Basic Electronics 

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Electric Current (I)  A net flow of Electrons. ( In warm conductors there is also a random movement of electrons. ) Currents only flow if there is a complete circuit (no gaps). Currents only flow if there is a potential difference causing it to flow. Currents (usually electrons) flow THROUGH circuits. 
Current Units  Amps or Amperes. 
Ammeter  Measures current in amps or amperes. The Voltage across an ideal ammeter is zero. Connect the ammeter in SERIES with the component who's current you want to measure. At a junction, if you add up all the currents entering the junction, this value equals the sum of all the currents leaving the junction. Cars at road junctions behave in the same way. 
Conductor  A material where the Electrons are free to move. 
Insulator  A material where the Electrons are NOT free to move. 
Voltage (V)  Electron Moving Force (EMF = Electromotive Force). Voltage is measured ACROSS circuits. 
Voltage Units  Volts 
Voltmeter  Measures Volts. The current through an ideal voltmeter is zero. Connect the voltmeter in PARALLEL with the component who's voltage you want to measure. The sum of the voltages in a series circuit is equal to the sum of the power supply or battery voltages. 
Potential  Is measured in Volts relative to the zero volt earth reference. 
Potential Difference  Is measured in Volts between any two nonearth points. 
Charge (Q)  Is measured in Coulombs. A neutral or uncharged object has the normal number of electrons. A negative charge occurs if extra electrons are added. A positive charge occurs if electrons are removed. A continuous nonrandom flow of charge is an electric current. 
Analogue Signal  A smoothly varying Voltage or Current with in infinite number of possible values. These values are proportional to measurements taken from the natural world. 
Digital Signal  This has only two values: ON/OFF or LOW/HIGH or 0/1 or TRUE/FALSE 
Transducer  A device that converts nonelectrical energy types to or from electrical energy. INPUT: A device that converts a measurement from the natural world into a Voltage or a Current. OUTPUT: A device that converts a Voltage or a Current back into the natural world as light, heat, sound or movement for example. 
Power (P)  Is measured in Watts. Power = Volts x Amps 
Resistance (R)  Is measured in Ohms Ω. Here is a way to think of resistance: It tells you how many volts are needed to make one amp flow. You'd need 10 Volts to make an amp flow through a circuit with a resistance of 10 Ohms. 
Frequency  The number of cycles per second measured in Hertz (Hz). 
Period  The length in seconds of a pulse or complete cycle of a wave. Period = 1 / Frequency 
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