Ohm's Law pt2

Ohm's Law Overview
  • Ohm's Law Formula: E = I * R
    • E = Voltage (Volts)
    • I = Current (Amps)
    • R = Resistance (Ohms)
Key Concepts
  • Voltage (E): The potential difference in electrical potential between two points in a circuit, measured in volts (V).
  • Current (I): The flow of electric charge in a circuit, measured in amperes (A).
  • Resistance (R): The opposition to current flow in an electrical circuit, measured in ohms (Ω).
Practical Application

  1. Finding Voltage: To calculate the voltage (E) in a circuit where the resistance (R) and the current (I) are known:

    • Use E = I * R

  2. Finding Current: To find the current (I) when voltage (E) and resistance (R) are known:

    • Use I = E / R

  3. Example Calculation:

    • Given a 208 volt circuit with 75,000 ohms:
      • Calculate current:
        I=20875000=0.00277 AI = \frac{208}{75000} = 0.00277 \text{ A}
    • To verify calculations, multiply current by resistance: 208=0.00277∗75000208 = 0.00277 * 75000
      • Result: Approximately 202.5V
Circuit Load Calculations
  • Total Wattage Calculation: For 11 fixtures with 150W each:
    • Total wattage:
      Total=11∗150=1650extWTotal = 11 * 150 = 1650 ext{ W}
  • To find required amperage on a 120V circuit:
    I=1650120=13.75extAI = \frac{1650}{120} = 13.75 ext{ A}
80% Rule for Circuit Loading
  • 80% Load Factor: Circuits should not exceed 80% of their rated capacity to ensure safety and prevent overheating:
    • Example: For a 20A circuit:
    • Maximum safe load = 20A * 80% = 16A
Circuit Sizing and Efficiency
  • When sizing circuits based on knowledgeable assumptions about amperage and wattage, electricians need to consider:
    • Safety margins to avoid nuisance tripping.
    • Upgrading to larger breakers or circuits as needed, while ensuring wiring capacity meets the demands.
Power Supply Considerations
  • Voltage and Current Relationship:
    • Increasing voltage reduces the current for the same power load.
    • Example Calculation: At 277V with the same wattage of 1650W:
      I=1650277=5.94 AI = \frac{1650}{277} = 5.94 \text{ A}
    • Dramatic reduction in current can allow for more fixtures without exceeding circuit limits.
Conclusion and Further Topics
  • Understanding Ohm's Law and the associated calculations are crucial for effective electrical work.
  • Future lessons will cover:
    • Series and parallel circuit calculations.
    • Sizing of conductors based on wire length, gauge, and circuit demand.
    • Voltage drop calculations during circuit operation, ensuring optimal performance.