Section Index Owatonna Steele County Amateur Radio 20-Jul-2007

The most fundamental understanding of Direct Current (DC) circuits starts with Voltage, Current, and Resistance. These three fundamentals are tied together so that a change of one requires one of the other fundamentals to change. VOLTAGE can be be considered the force that causes electricity to flow, and the unit of measure is volts. CURRENT refers to the flow of electrons, with a unit of measure called Amperes or Amps. Resistance is the opposition to flow, measured in Ohms.

One example of the relationship is pushing a car. The force used to push the car would be similar to Voltage. More force could be applied to pushing the car by using two people rather than one. When more force is used, the flow (speed) of the car becomes greater. In electricity, more CURRENT flows when more VOLTAGE is applied. The last component is RESISTANCE. When the car is pushed up a hill, the RESISTANCE of going up the hill would cause opposition to the flow (speed) of the car. In electricity, CURRENT decreases when more RESISTANCE is encountered.

The relationship between force, flow and opposition mean that if one component changes, then at least one other component must change:

  • With one person pushing the car, the flow must become smaller if the car meets any resistance. With constant force, the flow decreases when opposition increases.
  • If two people push the car on level ground, the flow of the car will be greater. With constant opposition, the flow increases when the force is increased.
When working with schematic diagrams and mathematical equations, symbols are often used:

Voltage:the letter E Volts:the letter V
Current: the letter I Amperes: the letter A
Resistance: the letter R Ohms: the Greek symbol Omega (Ω)
In DC electricity, a mathematical relationship relationship exists called Ohm's Law:
  • force = flow * opposition -- or
  • VOLTAGE (Volts) = CURRENT (Amperes) * RESISTANCE (Ohms) -- symbolically represented as
  • E = I * R
One other related term relates to POWER, or the amount of electricty being used. POWER is calculated as:
  • Power = force * flow -- or
  • POWER = VOLTAGE * CURRENT -- symbolically represented as
  • P = E * I.
The symbols used for poweron schematic diagrams and mathematical equations are:

Power: the letter P Watts: the letter W

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