Ohm’s Law is one of the most important relationships in electricity. It helps us understand how voltage, current, and resistance work together in an electrical circuit.
What Is Ohm’s Law?
Ohm’s Law describes the relationship between three electrical quantities:
- Voltage (V): the electrical potential difference that pushes charge through a circuit.
- Current (I): the rate at which electric charge flows.
- Resistance (R): the opposition to the flow of electric current.
For a material or component that behaves as an ohmic conductor, current is directly related to voltage when the resistance remains constant. The relationship is written as V = IR. ([openstax.org](https://openstax.org/books/physics/pages/19-1-ohms-law?utm_source=openai))
Ohm’s Law:
V = I × R
The Three Forms of Ohm’s Law
The formula can be rearranged to find any one of the three quantities:
| Quantity to find | Formula | Unit |
|---|---|---|
| Voltage | V = I × R | volt (V) |
| Current | I = V ÷ R | ampere (A) |
| Resistance | R = V ÷ I | ohm (Ω) |
Understanding Voltage, Current, and Resistance
Voltage
Voltage is the potential difference between two points. It provides the energy-per-charge difference that can drive current through a circuit. A battery, generator, or power supply can provide voltage.
Voltage is measured in volts (V).
Current
Electric current is the flow of electric charge. A larger current means that more charge passes a point in the circuit each second.
Current is measured in amperes, usually shortened to amps (A). One ampere is equal to one coulomb of charge flowing per second.
Resistance
Resistance describes how strongly a component or material opposes current. A higher resistance usually produces a smaller current for the same applied voltage.
Resistance is measured in ohms (Ω). One ohm is equal to one volt per ampere: 1 Ω = 1 V/A. ([openstax.org](https://openstax.org/books/physics/pages/19-1-ohms-law?utm_source=openai))
How Ohm’s Law Works
Ohm’s Law shows two important patterns:
- If resistance stays constant, increasing voltage increases current.
- If voltage stays constant, increasing resistance decreases current.
For example, if the voltage across an ohmic resistor is doubled while its resistance stays the same, the current also doubles. If the resistance is doubled while voltage stays the same, the current is reduced to half its original value. ([openstax.org](https://openstax.org/books/college-physics-2e/pages/20-2-ohms-law-resistance-and-simple-circuits?utm_source=openai))
Numerical Example 1: Finding Current
A 12 V battery is connected to a resistor with a resistance of 4 Ω. What current flows through the resistor?
Known values:
- V = 12 V
- R = 4 Ω
Use the formula: I = V ÷ R
Substitute the values:
I = 12 V ÷ 4 Ω
Answer: I = 3 A
The current through the resistor is 3 amperes.
Numerical Example 2: Finding Voltage
A current of 2 A flows through a resistor of 10 Ω. What voltage is across the resistor?
Known values:
- I = 2 A
- R = 10 Ω
Use the formula: V = I × R
V = 2 A × 10 Ω
Answer: V = 20 V
The voltage across the resistor is 20 volts.
Numerical Example 3: Finding Resistance
A device uses a current of 2.5 A when connected to a 12 V supply. What is its resistance?
Known values:
- V = 12 V
- I = 2.5 A
Use the formula: R = V ÷ I
R = 12 V ÷ 2.5 A
Answer: R = 4.8 Ω
Ohm’s Law Triangle
A useful way to remember the three formulas is the Ohm’s Law triangle:
V
I R
- Cover V to get I × R.
- Cover I to get V ÷ R.
- Cover R to get V ÷ I.
Ohmic and Non-Ohmic Components
Ohm’s Law is an experimentally observed relationship, but it does not apply in exactly the same way to every material or electrical component.
- Ohmic components have approximately constant resistance over a suitable range of voltage, current, and temperature.
- Non-ohmic components do not have a constant resistance under all operating conditions.
For an ohmic component, a graph of voltage against current is a straight line when conditions such as temperature remain controlled. Components such as lamps, diodes, and many electronic devices may not behave as simple ohmic components over their full operating range. ([openstax.org](https://openstax.org/books/physics/pages/19-1-ohms-law?utm_source=openai))
Ohm’s Law and Simple Circuits
In a simple circuit, a voltage source is connected to a component such as a resistor by conducting wires. The voltage source creates a potential difference, and the resulting electric field drives charge through the circuit. The resistor limits the current and produces a voltage drop.
For a single resistor connected to an ideal voltage source, the voltage supplied by the source equals the voltage drop across the resistor. ([openstax.org](https://openstax.org/books/college-physics-2e/pages/20-2-ohms-law-resistance-and-simple-circuits?utm_source=openai))
Common Mistakes to Avoid
- Using the wrong rearranged formula: Remember that current is voltage divided by resistance, not voltage multiplied by resistance.
- Mixing units: Convert milliamperes to amperes when necessary. For example, 250 mA = 0.250 A.
- Forgetting the unit: Voltage is measured in volts, current in amperes, and resistance in ohms.
- Confusing current with voltage: Voltage is the potential difference; current is the flow of charge.
- Assuming every component is ohmic: Some components have resistance that changes with temperature, voltage, or current.
- Ignoring temperature: The resistance of some conductors changes as their temperature changes, which can affect the current.
Important Points to Remember
- Ohm’s Law is written as V = IR.
- Voltage pushes or drives current through a circuit.
- Resistance opposes the flow of current.
- For constant resistance, current increases when voltage increases.
- For constant voltage, current decreases when resistance increases.
- The unit of resistance is the ohm (Ω).
- Ohm’s Law is most directly useful for components that behave as ohmic over the conditions being studied.
Quick Recap
Ohm’s Law explains the relationship between voltage, current, and resistance. Its main formula is V = I × R. Use I = V ÷ R to find current and R = V ÷ I to find resistance. Always use consistent units and check whether the component behaves as an ohmic device.