Copyright Goodheart-Willcox Co., Inc. 60 Electricity Copyright Goodheart-Willcox Co., Inc. In the series circuit, whatever level of current enters one resistor, that same current level must fl ow out. In turn, the current enters the next resistor, fl ows out to the next resistor, and so on. In Figure 7-3, the circuit of Figure 7-2 has been redrawn to include ammeters to measure current. Every meter will indicate the same current fl ow, regardless of the value of any one of the resistances. Of course, if any resistor is changed in value, the total current in the circuit will also change. Ohm’s law shows this to be true. IT = ET ___ RT . Note that ammeters are always connected in series in the circuit. Voltage Drop In our study of the volt, we learned that it was the unit of measurement for potential difference or electromotive force (voltage). A force is required to cause a current to fl ow through a circuit. In Figure 7-4, the circuit has been redrawn and values assigned. The total resistance of this circuit is equal to: RT = 100 Ω + 100 Ω + 100 Ω + 100 Ω = 400 Ω The current fl owing in all parts of the circuit is: I = 100 V ______ 400 Ω = 0.25 amps But point A has a potential of 100 volts and point B is zero. Has the 100 volts been lost somewhere along the circuit between A and B? You can understand how this can happen when you think of a resistance as something that must be overcome by a force. Actually, the resistance has been overcome, but some of the force or voltage has been lost. The voltage lost by resistance in a circuit is called the voltage drop, or IR drop. It is called IR drop because Ohm’s law states that: E = IR To fi nd the voltage drop across each resistor is relatively simple. First fi nd the current, which we have computed as 0.25 A, then: ER1 = 0.25 A × 100 Ω = 25 V ER2 = 0.25 A × 100 Ω = 25 V ER3 = 0.25 A × 100 Ω = 25 V ER4 = 0.25 A × 100 Ω = 25 V A A R1 R2 R3 R4 A A A 1 2 3 4 5 Ammeter Goodheart-Willcox Publisher Figure 7-3. All of the ammeters in the circuit will read the same current value. R4 = 100 Ω R3 = 100 Ω C D B A E E = 100 V R2 = 100 Ω R1 = 100 Ω Goodheart-Willcox Publisher Figure 7-4. The resistance and voltage are known. What are the voltage drops across each resistor?