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VOLTAGE DIVIDER RULE

When a voltage is applied across the two resistors connected in series, the voltages across each resistor are dependent upon the values of the resistors. You can say that the voltage is divided (or distributed) into two resistors, and the sum of two voltages is the same as the applied voltage. How much voltage is developed across a resistor is what the voltage rule is all about. What it says is the amount of the voltage (or the portion of the applied voltage) developed across a resistor is directly proportional to the value of the resistor. After all, according to the Ohm's Law, when current is the same, voltage is proportional to resistance, V=IR or V=I*R1 + I*R2 = V1 + V2

If a resistor (R1) has 20% of the total combined resistor, then the voltage across the resistor (V1) is 20% of the applied voltage (V) across the two resistors connected in series. The other resistor (R2)'s value is 80% (100% - 20%) and the voltage across the other resistor (V2) is 80% of the applied voltage (V).

Consider a resistor of 20W and the other resistor of 30W are serially connected and a voltage of 100V is applied at the ends of the two serially connected resistors. The voltage across 20W is then [20/(20+30)] (= 40%) of the 100V, which is 40V. The voltage across 30W is [30/(20+30)] (=60%) of 100V, which is 60V. So the formula can be said: When a voltage is applied across the two ends of the serially connected two resistors, the voltage developed across a resistor is determined by the portion of the applied voltage which is calculated by dividing its resistance by the sum of its resistance and the resistance of the other resistor.

Warning: Apply this voltage divider rule only when a voltage (whether a voltage source or developed by other means) is applied to the ends of two (or more) serially connected resistors.

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