ELEC 105/ELEC 106
6 February 2004
ELEC 105 / ELEC 106
Voltage, Current, and Resistance
Objectives
• Define voltage and discuss its characteristics
• Define current and discuss its characteristics
• Define resistance and discuss its characteristics
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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
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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
Schematic Symbol for the DC Voltage Source
Anode
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Andrew H. Andersen
Cathode
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Batteries
• A battery is a voltage source that converts chemical energy
into electrical energy
• The way cells are connected, and the type of cells, determines
the voltage and capacity of a battery
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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
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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
Q
C
I=
A=
t
s
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Ampere: The Unit of Current
• One ampere is the amount of current that exists when
a number of electrons having a total charge of one
coulomb pass a given cross-sectional area in one
second
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Resistance
• The property of a material that restricts the flow of
electrons is called resistance (R)
• 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
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Ohm: The Unit of Resistance
• By definition:
One ohm of resistance exists if there is one ampere
of current in a material when one volt is applied
across the material
The unit of measurement for resistance is the ohm (Ω)
Schematic Symbol for a Resistor
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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
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Common Fixed Resistors
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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
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Precision Resistors
• Precision resistors will have a tolerance of 2%, 1%,
0.5%, 0.25% or 0.1%
• Some precision resistor values are identified using the
color code with 5 color bands,
– the first 3 bands indicate the rated resistance value
– the 4th band is the multiplier
– the 5th band indicates the tolerance
• Other precision resistors use an alphanumeric coding
• The cost of the device is directly related to the
precision – higher precision = higher cost
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Color Coded Precision Resistors
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Alphanumeric Labeling
• Two or three digits, and one of the letters R, K, or M
are used to identify a resistance value
• The letter is used to indicate the multiplier, and its
position is used to indicate decimal point position
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Fixed IC Resistors
• Often it is convenient to use multiple resistors mounted in IC
packages in many of today's modern circuits
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Cutaway View of a Resistor
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Variable Resistors
• Variable resistors are designed so that their resistance
values can be changed
• A potentiometer is a three terminal variable resistor
used to divide voltage
• A rheostat is a variable resistor used to control
current
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Examples of Variable Resistors
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Linear vs Logarithmic Taper Potentiometers
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Power Resistors
• Used when the device must accommodate high
currents
• Physically large to surface can dissipate heat into the
environment
• Heats the air around the device – may be very hot
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