What Are Current, Voltage and Resistance?

The first three concepts to learn when starting out in electronics are current, voltage and resistance. These three are the key to understanding how a circuit works, and they're all tied together by Ohm's law.

What Is Voltage?

Voltage is the electrical potential difference between two points; the "push" that moves electrons. Its unit is the volt (V). The 1.5 V across a battery's terminals is that battery's "pressure" to push electrons through the circuit.

What Is Current?

Current is the amount of charge passing through a conductor per unit time; that is, the flow rate of electric charge. Its unit is the ampere (A). In small circuits it's usually discussed at the milliampere (mA) level (1 A = 1000 mA).

What Is Resistance?

Resistance is the opposition to the flow of current. Its unit is the ohm (Ω). The higher the resistance, the less current flows under the same voltage.

The Water-Pipe Analogy

The easiest way to understand the three is plumbing:

• Voltage = the water pressure (how hard it pushes)
• Current = the amount of water through the pipe (flow rate)
• Resistance = the narrowness of the pipe (how much it hinders flow)

Increase the pressure (voltage) and more water (current) flows; narrow the pipe (increase resistance) and the flow drops.

The Relationship Between the Three

These three quantities are tied together by Ohm's law: V = I × R. Know two and you can calculate the third. For example, connect a 450 Ω resistor to a 9 V source and the current is I = V / R = 9 / 450 = 0.02 A = 20 mA.

How Are They Measured?

All three are measured with a multimeter: voltage is measured in parallel with the circuit, current in series; resistance is measured with the component out of the circuit. For the details, see How Do You Measure Voltage, Current and Resistance?

Once you've grasped this trio, for the math that ties them together: What Is Ohm's Law?