(455) Ohm’s law, resistance and power [IB Physics SL/HL]

Ohm's Law

• Definition: Ohm's Law relates voltage (V), current (I), and resistance (R).

  • Formulas:

    • R = V / I

    • V = I

• Units:

  • Resistance (R): Ohms (Ω)

  • Voltage (V): Volts (V)

  • Current (I): Amperes (A)

Linearization of Ohm's Law

• Linear Graphing:

  • Rewrite Ohm's Law: R * I = V

  • This represents a straight-line equation: y = mx + b

  • Gradient of the graph correlates with resistance, which is defined as ΔV / ΔI.

Characteristics of Resistance

• Ohmic vs Non-Ohmic:

  • Ohmic behavior: Linear relationship (�V vs I graph passes through origin)

  • Non-ohmic behavior: Curved relationship

Resistivity and Wire Resistance

• Resistance in a Wire:

  • Defined by resistivity (ρ), length (L), and cross-sectional area (A)

  • Formula: R = ρ (L / A)

  • Units for resistivity: Ohm-meters (Ω·m)

Power in Electrical Circuits

• Definition: Power relates to energy loss (heating) in resistors.

• Formulas:

  • Power (P) = I * V

  • Alternative forms: P = V² / R, P = I² * R

  • Units for power: Watts (W) = Joules/second (J/s)

Example Problem: Finding Power

• Step 1: Identify given values (current or resistance needed).

• Step 2: Redraw circuits for clarity (especially with parallel and series resistors).

• Step 3: Simplify the circuit to find equivalent resistance.

• Step 4: Use the power formulas based on known quantities (P = V²/R or P = I * V).

• Final Calculation: E.g., using P = V²/R with V = 8V and R = 4Ω, yields P = (8²)/4 = 16 W.

Conclusion

• Understanding the relationships in electrical circuits is crucial for solving problems involving Ohm's Law, resistance, and power.