Exam Tidbits: Ohm's Law

Ohm's Law

• Ohm's Law is frequently tested on the NASPExAM AP.
• Understanding Ohm's Law requires careful attention to the following information.

Ohm's Law Formula

• Traditional formula:
  • $V = IR$
    • Where:
      • V = Voltage (electromotive force)
      • I = Current (flow of electrons)
      • R = Resistance (measured in ohms)
• Furman's formula:
  • $E = IR$
    • Where:
      • E = Electromotive Force
• Energy formula (Furman):
  • $ENERGY = EIt$
  • $ENERGY = voltage \times current \times time (pulse width)$

Resistance

Series

• Definition: The beginning of one resistance is connected to the end of another.
• Calculation: Sum of the resistances.
  • $R1 + R2 = Total resistance$
• Voltage Drop:
  • The percentage of voltage drop across a resistance can be determined by the ratio of individual resistances to the total resistance.
  • Example: If there are two wires, one with $10$ ohms and the other with $40$ ohms, total resistance is $50$ ohms.
    • Voltage drop across $10$ ohm wire: $20\%$ of the total
    • Voltage drop across $40$ ohm wire: $80\%$ of the total
  • The sum of percentage drops must equal $100\%$.
• Example: Lead fracture (increases resistance).

Parallel

• Definition: Resistances are connected to the SAME point.
• Example: Lead insulation defect.
• Calculation: Product of the resistances divided by their sum.
  • $(R1 \times R2) / (R1 + R2)$

Impedance

• Lead fractures: INCREASE impedance.
• Lead insulation defects: DECREASE impedance.
• Normal lead impedance range (Furman, 3rd edition): $400-600$ ohms (Note: modern leads can range from mid $300$s to over $1000$ ohms).

Pulse Generators

• Permanent pacemakers: ALL are constant voltage devices.
• Temporary pacemakers: SOME are constant voltage, but most are constant current.

Load

• Definition: Impedance (or resistance) applied to a circuit.
• Small Load (low impedance): Constant current device.
• Large Load (high impedance): Constant voltage device.