Chapter 7 Series Circuits 61 Copyright Goodheart-Willcox Co., Inc. Copyright Goodheart-Willcox Co., Inc. The voltage lost or dropped across R1 is 25 V, so the voltage across or between point C and point B is 100 V – 25 V or 75 V. The voltage lost or dropped across R2 is 25 V, so the voltage between point D and point B is 75 V – 25 V or 50 V. The voltage lost or dropped across R3 is 25 V, so the voltage between point E and point B is 50 V – 25 V or 25 V. The voltage lost or dropped across R4 is 25 V, so the voltage at point B is 25 V – 25 V or 0 V. Add up all the voltage drops and they will total 100 V, which is the applied voltage of the circuit. 25 V + 25 V + 25 V + 25 V = 100 V In addition to his current law, Kirchhoff also formulated a voltage law that bears his name as well. Kirchhoff’s voltage law states: The sum of the voltage drops in a circuit must equal the applied, or source, voltage. Or: ET = ER1 + ER2 + ER3 + ER4… where ET is the total applied voltage in the circuit, ER1 is the voltage drop across resistor 1, ER2 is the voltage drop across resistor 2, etc. In Figure 7-5, we know the total current (0.5 A) and the total resistance (R1 + R2 + R3 = RT or 50 Ω + 150 Ω + 200 Ω = 400 Ω), but we do not know the applied voltage. Ohm’s law states that: E = IR so, E = 0.5 A × 400 Ω = 200 V The individual voltage drops now can be easily found: ER1 = 0.5 A × 50 Ω = 25 V ER2 = 0.5 A × 150 Ω = 75 V ER3 = 0.5 A × 200 Ω = 100 V Total = 200 V The sum of the voltage drops is 200 volts. It is the same as we computed for the applied (source) voltage. Never use a lamp or appliance cord in your home if the insulation covering is worn out! If insulation from the cord is damaged, unplug the device and have the cord replaced. Never remove a plug from an outlet by pulling on the cord. Always pull by the plug. This prevents the cord from separating from the plug. Safety Note R3 = 200 Ω I = 0.5 amps E = ? R2 = 150 Ω R1 = 50 Ω Goodheart-Willcox Publisher Figure 7-5. The resistance and current are known. What is the value of the voltage applied to the circuit?