charge that passes through it.

A 9V battery has a voltage of 9V DC, and may be positive or negative depending on the terminal that is used as the reference.

The mains has a voltage of 220, 240 or 110V depending where you live – this is AC, and alternates between positive and negative values. Voltage is also commonly measured in millivolts (mV), and 1,000 mV is 1V. Microvolts (uV) and nanovolts (nV) are also used.

6. Coulomb ( C )

Coulomb, unit of electric charge in the metre-kilogram-second-ampere system, the basis of the SI system of physical units.

It is abbreviated as C.

The coulomb is defined as the quantity of electricity transported in one second by a current of one ampere.

It is approximately equivalent to 6.24 × 10¹⁸ electrons, with the charge of one electron, the elementary charge, being defined as 1.602176634 × 10⁻¹⁹ C.

It is the quantity of electricity conveyed in one second by the current produced by an electro-motive force of one volt acting in a circuit having a resistance of one ohm, or the quantity transferred by one ampere in one second.

                      Q = I t   

7. Resistance ( Ohm )

Resistance is a measure of the opposition to current flow in an electrical circuit.

Resistance is measured in ohms, symbolized by the Greek letter omega (Ω) .

Resistance measurements are normally taken to indicate the condition of a component or a circuit.

• The higher the resistance, the lower the current flow. If abnormally high, one possible cause (among many) could be damaged conductors due to burning or corrosion. All conductors give off some degree of heat, so overheating is an issue often associated with resistance.

• The lower the resistance, the higher the current flow. Possible causes: insulators damaged by moisture or overheating.

Copper wire has a very low resistance, so a small voltage will allow a large current to flow.

Likewise, the plastic insulation has a very high resistance, and prevents current from flowing from one wire to those adjacent.

8. Capacitance ( Farad )

Capacitance is a measure of stored charge. Unlike a battery, a capacitor stores a charge electrostatically rather than chemically, and reacts much faster.

It's unit is Farad and Symbol is C.

A capacitor passes AC, but will not pass DC (at least for all practical purposes). The reactance or AC resistance (called impedance) of a capacitor depends on its value and the frequency of the AC signal. Capacitance is always a positive value.

A 1 farad capacitor, when charged with 1 coulomb of electrical charge, has a potential difference of 1 volt between its plates. The reciprocal of capacitance is called elastance.

9. Inductance ( Henry )

An inductor stores a charge magnetically, and presents a low impedance to DC (theoretically zero), and a higher impedance to AC dependent on the value of inductance and the frequency.

It is measured in Henry.

In this respect it is the electrical opposite of a capacitor. Inductance is always a positive value.

10. Impedance ( Z )

Unlike resistance, impedance is a frequency dependent value, and is specified for AC signals. Impedance is made up of a combination of resistance, capacitance, and/ or inductance.

It's unit is ohm and symbol is Z.

11. Decibel ( Z )

The decibel (dB) is a logarithmic unit used to measure sound level. It is also widely used in electronics, signals and communication. The dB is a logarithmic way of describing a ratio. The ratio may be power, sound pressure, voltage or intensity or several other things.

A 3dB change is half or double the power (0.707 or 1.414 times voltage or current respectively).