2.2 Relationships between quantities
For many materials, current and voltage are directly proportional to each other over a wide range of values, with the resistance as the constant of proportionality, so
voltage = current × resistance
Such materials are said to obey Ohm’s law and are said to be ohmic. However, not all materials are ohmic in nature.
As current flows through a circuit, it transfers energy. When it flows through a material that has a non-zero resistance, this energy is used (for instance, to light a bulb or run a motor) or dissipated as heat (which is why electronics circuits sometimes feel warm, and why computers need cooling fans). As in every other context, the rate of change of energy is known as power, and can be measured in watts (symbol W).
In the context of an electrical current flowing through a resistor, the power used or dissipated can be calculated by multiplying the current flowing through it by the potential difference across the resistor, giving
power (watts) = current (amps) × potential difference (volts)
Before you move on, Table 1 recaps the electrical quantities mentioned so far, their units and how they relate to each other.
| Quantity | Unit | Equation |
|---|---|---|
| energy | joules, J | |
| voltage (potential difference) | volts, V or energy per charge, joules per coulomb, J C−1 |
|
| current | amps, A or charge per second, coulombs per second, C s−1 |
|
| power | watts, W or energy per second, joules per second, J s−1 |
power = voltage × current |
| resistance | ohms, Ω | resistance = voltage ÷ current |
All the quantities listed in Table 1 can be measured in larger or smaller multiples of their standard unit using SI prefixes, which make it easier to read values at a glance. Some of these are listed in Table 2.
| Prefix | Symbol | Multiple of standard unit | Example | |
|---|---|---|---|---|
| micro | µ | one millionth | 10−6 | microamp, µA |
| milli | m | one thousandth | 10−3 | millivolt, mV |
| kilo | k | one thousand | 103 | kilo-ohm, kΩ |
| mega | M | one million | 106 | megawatt, MW |
As you gain experience in electronics, try to notice the values of currents, voltages and other quantities for the components you see, and note the associated effects they are having on the circuit. This will help you to choose values when you design, and to troubleshoot when designs or circuits do not work.