Ohm's Law Calculator
Basic Ohm's Law (V = I × R)
Enter two values to calculate the third. Leave one blank.
Power Calculation
Calculate power from voltage and current (P = V × I)
Series Resistance Calculator
Calculate total resistance in series (R_total = R1 + R2 + R3...)
Parallel Resistance Calculator
Calculate total resistance in parallel (1/R_total = 1/R1 + 1/R2 + 1/R3...)
Calculation Results
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Ohm's Law states that the current through a conductor is directly proportional to the voltage and inversely proportional to the resistance. These calculations assume ideal conditions. Real-world circuits may have additional factors like temperature, frequency, and material properties.
Ohm's Law & Electrical Circuits Guide
Ohm's Law is one of the fundamental principles of electrical engineering. Understanding voltage, current, and resistance relationships is essential for circuit design, troubleshooting, and electrical calculations.
Ohm's Law Formula
Voltage (volts) = Current (amperes) × Resistance (ohms)
I = V / R
Current = Voltage ÷ Resistance
R = V / I
Resistance = Voltage ÷ Current
Electrical Units
| Quantity | Symbol | Unit | Definition |
|---|---|---|---|
| Voltage | V | Volt (V) | Electrical potential difference (pressure) |
| Current | I | Ampere (A) | Flow of electrical charge |
| Resistance | R | Ohm (Ω) | Opposition to current flow |
| Power | P | Watt (W) | Rate of energy consumption |
| Energy | E | Joule (J) | Total work done by electrical power |
| Frequency | f | Hertz (Hz) | Cycles per second (AC circuits) |
Power Formulas
Power = Voltage × Current
P = I² × R
Power = Current² × Resistance
P = V² / R
Power = Voltage² ÷ Resistance
E = P × t
Energy = Power × Time
Series vs Parallel Circuits
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Current Path | Single path (same current everywhere) | Multiple paths (current splits) |
| Total Voltage | V_total = V1 + V2 + V3... | V_total = V1 = V2 = V3... (same) |
| Total Current | I_total = I1 = I2 = I3... (same) | I_total = I1 + I2 + I3... |
| Total Resistance | R_total = R1 + R2 + R3... | 1/R_total = 1/R1 + 1/R2 + 1/R3... |
| Failure Impact | One break stops all | Others continue working |
| Common Use | Battery and cells in devices | Home electrical outlets |
Common Voltage Standards
| Application | Voltage | Type | Common Use |
|---|---|---|---|
| Batteries (AA/AAA) | 1.5V | DC | Remote controls, toys |
| Car Battery | 12V | DC | Automobiles |
| Phone/USB | 5V | DC | Smartphones, tablets |
| Laptop | 15-20V | DC | Laptop power adapters |
| Home (USA) | 120V / 240V | AC | Household outlets |
| Home (Europe) | 230V | AC | Household outlets |
| Industrial | 400-480V | AC | Three-phase power |
Resistance Color Code
Resistors use color bands to indicate their value. The first two bands represent significant digits, the third band is the multiplier, and the fourth is tolerance.
| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1,000 | — |
| Yellow | 4 | ×10,000 | — |
| Green | 5 | ×100,000 | ±0.5% |
| Blue | 6 | ×1,000,000 | ±0.25% |
| Violet | 7 | ×10,000,000 | ±0.1% |
| Grey | 8 | ×100,000,000 | — |
| White | 9 | ×1,000,000,000 | — |
Power Dissipation
When current flows through a resistor, power is dissipated as heat. Understanding power dissipation is important for component selection and heat management.
AC vs DC Circuits
- DC (Direct Current): Current flows in one direction. Voltage is constant. Used in batteries, electronic devices. Ohm's Law applies directly.
- AC (Alternating Current): Current changes direction periodically (60 Hz in USA, 50 Hz in Europe). Voltage varies sinusoidally. Uses RMS (root mean square) values for calculations. Includes reactance in addition to resistance.
- RMS Voltage: For AC, RMS = Peak × 0.707. A 120V AC outlet is RMS voltage, peak is ~170V.
Circuit Analysis Tips
- Voltage Drop: In series circuits, voltage drops across each component proportionally to its resistance
- Kirchhoff's Voltage Law: Sum of voltage drops in a closed loop equals supply voltage
- Kirchhoff's Current Law: Current entering a node equals current leaving it
- Power Dissipation: Choose resistor ratings higher than calculated dissipation (typically 2-4x)
- Impedance: In AC circuits, impedance combines resistance and reactance (Z² = R² + X²)
Common Scenarios
| Scenario | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| LED with 220Ω resistor | 5V | ~23mA | 220Ω | 0.11W |
| Phone charger | 5V | 2A | 2.5Ω | 10W |
| Light bulb (100W) | 120V | 0.83A | 144Ω | 100W |
| Hair dryer | 120V | 15A | 8Ω | 1800W |
| Car starter motor | 12V | 100A | 0.12Ω | 1200W |
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Frequently Asked Questions
1. What is Ohm's Law?
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. Formula: V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.
2. Why is Ohm's Law important?
Ohm's Law is fundamental to understanding electrical circuits. It allows engineers and technicians to calculate unknown values, design circuits, select components, troubleshoot problems, and ensure safety. Almost all electrical calculations start with Ohm's Law.
3. What's the difference between voltage, current, and resistance?
Voltage is the electrical pressure pushing electrons (like water pressure). Current is the flow of electrons (like water flow). Resistance opposes current flow (like friction). An analogy: water in a hose with pressure (voltage), flow rate (current), and a narrow nozzle (resistance).
4. Why do resistors get hot?
When current flows through a resistor, it dissipates energy as heat according to P = I² × R. The higher the current or resistance, the more heat is generated. This is why power rating is critical - exceeding it causes the resistor to burn.
5. How do I choose resistor values for an LED?
Example: 5V supply, LED rated 3.2V @ 20mA. Voltage drop across resistor: 5V - 3.2V = 1.8V. Resistance needed: R = V / I = 1.8V / 0.02A = 90Ω. Use 100Ω standard resistor. Power: P = 1.8V × 0.02A = 0.036W, so 1/4W resistor is safe.
6. What happens in a short circuit?
A short circuit occurs when resistance approaches zero. Current becomes extremely high (I = V / R approaches infinity). This generates massive heat, can destroy components, and is a fire hazard. Always use fuses or circuit breakers to protect against shorts.
7. Why are household circuits parallel?
Parallel circuits ensure all outlets get full voltage (120V or 230V). If they were series, voltage would drop across each device. Parallel also allows independent operation - you can turn off one device without affecting others.
8. What is impedance in AC circuits?
Impedance (Z) combines resistance (R) and reactance (X). Formula: Z = √(R² + X²). Reactance comes from capacitors and inductors which behave differently in AC vs DC. For DC, impedance equals resistance.
9. What's RMS voltage?
RMS (Root Mean Square) is the effective value of AC voltage. A 120V RMS AC outlet has the same power delivery as a 120V DC source. Peak voltage = RMS × √2 ≈ RMS × 1.414. So 120V RMS = ~170V peak.
10. How do I calculate total resistance in complex circuits?
Break the circuit into sections: series resistances add (R_t = R1 + R2...). Parallel resistances use 1/R_t = 1/R1 + 1/R2.... Simplify step-by-step from the most complex part to the simplest until you get total resistance.
11. What is power factor?
Power factor is the ratio of real power to apparent power in AC circuits. Perfect factor = 1.0 (pure resistance). Capacitive/inductive loads have lower factors. Low power factor means more current needed for same power, increasing losses.
12. Can I use a higher power-rated resistor as a replacement?
Yes, higher power-rated resistors are safe (they dissipate heat better). However, don't use lower power-rated resistors - they'll overheat and fail. The resistance value must be the same or you'll change circuit behavior.