Ohm’s Law Current 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: A taxi light has a resistance of 4.9Ω and a total current of  2.85 amps. Calculate the system voltage. 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.  A 28-volt landing light circuit has a lamp with 4 ohms of  resistance. Calculate the total current of the circuit. 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. 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 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 Example 2 Example 3 A 28-volt deice boot circuit has a current of 6.5 amps.  Calculate the resistance of the deice boot. I = E R R =  E I I = E R I = 28 volts 4Ω I = 7 amps R = 4.31Ω E = 14 volts E = I  ×  R E = 2.85  ×  4.9Ω E = 24 V DC  R = 12Ω I = 2A 9-2 R =  E I R =  28 volts 6.5 amps