05-12-2012, 06:39 PM
Voltage, Current, and Resistance
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Categories of Materials
• Conductors readily permit current flow, due to a large number of free electrons in the material
– Conductors are characterized by 1, 2, or 3 valence electrons in their atomic structure
• Semiconductors have 4 valence electrons
• Insulators have few free electrons, and tend not to permit current to flow through them
Current
• Electron Current Flow is the movement of free
electrons from negative terminal of the source to
positive terminal
• We may also define Conventional Current Flow as
the flow of positive free charges from positive
terminal of the source to negative terminal
• Both are called electrical current (I) which has the
Ampere (A) as the unit of measurement
• By definition: electrical current is the rate of flow
of charge
Voltage
• The unit of measurement for voltage is the volt (V)
• It is the source of electrical energy in a circuit
• By definition:
One volt is the potential difference (voltage) between two
points when one joule of energy is used to move one
coulomb of charge from one point to the other
Other Voltage Sources
• Solar Cells convert light energy into electrical energy
• Generators convert mechanical energy into electrical
energy
• Electronic power supplies do not produce electrical
energy, but they convert the ac voltage from an outlet
into a constant dc voltage for use in our circuits
Resistance
• The property of a material that restricts the flow of
electrons is called resistance ®
• By definition:
Resistance is the opposition to current
• Where there is current through any material that has
resistance, heat is produced by the collisions of
electrons and atoms
Resistors
• Resistors are used to limit current or divide voltage,
and in some cases, generate heat
• Common resistors are carbon-composition, carbon
film, metal film, and wirewound
– surface mount resistors are available as small resistor chips
– wirewound resistors are used where high power ratings are
required
Color-code Bands on a Resistor
• This coding is used for 5%, 10%, and 20% tolerance resistors
• The first three bands are used to denote the rated value
• The fourth denotes how much the device may vary from the
rated value
1st band is the first digit of the resistance value
2nd band is the second digit of the resistance value
3rd band is the multiplier (number of zeros)
4th band indicates the tolerance