# Ohm’s Law, Kirchhoff’s Laws, and Power Equations

March 22, 2020 by Robert Keim
This video tutorial introduces fundamental laws and properties of electrical circuits and systems: Ohm's law, Kirchhoff's laws, and power equations.

### Video Transcript

Electronic design is fundamentally about electric circuits. That is, networks that allow electric charge to start at a source, move through a continuous pathway of conductors and components and return to the source.

### Electron Flow Versus Current Flow

If you're accustomed to thinking about electricity as the movement of electrons which flow from lower to higher voltage, you need to remember that in the context of circuit design and analysis, we assume that current is the movement of electric charge from higher voltage to lower voltage (Figure 1).

### Kirchhoff's Current Law

Kirchhoff's current law, abbreviated KCL, states that the total current entering a node is equal to the total current exiting a node. KCL allows us to use known currents to determine an unknown current. In this example (Figure 2), if we measure I1 and I2, we can calculate I3, because I1= I2 +  I3 .

Figure 2. Kirchhoff's current law (KCL): the total current entering a node equals the total current exiting a node

### Kirchhoff's Voltage Law

Voltage changes between one terminal of a component and another terminal of the same component. This is called a voltage drop. Kirchhoff's voltage law, abbreviated KVL, tells us that the sum of the voltage drops in a closed circuit is equal to the voltage supplied by the source.

In this example, since we know VR1 equals 2 V and Vsupply equals 5 V, we know from KVL that VR2 equals 3 V.

### Ohm’s Law

Ohm's law states that the current through a resistive component is equal to the voltage across the component divided by the resistance.

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

We can rearrange this equation to show that the voltage across a resistive component is equal to the current flowing through the component multiplied by the resistance.

$$V =IR$$

This diagram (Figure 4) shows how Ohm's law can be used to determine, first, the total current (I) in the circuit and, second, the voltage (V) dropped across each resistor.

Figure 4. Ohm’s law defines the relationship between current, voltage, and resistance

### Electrical Power

Neither voltage nor current is a direct indication of the way in which a circuit is using energy. However, energetic calculations are very important because energy, not voltage or current, is the quantity that corresponds to a system's ability to perform useful work. The energetic characteristics of an electric circuit or component are analyzed by means of power, which tells us the rate at which energy is consumed or transferred. Electric power in watts (W) is the product of voltage in volts (V) and current in amperes (A):

$$P=IV$$

By combining this equation with Ohm's law, we can create alternative expressions that emphasize the relationship between power and voltage or between power and current:

$$P=\frac {V^2}{R}$$

$$P=I^2R$$

### Power Dissipation

Resistive materials convert electrical energy into heat. In most cases, this heat is not desired. Consequently, the power associated with a resistive material is dissipated into the environment, and we often refer to electrical power as power dissipation.

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