What SI Units Are Used for Voltage, Current, Resistance, and Power?

Voltage is measured in volts $\left(\text{V}\right)$, current in amperes $\left(\text{A}\right)$, resistance in ohms $\left(\Omega\right)$, and power in watts $\left(\text{W}\right)$. These SI units are essential in A3.4 Introduction to electronic systems at HL.

These definitions also show how the units relate. One volt equals one joule of energy transferred per coulomb of charge, while one ampere equals one coulomb of charge per second. One ohm is the resistance that permits a current of one ampere when one volt is applied, and one watt equals one joule transferred per second.

The quantities are connected by Ohm's law:

$$V = IR$$

Therefore, resistance can be calculated using:

$$R = \frac{V}{I}$$

Electrical power is commonly calculated using:

$$P = VI$$

Metric prefixes are important in electronic systems: $1\text{ mA} = 0.001\text{ A}$ and $1\text{ k}\Omega = 1000\Omega$. Convert all values to compatible units before substituting them into an equation. For example, $500\text{ mA} = 0.500\text{ A}$, not . This prevents errors of factors of one thousand.

For example, if an electronic component operates at $12\text{ V}$ and draws a current of $3\text{ A}$, its resistance is:

$$R = \frac{12\text{ V}}{3\text{ A}} = 4\Omega$$

Its power consumption is:

$$P = 12\text{ V} \times 3\text{ A} = 36\text{ W}$$

A common misconception is that the quantity and its unit use the same symbol. They do not: voltage is represented by $V$, but its numerical value is measured in volts, also symbolized by $\text{V}$. Current is represented by $I$, while its unit is the ampere $\left(\text{A}\right)$.

For an IB Design Technology response, state both the full unit name and its correct symbol. In calculation questions, write the equation, substitute values with units, and give the final answer with the appropriate SI unit.