Basic Circuit Laws

These principles undergird much of practical electronics applications

The basic laws of electrical circuits focus on the basic circuit parameters of voltage, current, power, and resistance. These laws define how each circuit parameter is interrelated. These laws were discovered by Georg Ohm and Gustav Kirchhoff, and are known as Ohm's law and Kirchhoff's laws.

A white background with a circuit design shown in black. At the top and bottom are arrows, indicating that a current I flows clockwise through the circuit. On the right is a jagged section of line, indicating a resistor, R. On the left is a voltage, V, with a positive on top and a negative on bottom.
Wikimedia Commons

Ohm's Law

Ohm's law is the relationship between voltage, current, and resistance in a circuit. It is the most common (and most simple) formula used in electronics. Ohm's law can be written in several ways, all of which are commonly used.

  • The current flowing through a resistance is equal to the voltage across the resistance divided by the resistance (I=V/R).
  • Voltage is equal to the current flowing through a resistor times its resistance (V=IR).
  • Resistance is equal to the voltage across a resistor divided by the current flowing through it (R=V/I).

Ohm's law is also useful in determining the amount of power a circuit uses because the power draw of a circuit is equal to the current flowing through it, multiplied by the voltage (P=IV). Ohm's law determines the power draw of a circuit as long as two of the variables in Ohm's law are known for the circuit.

One basic application of Ohm's law and the power relationship is to determine how much power is dissipated as heat in a component. This information helps you choose the right-size component with the proper power rating for a given application.

For example, when you select a 50-ohm surface mount resistor that will see 5 volts during normal operation, it must dissipate half a watt when it incurs 5 volts. The formula, with progressive substitutions, is:

  • P=I×V → P=(V÷R)×V → P=(5 volts)² ÷ 50 ohms → 0.5 watts

Therefore, you'll require a resistor with a greater power rating than 0.5 watts. Knowing the power usage of the components in a system lets you know if additional thermal problems or cooling may be required. It also dictates the size of the power supply for the system.

Kirchhoff's Circuit Laws

Kirchhoff's circuit laws tie Ohm's law into a complete system. Kirchhoff's Current Law follows the principle of conservation of energy. It states that the total sum of all current flowing into a node (or point) on a circuit is equal to the sum of the current flowing out of the node.

A simple example of Kirchhoff's Current Law is a power supply and resistive circuit with several resistors in parallel. One of the nodes of the circuit is where all the resistors connect to the power supply. At this node, the power supply generates current into the node and the current divides among the resistors and flows out of that node and into the resistors.

Kirchhoff's Voltage Law also follows the principle of conservation of energy. It states that the sum of all the voltages in a complete loop of a circuit must equal zero.

Extending the previous example of a power supply with several resistors in parallel between the power supply and ground, each individual loop of the power supply, a resistor, and ground sees the same voltage across the resistor since there is only one resistive element. If a loop has a set of resistors in series, the voltage across each resistor is divided according to the Ohms law relationship.