Basic Electrical Quantities and Properties

Voltage is the driving force, caused by the potential energy of a chemical or mechanical reaction, that causes electrons to conduct if given a continuous path (circuit). Sometimes called ‘electro-motive force’, EMF, or simply E.

Volts (V)

Other common values:

millivolt (mV)

microvolt (µV)

kilovolt (kV)

Current is the collective unified motion of electrons past a point in a wire, driven by a voltage, and only occurring if given a path, or circuit, from one side of the potential energy source to the other.

Amp (A)

Other common values:

milliamp (mA)

microamp (µA)

kiloamp (kA)

Resistance is the opposition to the flow of electrons by causing a change in the form of energy from electrical energy into some other form (commonly heat or light), increasing the voltage required to drive the current.

Ohm (Ω)

Other common values:

kilo-ohm (kΩ)

megaohm (MΩ)

gigaohm (GΩ)

Power is the work done, or the energy consumed over a given period of time. Power can be delivered, or supplied into a circuit or system. Power will almost certainly be lost within any real system (efficiency), and as the energy is converted from electrical energy into light, heat, or motion by the ‘load device’, we say the power is dissipated.

Watt (W)

Other common values:

milliwatt (mW)

kilowatt (kW)

megawatt (MW)

Reactance, abbreviated ‘X’

Inductors supply inductive reactance (XL)

Capacitors supply capacitive reactance (XC)

X is measured in Ω and kΩ

Impedance, abbreviated ‘Z’

Z is measured in Ω and kΩ

AC circuits only

A transformer steps voltage up and down, and delivers power to a load device at a different voltage and current with the same power from input to output. Therefore, a transformer does not dissipate virtually any power, and so is not rated in watts (W), but rather in volt-amps (VA).

AC voltage only.

The transformer is an inductor, transferring energy by generating a fluctuating magnetic field that must constantly rise and fall to drive current.

This will not work with DC voltage, for the same reason given in a previous question in regards to inductors in a DC circuit.

The total resistance will increase, and the total current will decrease.

Since the supply voltage did not change, but the current decreased, the total power dissipation will also decrease.

The total resistance will decrease, and the total current will increase.

Since the supply voltage did not change, but the current increased, the total power dissipation will also increase.

As frequency increases, the capacitive reactance (and overall circuit impedance) will decrease, and therefore current will increase.

As frequency increases, the inductive reactance (and overall circuit impedance) will increase, and therefore current will decrease.