content_image/577263.jpg content_image/577264.jpg content_image/577272.jpg content_image/577265.jpg content_image/577266.jpg content_image/577274.jpg content_image/577275.jpg content_image/577270.jpg content_image/577271.jpg
## Ohm’s Law 

Ohm’s Law describes the basic mathematical relationships of electricity. The law was named after German Physicist George Simon Ohm (1789–1854). Basically, Ohm’s Law states that the current (electron flow) through a conductor is directly proportional to the voltage (electrical pressure) applied to that conductor and inversely proportional to the resistance of the conductor. The unit used to measure resistance is called the ohm. The symbol for the ohm is the Greek letter omega (Ω). In mathematical formulas, the capital letter R refers to resistance. The resistance of a conductor and the voltage applied to it determine the number of amperes of current flowing through the conductor. Thus, 1 ohm of resistance limits the current flow to 1 ampere in a conductor to which a voltage of 1 volt is applied. The primary formula derived from Ohm’s Law is: E = I × R (E = electromotive force measured in volts, I = current flow measured in amps, and R = resistance measured in ohms). This formula can also be written to solve for current or resistance: 

<img src='content_image/577263.jpg'>

<img src='content_image/577264.jpg'>

Ohm’s Law provides a foundation of mathematical formulas that predict how electricity responds to certain conditions. [Figure 9-1] For example, Ohm’s Law can be used to calculate that a lamp of 12 Ohms (Ω) passes a current of 2 amps when connected to a 24-volt direct current (DC) power source. 

## Example 1 

A 28-volt landing light circuit has a lamp with 4 ohms of resistance. Calculate the total current of the circuit. 

<img src='content_image/577272.jpg'>

<img src='content_image/577265.jpg'>

<img src='content_image/577266.jpg'>

I = 7 amps 

## Example 2 

A 28-volt deice boot circuit has a current of 6.5 amps. Calculate the resistance of the deice boot. 

<img src='content_image/577274.jpg'>

<img src='content_image/577275.jpg'>

R = 4.31Ω 

## Example 3 

A taxi light has a resistance of 4.9Ω and a total current of 2.85 amps. Calculate the system voltage. 

<img src='content_image/577270.jpg'>

<img src='content_image/577271.jpg'>

E = 14 volts 

Whenever troubleshooting aircraft electrical circuits, it is always valuable to consider Ohm’s Law. A good understanding of the relationship between resistance and current flow can help one determine if a circuit contains an open or a short. Remembering that a low resistance means increased current can help explain why circuit breakers pop or fuses blow. In almost all cases, aircraft loads are wired in parallel to each other; therefore, there is a constant voltage supplied to all loads and the current flow through a load is a function of that load’s resistance. 

Figure 9-2  illustrates several ways of using Ohm’s Law for the calculation of current, voltage, and resistance. 

## Current 

Electrical current is the movement of electrons. This electron movement is referred to as current, flow, or current flow. In practical terms, this movement of electrons must take place within a conductor (wire). Current is typically measured in amps. The symbol for current is I and the symbol for amps is A. 

The current flow is actually the movement of the free electrons found within conductors. Common conductors 

