Theory

Introduction to Ohm's Law

Ohm's Law is the fundamental principle that describes the relationship between voltage, current, and resistance in an electrical circuit. Named after the German physicist Georg Simon Ohm, this law forms the foundation of all electrical circuit analysis.

V = I × R
Voltage (V) = Current (I) × Resistance (R)

Where:

  • V = Voltage (measured in Volts, V)
  • I = Current (measured in Amperes, A)
  • R = Resistance (measured in Ohms, Ω)

The Ohm's Law Triangle

A helpful memory tool is the Ohm's Law triangle. Cover the variable you want to solve for, and the remaining positions show how to calculate it:

Find Voltage

V = I × R

Find Current

I = V ÷ R

Find Resistance

R = V ÷ I

Key Concept:

Ohm's Law only applies to ohmic materials—components where resistance remains constant regardless of voltage or current changes. Some components like diodes and bulbs are non-ohmic.

Electrical Power

Power is the rate at which electrical energy is consumed or transferred. The unit of power is the Watt (W). There are three equivalent formulas for calculating power in a circuit:

P = V × I
Power = Voltage × Current
P = I² × R
Power = Current² × Resistance
P = V² ÷ R
Power = Voltage² ÷ Resistance

Electrical Energy

Energy is the total amount of power consumed over time. The unit of energy is the Joule (J), but in electrical billing, we commonly use Kilowatt-hours (kWh).

W = P × t
Energy (W) = Power (P) × Time (t)

Where:

  • W = Energy (Joules or Watt-hours)
  • P = Power (Watts)
  • t = Time (seconds or hours)

Practical Applications

Real-World Example:

A 100W light bulb connected to 120V mains circuit draws:

Current I = P ÷ V = 100W ÷ 120V = 0.83A

Resistance R = V ÷ I = 120V ÷ 0.83A ≈ 144Ω

Step-by-Step Problem Solving

1

Identify known values

List all given quantities from the problem statement

2

Select appropriate formula

Choose the formula that relates your known and unknown quantities

3

Rearrange if needed

Solve the formula for the unknown variable

4

Calculate and verify units

Perform the calculation and ensure correct units

Interactive Simulation - Ohm's Law Calculator

Experiment with the circuit parameters below to see how voltage, current, and resistance affect each other in real-time. The simulation will automatically calculate the power consumption.

Practice Problems

Test your understanding by solving these problems. Enter your answers and check them against the solutions.

Problem 1

Easy

A circuit has a voltage source of 24V and a resistor of 480Ω. Calculate the current flowing through the circuit.

Problem 2

Medium

A 60W light bulb is connected to a 120V power supply. Calculate:
a) The current flowing through the bulb
b) The resistance of the bulb's filament

Problem 3

Hard

An electric heater has a resistance of 20Ω and draws a current of 6A. Calculate:
a) The power consumed by the heater
b) The energy consumed in kWh if it operates for 4 hours

Knowledge Check Quiz

Test your understanding of Ohm's Law and power calculations with this 5-question quiz.

Key Takeaways

  • Ohm's Law states: V = I × R, where voltage equals current times resistance
  • Any of the three variables can be found if the other two are known
  • Power in a circuit can be calculated using P = VI, P = I²R, or P = V²/R
  • Electrical energy is power consumed over time: W = P × t
  • 1 kilowatt-hour (kWh) = 1000 watts × 1 hour = 3,600,000 Joules
  • Always verify units before performing calculations
  • Use the interactive simulation to visualize relationships between V, I, and R