How Do You Calculate Voltage Using V = IR?

Use Ohm's law: multiply the current by the resistance. The voltage across a component is calculated using $V = IR$, where $V$ is voltage, $I$ is current, and $R$ is resistance.

Applying the Equation

First, identify the current and resistance, convert them into standard SI units, and then substitute them into $V = IR$.

For example, suppose a resistor has a resistance of $120\,\Omega$ and carries a current of $0.25\,\text{A}$. Substitute these values:

$$V = IR$$ $$V = 0.25 \times 120 = 30\,\text{V}$$

Therefore, the potential difference across the resistor is $30\,\text{V}$.

Unit conversion is important. If the current is given as $250\,\text{mA}$, convert it before calculating:

$$250\,\text{mA} = \frac{250}{1000}\,\text{A} = 0.250\,\text{A}$$

Prefixes must be handled before substitution. The prefix milli means one thousandth, so a value in milliamperes is divided by $1000$ to obtain amperes. Likewise, $1\,\text{k}\Omega = 1000\,\Omega$. As a quick check, increasing either current or resistance while the other remains constant must increase voltage. You can also check units: $\text{A} \times \Omega = \text{V}$. These checks help identify calculator and conversion errors before you evaluate whether the chosen component operates within its rated voltage. This is especially important when datasheets specify maximum operating limits.

In electronic systems, this calculation helps designers select suitable power supplies, resistors, and other components. For an ohmic resistor, resistance remains constant provided physical conditions, particularly temperature, remain constant.

A common misconception is to add current and resistance because voltage, current, and resistance appear together in the equation. This is incorrect: $I$ and $R$ must be multiplied when calculating $V$.

IB Exam Technique

For B3.4 Electronic systems application and selection (HL), show the formula, substitute values with units, perform any prefix conversion, and state the final answer in volts. Examiners may also expect you to use the result to justify whether a component or power supply is suitable for the electronic system.