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:

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.

Figure 9-1. Ohm's Law used to calculate how much current a lamp will pass when connected to a 24-volt DC power source.

Figure 9-1. Ohm’s Law used to calculate how much current a lamp will pass when connected to a 24-volt 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.

Example 2

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

Example 3

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

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.

Figure 9-2. Ohm's Law chart.

Figure 9-2. Ohm’s Law chart.