Electrical principles

[Ref: DK5:chp14; TF8:p179-180]

Units

Currents

Unit for current: ampere

--> represents 6.24 x 10¹⁸ electrons passing some point per second

Electric charge

Unit for electric charge: coulomb

--> the quantity of electric charge which passes some point when a current of 1 ampere flows for a period of one second.
* equivalent to 6.24 x 10¹⁸ electrons

Potential difference (voltage)

Unit for voltage: volt

--> the potential difference which produces a current of 1 ampere in a substance where the rate of energy dissipation is 1 watt.

Resistance

Unit for resistance: ohm

--> the resistance which will allow 1 ampere of current to flow under the influence of a potential difference of 1 volt.

Capacitance

A measure of the ability of an object to hold electrical charge

Unit for capacitance: farad

--> the capacitance of an object for which the electrical potential increases by 1 volt when one coulomb of charge is added to it.

Relations

Charge

Charge = Voltage x Capacitance

Stored energy

Stored energy
= 1/2 Charge x Voltage
= 1/2 Capacitance x Voltage^2

Ohm's law

Voltage = Resistance x Current

(cf: pressure = flow x resistance)

Resistance and impedance

Impedance - resistance that is dependent on frequency of the alternating current.
* symbol Z
* unit: ohm

Different types of "resistors"

• resistor - resistance does not vary with frequency of alternating current
  * resistance
• capacitor - resistance is lower with high frequency current
  * capacitance
• inductor - resistance is lower with low frequency current
  * inductance
  * Resistance is "induced" as frequency increases