Ohm's Law

Ohm's Law (V = IR) is the most fundamental equation in electronics. Discovered by German physicist Georg Ohm in 1827, it describes how voltage, current, and resistance relate to each other. If you know any two of these values, you can calculate the third. Master this equation, and you can analyze almost any circuit.

Objectives

• Master all three forms of Ohm's Law (V=IR, I=V/R, R=V/I)
• Use the VIR triangle as a quick reference tool
• Calculate power dissipation using P=VI, P=I²R, and P=V²/R
• Design voltage dividers for specific output voltages
• Calculate current-limiting resistors for LEDs
• Understand component power ratings and why they matter

Key Takeaways

• V = IR is the foundation of circuit analysis
• Use the VIR triangle: cover what you want to find
• Power formulas: P = VI = I²R = V²/R
• Voltage divider: V_out = V_in × R2/(R1+R2)
• LED resistor: R = (V_supply - V_LED) / I_LED
• Always check power ratings exceed your calculated dissipation
• Convert units: 1A = 1000mA, 1kΩ = 1000Ω

Theory

Ohm's Law

The current through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance.

V = I × R

Practice Problems

Problem 1: A 12V power supply is connected to a 220Ω resistor. What current flows?

Problem 2: What power does a 470Ω resistor dissipate with 9V across it?

Problem 3: A voltage divider uses R1 = 10kΩ and R2 = 5kΩ with 12V input. What is V_out?

Problem 4: A 1kΩ resistor is rated for 0.25W. What is the maximum safe voltage across it?

Problem 5: An LED needs 15mA and has V_f = 2.1V. What resistor is needed for 5V USB power?