The International System ( SI ) has established internationally accepted values for physical quantities. These agreed-upon values constitute the building blocks of many scientific fields of study, including electronics. Each quantity in question is accompanied by a symbol, and each symbol can be substituted with an SI unit that gives it meaning. Consider the following example:
V = IR
This equation states that the voltage ( V ) of a system can be evaluated by multiplying a current ( I ) value by a measure of resistance ( R ). At least two variables must be known to solve for an unknown. In order to determine the value of current in such a system, we first algebraically manipulate the equation’s symbols. Once we arrive at a derivation of choice, we substitute SI values into the equation to derive an answer:
( V / R ) = I
Voltage is defined as the quantity of joules ( J ) lost ( or gained ) by a coulomb ( C ) of charge as it traverses a point of reference. Voltage values are oftentimes listed “ as is “, but an analysis of units can help determine if an equation makes sense. In this case, the use of SI units to describe resistance would also be necessary. Resistance is comprised of units called ohms ( Ω ) which can be broken down further ( if necessary ):
Ω = ( V / A ) = ( 1 / S ) = ( W / A2 ) = ( V2 / W ) = ( s / F ) = ( H / s ) = ( J*s / C2 ) = ( kg*m2 / s*C2 ) = ( J / s*A2 ) = ( kg*m2 / s3*A2 )
Fortunately, extreme derivations like the one above are generally reserved for academic purposes that are outside of the normal problem-solving etiquette. What is most practical is the development of confidence that evolves with problem-solving. The practical way of solving such a problem is as follows:
( V / R ) = I = ( 1V / 1Ω ) = I
In this example, the SI units of voltage and resistance had values of “ 1 “, but SI units can have a wide range of values assigned to them. Familiarity with the electrical quantities listed below is essential to the study of electronics:
- Capacitance ( C )
Capacitance is a measure of how much charge ( Q ) is stored per unit voltage ( V ) applied to a capacitor ( C = Q / V ). The SI unit of capacitance is the Farad ( F ).
- Charge ( Q )
Charge is a property of matter commonly associated with protons and electrons. Electrons and protons carry charges of 1.602 x 10-19 C. The SI unit of charge is the coulomb ( C ), and the number of charges contained by a coulomb is defined as 1C divided by ( 1.602 x 10-19 C / elementary charge ):
[ 1C / ( 1.602 x 10-19 C / elementary charge ) ] = 6.24 x 1018 elementary particles.
- Conductance ( G )
The SI unit of conductance is siemens ( S ).
- Current ( I )
The SI unit of current is the ampere ( A ). An ampere is a measure of how many coulombs ( C ) of charge pass of point-of-reference in one second ( s ):
I = ( C / s )
- Energy ( J ) and/or Work
The SI unit of energy is the joule ( J ). In terms of work ( W ), a joule is defined as follows:
W = Fd, where “ F “ quantifies force, and “ d “ is the distance over which a force acts. It is important to note that energy can be that of an object in motion ( kinetic energy = KE ), or it can be stored as potential energy ( PE ).
Force is a product of mass ( m ) and acceleration ( a ), so W = ( m )( a )( d ) = ( kg )( m / s2 )( m ) = kg*m2 / s2. When the force pairs acting upon a system are balanced, the system is in translational equilibrium, and it will either “ sit still “ with respect to an observer or move with a constant velocity. When an unbalanced force acts upon a system, it will accelerate until it is acted upon by some other force that establishes balance.
- Frequency ( f )
Frequency is defined by a number of complete cycles that occur per second in an oscillating system:
f = ( cycles / second ) = ( 1 / s ) = ( cycles*s-1 )
The SI unit of frequency is hertz ( Hz ).
- Impedance ( Z )
The SI unit of impedance is the ohm ( Ω ).
- Inductance ( L )
The SI unit of inductance is the henry ( H ).
- Power ( P )
Power is defined as the amount of energy ( J ) liberated or stored per second ( s ):
P = ( J / s )
The SI unit of power is the watt ( W ).
- Reactance ( X )
The SI unit of reactance is the ohm ( Ω ).
- Resistance ( R )
Resistance is a measure of a material’s ability to impede or restrict a flow of electrons ( I ). The resistance of a material determines how much energy a coulomb of charge deposits as it traverses the material. The SI unit of resistance is the ohm ( Ω ).
- Voltage ( V )
Interestingly, the SI unit of voltage is the volt ( V ).
Emphasis has been placed upon electrical units of measure that will be encountered relatively early in the study of electronics. Once students become familiar with these basic building blocks, the remaining units will be less difficult to assimilate into their knowledge base.
RENAISSANCE PHYSICS 