Voltage is a measure of electric energy per unit charge. Electrical current, the flow of electrons, is powered by voltage and travels throughout a circuit and becomes impeded by resistors, such as light bulbs. Finding the voltage drop across a resistor is a quick and simple process.
Look at the structure of the circuit to identify a parallel circuit. If a resistor is connected to other resistors and battery at both nodes, then you have a parallel circuit.
Find the sum of the reciprocals of all the resistors in the parallel resistor. The total resistance formula can be found at the Electronic-Tutorials website. Check that you accounted for all the resistors in the circuit as there may be other resistors connected in a series to the parallel resistor. If this is the case, add the total resistance of the parallel resistor to all the other resistors connected in series in the circuit.
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Multiply the amount of current (which should be given) by the total resistance of the parallel resistor. The voltage drop across a resistor is equal to the product of the amount of current passing through it and its resistance. The voltage drop across a particular resistor in the parallel resistor is equal to the voltage drop across the entire parallel resistor.
All formulas can be found at the Electronics-Tutorials website.