DC electronics

Resistance

• Because electrons have mass and are held in place by polarity “bonds” – energy is consumed to dislodge electrons

• Resistance is affected by – Composition of material– Length of material– Cross-sectional area of material– Temperature of material

Resistance

• Composition– Resistance is the opposite of conductivity so

resistance is lowest in silver, gold, copper, etc.• Length – Resistance increases as length increases– Causes voltage drop in long cables

• Cross-sectional area– Resistance increases as cross section decreases– Causes heating of under-sized cables

Resistance

• Cross-sectional area– Wire diameter is described by system of “gauges”

called American Wire Gauge (AWG)– Bigger number equals smaller diameter• 28 is used for telephone, 12 or 14 is for household

current, 00 is for entrance cable, etc

• Temperature– As temperature increases, so does resistance– “Super conductors” are cooled to absolute zero

AC versus DC+120 Volts

-120 Volts

0Volts

+12Volts

-12Volts

0Volts

Capacitance

• There is a field of force surrounding the electrons

• Current can be caused to flow even when there is a “break” in a circuit

• When enough electrons, or negative force, build up – adjacent electrons can be caused to move

Capacitance

• Capacitors are constructed of two plates in close proximity

• Plates are separated by an insulator called a dielectric

• Current can flow through a capacitor even though the plates are separated by an insulator

Capacitance

Plate Plate

Dielectric

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Capacitance

Plate Plate

Dielectric

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Capacitors

• Capacitors have the ability to store electrons• The quantity of electrons in a capacitor is

measured in farads• Farad is the measure of capacitance• 1 farad = 6.28 X 10• Size of plates and the dielectric constant

determine capacitance

18

Capacitors

• Dielectric made be made of– Wax paper– Ceramic– Mylar– Mica– Electrolyte– Air (

Capacitors

• One application of capacitors is to convert AC to DC

• During each phase of AC when electrons are pushed onto the “in” plate – some current flows off the “out” plate

• During the opposite phase when no electrons are moving into capacitor – no current flows out

Capacitors

Output current from a capacitor

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Capacitors