<P> National and local electrical codes may set guidelines for the maximum voltage drop allowed in electrical wiring, to ensure efficiency of distribution and proper operation of electrical equipment . The maximum permitted voltage drop varies from one country to another . In electronic design and power transmission, various techniques are employed to compensate for the effect of voltage drop on long circuits or where voltage levels must be accurately maintained . The simplest way to reduce voltage drop is to increase the diameter of the conductor between the source and the load, which lowers the overall resistance . In power distribution systems, a given amount of power can be transmitted with less voltage drop if a higher voltage is used . More sophisticated techniques use active elements to compensate for the undesired voltage drop . </P> <P> Consider a direct - current circuit with a nine - volt DC source; three resistors of 67 ohms, 100 ohms, and 470 ohms; and a light bulb--all connected in series . The DC source, the conductors (wires), the resistors, and the light bulb (the load) all have resistance; all use and dissipate supplied energy to some degree . Their physical characteristics determine how much energy . For example, the DC resistance of a conductor depends upon the conductor's length, cross-sectional area, type of material, and temperature . </P> <P> If the voltage between the DC source and the first resistor (67 ohms) is measured, the voltage potential at the first resistor will be slightly less than nine volts . The current passes through the conductor (wire) from the DC source to the first resistor; as this occurs, some of the supplied energy is "lost" (unavailable to the load), due to the resistance of the conductor . Voltage drop exists in both the supply and return wires of a circuit . If the voltage across each resistor is measured, the measurement will be a significant number . That represents the energy used by the resistor . The larger the resistor, the more energy used by that resistor, and the bigger the voltage drop across that resistor . </P> <P> Ohm's Law can be used to verify voltage drop . In a DC circuit, voltage equals current multiplied by resistance . V = I R (\ displaystyle V = IR). Also, Kirchhoff's circuit laws state that in any DC circuit, the sum of the voltage drops across each component of the circuit is equal to the supply voltage . </P>

Why is there a potential drop across a resistor
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