In this article we will basically discuss the various units of electronic parameters, and study their following mentioned aspects:
- the unit's designation
- the unit's graphical representation
- the physical quantity measured by the unit
- In equations, the symbol representing the physical quantity.
Unit symbols are always printed in roman (upright) characters, whereas quantity symbols are written in italic (sloping) characters.
Ampere (= amp) A current I
It is a fundamental electrical unit, determined by the force (in newtons) between two conductors or inductors of certain dimensions (in metres) positioned at a specific distance away from each other (in metres).
Coulomb C charge Q
One coulomb of charge travels across point in a circuit while a current of 1 ampere passes for one second.
charge = current x time
Q = It
I = Q/t
Potential V
The voltage difference across any point in space can be defined as the work done (in joules) in delivering a unit charge (1C) to this point, from an infinite distance.
Typically, when we relate to the potential in a point we actually imply the potential difference relative to another point that we consider to be at zero potential (0V).
The reference potential is usually earth or ‘ground’ level. Occasionally it may be referred to as the potential of the 'negative' terminal of a battery.
The potential of a point is recognized as positive in case work needs to be completed to transfer a positive charge toward it.
Most of the time, the ‘points’ as mentioned above are areas within an electrical circuit.
potential = energy / charge
V = J / Q
While no current flows, the electromotive force (e.m.f.) of a DC source, for example an electric battery or dynamo, can be determined across its terminals.
When a cell or generator is hooked up to an electric circuit and current travels through it, there is a reduction in potential difference across the battery terminals or generator's internal resistance.
As a result, the potential difference in between its terminals is smaller than the e.m.f. of the circuit. The potential difference decreases as the current increases. The sign E is occasionally used to denote E.m.f.
Ohm Ω
- impedance Z
- reactance X
- resistance R
A circuit's impedance (Z) is made up of two parts: resistance (R) and reactance (X), both of which are measured in ohms. Capacitive reactance (Xc) and inductive reactance (XL) are two types of reactance. Resistance is frequency independent, however both forms of reactance are frequency dependent:
Xc = 1 / 2πfC
XL = 2πfL
When a current I travels through a conductor, the conductor's resistance converts electrical energy to heat energy. As a result, the conductor's potential drops, causing a potential difference V in between terminals.
The conductor's resistance R is given by:
resistance = potential difference (p.d,) / current
R = V / I
Alternatively, the above equation could be also as:
V = IR
I = V / R
WATT W (power P)
When energy is transformed from one form to another, power is the rate at which work is done.
Energy conversions involving electric current can be expressed as:
P = IV
Considering that V = IR and P = IV, then the power getting dissipated (i.e. transformed into heat) within a resistance can also be written as:
P = IV = I x IR = I2R
Likewise, P = IV = V/R x V = V2/R
Siemens S
admittance Y
susceptance B
conductance G
The inverse of resistance is conductance. The mho is an older term for this unit. It is expressed through the following formula:
G = I / V which appears just the opposite of resistance formula.
A circuit's admittance (T), measured in siemens (or mhos), may be split into two parts: conductance (G) and susceptance (B), both measured in siemens.
Capacitive susceptance (BC) and inductive susceptance (BL) are two types of susceptance.
Although conductance is frequency independent, both forms of susceptance are frequency dependent:
Bc = 2π/C
BL = 1 / 2πfL
The alternating current across a conductance is in phase with the voltage when an alternating voltage is applied to it.
The current through a capacitor is 90 degrees ahead of the voltage.
The current via an inductance is 90 degrees behind the voltage.
The admittance is not actually the simple sum of the values of conductance and susceptance when they are in parallel. Rather, at any given frequency, this can be expressed as:
Y = √(G2 + B2)
The siemens is also used to indicate the gain, or transconductance, of devices like FETs, which is used to associate the current flowing through the transistor to its gate potential.
Ohm-meter Ωm resistivity ρ
Any circuit component for example a length of wire, a resistor, a cell, or a bulb is referred to as resistance. The element from which this component is constructed is referred to as resistivity.
The relationship between resistance and resistivity can be expressed using the following formula:
resistance, R = ρℓ / A
where A denotes the conductor's cross-sectional area, in square metres, and ℓ indicates its length, in metres.
Farad F capacitance C
A potential difference (p.d.) develops across a capacitor or any other capacitive devices as charge accumulates into it. The capacitance C is calculated as follows:
capacitance = charge / p.d.
C = Q / V
Tesla T magnetic flux density B
Magnetic field is another name for the above unit.
If a current is conducted by a wire of length ℓ and the angle between the magnetic flux field B and the wire is θ, then the force F (in newtons) on the wire is determined by:
F = BI. ℓ sinθ
When the direction of the force is at right angles to the magnetic field, than when θ = 90°, then the above equation becomes:
F = BI. ℓ
In case of a circular coil, having N number of turns and radius r, the magnetic density around the center of this coil could be calculated with the following equation:
B = μ0NI/2r
Where μ0 stands for the permeability of the free space. This carries a value of 4π x 10-7 = 1.2566 x 10-6
The above unit can be understood with the following example:
Let's consider a circular coil with 25 no of turns and radius of 0.05 m. When a current of 1.5 amps is passed through it, a magnetic flux of (1.2566 x 10-6 x 1.5 x 25) / (2 x 0.05) = 4.7 x 10-4 T develops at the center of the coil.
At some point over the coil's axis, which is at a distance x from the center of the coil, we find:
B = μ0NI/2r x [ r2 / (r2 + x2)3/2 ]
Weber Wb magnetic flux
Considering the magnetic flux density is IT and the magnetic flux as 1 Wb per square metre. A magnetic flux of 1 Wb associated with a coil having 1 turn, will generate an e.m.f. of 1 V, when the magnetic flux around it is adjusted to zero, for 1 second.
Henry H
- self-inductance L
- mutual inductance M
When a current of 1 A running through a coil generates a magnetic flux of 1 Wb, the coil's self-inductance L is 1 H. This can be expressed with the following equation.
L = NΦ/I
Put another way, the self inductance of a coil may be defined as the e.m.f. generated in the coil when the current is varied at a rate of 1A per second:
E = - L.dI/dt
In the above equation, the induced e.m.f. tries to stop any change in the current, which is indicated by the negative sign.
When a current of 1A passing through a coil generates a magnetic flux of 1 Wb in another coil, then the mutual inductance M between the two coils will be 1H. This can be written with the following formula:
M = NsΦs/Ip
In the above equation, the letters s and p denote secondary and primary winding of the coil.
Conversely, when the current in one coil varies at 1A per second, the mutual inductance is equivalent to the e.m.f. generated in the second coil. This can shown with the help of the following formula:
Es = - M.dIp/dt
Hertz Hz frequency f
One hertz represents one occurrence per second. Frequency in electronics refers to the number of electrical pulses or waveform cycles per second.
Kelvin K temperature t
The kelvin is a temperature unit which is measured on the absolute scale, where 0 K equals absolute zero (-273°C) and 273K equals 0°C. You just need to a dd 273 to convert kelvin to degrees Celsius.
Decibel dB ratio between two quantities n
If we consider two quantities x1 and x2 (this may be for example, in the form of two currents or maybe two potential differences), then the ratio between them in decibels can be expressed with the following formula:
n = 10 x log10(x1 / x2)