Ohm's Law: Definition, Formula, Examples and Calculator
Ohm's law explains the relationship between voltage, electric current and resistance. It helps us calculate how much current flows through a conductor when the voltage and resistance are known.
This formula applies to an ohmic conductor when its physical conditions, such as temperature, remain constant.
What Is Ohm's Law?
Ohm's law is a fundamental principle of electricity. It states that, for a conductor under constant physical conditions, the electric current flowing through it is directly proportional to the potential difference (voltage) across its ends.
In simple words, voltage provides the electrical push, current represents the flow of electric charge, and resistance opposes that flow.
If the resistance remains constant, increasing the voltage increases the current in the same proportion. For example, doubling the voltage across a 5-ohm resistor doubles its current from 2 A to 4 A.
Voltage, Current and Resistance Explained
1. Voltage (V)
Voltage is the electrical potential difference between two points. It provides the push that drives current through a circuit.
Example: A 6 V battery provides a potential difference of 6 volts.
2. Current (I)
Electric current is the rate at which electric charge passes a point in a circuit.
Example: A current of 2 A means 2 coulombs of charge pass a point each second.
3. Resistance (R)
Resistance measures how strongly a component opposes the flow of electric current.
Example: A 10 Ω resistor has a resistance of ten ohms.
A simple water-flow analogy
Imagine water flowing through a pipe. Pressure difference is similar to voltage, the rate of water flow is similar to current, and a narrow or obstructed pipe is similar to electrical resistance.
This comparison helps explain the basic idea, although electrical circuits and water systems do not behave identically in every situation.
Ohm's Law Formula and Its Three Forms
The main formula of Ohm's law is:
Here, V represents voltage in volts, I represents current in amperes, and R represents resistance in ohms.
Calculate voltage
Multiply current by resistance.
Example: I = 2 A, R = 5 Ω
V = 2 × 5 = 10 V
Calculate current
Divide voltage by resistance.
Example: V = 12 V, R = 6 Ω
I = 12 / 6 = 2 A
Calculate resistance
Divide voltage by current.
Example: V = 12 V, I = 3 A
R = 12 / 3 = 4 Ω
Ohm's law formula table
| Quantity | Formula | SI unit |
|---|---|---|
| Voltage | V = I × R | Volt (V) |
| Current | I = V / R | Ampere (A) |
| Resistance | R = V / I | Ohm (Ω) |
Ohm's Law Calculator
Use this free calculator to find voltage, current or resistance. Enter the two known quantities and select the quantity you want to calculate.
Use non-negative values. For this basic calculator, enter exactly two known quantities and calculate the third. The calculation assumes a component that follows Ohm's law under the conditions considered.
Solved Numerical Examples of Ohm's Law
Example 1: Find the current
Question: A 12 V battery is connected across a 4 Ω resistor. Calculate the current.
Step 1: Write the given values.
Voltage, V = 12 V; Resistance, R = 4 Ω.
Step 2: Select the formula.
Step 3: Substitute the values.
I = 12 / 4 = 3 A.
Example 2: Find the voltage
Question: A current of 2 A flows through a 6 Ω resistor. Find the voltage across it.
Step 1: I = 2 A and R = 6 Ω.
Step 2: Use V = I × R.
Step 3: V = 2 × 6 = 12 V.
Example 3: Find the resistance
Question: A conductor has a potential difference of 24 V across it and carries a current of 3 A. Calculate its resistance.
Step 1: V = 24 V and I = 3 A.
Step 2: Use R = V / I.
Step 3: R = 24 / 3 = 8 Ω.
Example 4: A resistor connected to a 9 V battery
Question: What current flows through a 3 Ω resistor connected across a 9 V supply?
Using I = V / R:
I = 9 / 3 = 3 A.
Example 5: Convert kilohms into ohms
Question: A 2 kΩ resistor is connected across 10 V. Find the current in milliamperes.
Step 1: Convert 2 kΩ into ohms.
R = 2 × 1000 = 2000 Ω.
Step 2: Calculate current.
I = V / R = 10 / 2000 = 0.005 A.
Step 3: Convert amperes into milliamperes.
0.005 A × 1000 = 5 mA.
V-I Graph of Ohm's Law
For an ohmic conductor at constant temperature, voltage is directly proportional to current. Therefore, a graph of voltage against current is a straight line passing through the origin.
The slope of a voltage-versus-current graph equals resistance:
A steeper straight line means a higher resistance, provided voltage is plotted on the vertical axis and current on the horizontal axis.
Applications of Ohm's Law
Ohm's law is useful in basic circuit calculations and electrical troubleshooting.
- Finding current: Calculate the current through a resistor when its voltage and resistance are known.
- Choosing resistors: Estimate the resistance required for a desired current at a given voltage.
- Checking circuits: Compare measured voltage, current and resistance to identify possible circuit problems.
- Electronic projects: Calculate values for simple circuits containing resistors and other suitable ohmic elements.
- Power calculations: Combine Ohm's law with electrical power formulas to estimate power in a resistor.
Ohm's law and electrical power
Electrical power is calculated using:
For a resistor that obeys Ohm's law, substituting V = IR gives two additional formulas:
Using current
Using voltage
Power unit
Example: A 4 Ω resistor carries a current of 2 A.
P = I²R = 2² × 4 = 16 W.
The electrical power dissipated in the resistor is 16 watts.
Limitations of Ohm's Law
Ohm's law does not describe every electrical component in every situation. It is applicable when the voltage-current relationship is proportional under the conditions being considered.
- Temperature changes: A resistor's resistance may change as it heats up, so the original voltage-current relationship may no longer hold.
- Semiconductor devices: Diodes and many transistor junctions have nonlinear voltage-current relationships.
- Filament lamps: The filament becomes hotter as current increases, changing its resistance.
- Other non-ohmic materials: Some materials have current-voltage relationships that depend on factors such as the applied voltage, direction, temperature or illumination.
Test Your Understanding of Ohm's Law
Choose an answer for each question and check your score.
1. What is the SI unit of resistance?
2. A 12 V supply is connected across a 6 Ω resistor. What is the current?
3. Which formula calculates voltage using current and resistance?
Frequently Asked Questions About Ohm's Law
1. What is Ohm's law in simple words?
Ohm's law explains how voltage, current and resistance are related. For an ohmic conductor under constant physical conditions, voltage equals current multiplied by resistance.
2. What is the formula of Ohm's law?
The main formula is V = IR. The equivalent forms are I = V/R and R = V/I.
3. Who gave Ohm's law?
Ohm's law is named after German physicist Georg Simon Ohm, who published his work on the relationship between voltage and current in 1827.
4. What are the SI units used in Ohm's law?
Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω).
5. What happens to current when voltage increases?
If resistance remains constant, current increases in direct proportion to voltage. Doubling voltage doubles current.
6. What happens to current when resistance increases?
If voltage remains constant, increasing resistance reduces current. For example, doubling resistance halves current in an ohmic circuit.
7. Does Ohm's law apply to all electrical devices?
No. It applies to ohmic conductors under suitable conditions. Devices such as diodes and filament lamps may have nonlinear voltage-current relationships.
8. What is the slope of a V-I graph?
When voltage is plotted on the vertical axis and current on the horizontal axis, the slope of a straight-line V-I graph represents resistance.
9. What is the relationship between Ohm's law and power?
Electrical power is P = VI. For an ohmic resistor, it can also be expressed as P = I²R or P = V²/R.
10. How can I calculate current from voltage and resistance?
Divide the voltage by the resistance using I = V/R. For example, 10 V across a 5 Ω resistor produces a current of 2 A.
Related Physics Topics
Continue learning about electricity with these related lessons:
Explore these topics to build a stronger understanding of current electricity and electrical circuits.