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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
Capacitance
• There is a field of force surrounding the electrons known as electrostatic
• Current can be caused to flow even when there is a “break” in a circuit
• When enough electrons, or negative electrostatic 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
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 may be made of– Wax paper– Ceramic– Mylar– Mica– Electrolyte– Air (variable)
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
Resistance
• Resistance is a measure of the reluctance of the electrons to leave their orbits
• Resistors are devices that limit current flow • Resistors convert electrical energy into heat• If current flow is too high, and the resistor
cannot dissipate sufficient heat – it will burn up
Resistors
• Common types of resistors– Carbon composition– Carbon film– Metal film– Wire wound– Foil – Grid– Variable
Variable resistor
As the wiper is moved further from the current input, the resistance is
increased and the volume level is decreased.
Calculating resistance
• Series = additive R + R + R = R• Parallel = combined resistance is always lower than the
lowest value
1 + 1 + 1
1
R R R1 2 n
R R
R R
1 2
21
X
+
1 2 n t
To solve for a pair of resistors in parallel