http://www.ck12.org Chapter 17. Circuits
The symbol for a resistor:
The symbol for a capacitor:
The symbol for a switch:
Assumptions Regarding Wires and Batteries
The wires used to connect the elements of a circuit are usually assumed to have much smaller resistance than the
elements themselves. We will assume that the resistances of the wires are negligible unless otherwise stated.
Voltage Drop
The phrase voltage drop is used when a current passes through a resistor. Let us “follow” the conventional current
from the positive terminal of the battery where the potential is highest (understanding that the current in an electrical
circuit is established almost instantaneously). If a current passes through a resistor, there must be a drop in potential
across the resistor. The drop in potential is analogous to the drop in gravitational potential for a series of descending
stairs. The gravitational potential of the highest stair is similar to the potential at the positive battery terminal. Each
descending stair signifies another drop in potential, analogous to a sequence of resistors over which the voltage drops
as the current moves from one resistor to another. The “ground level” is the “negative side” of the battery-that is, the
lower-potential terminal.
Resistors in Series
Resistors are connected in series if the current through them is the same, as shown inFigure17.11. If any one
element in a series circuit fails (or is no longer capable of conducting current), none of the other elements in the
series circuit will conduct current either.Figure17.11 shows two resistors in series. Notice how the current has no
choice but to flow through each resistor.
Computing the Equivalent Resistance of a Series Circuit
Figure17.12 showsFigure17.11 with a 12Vbattery and with a resistor ofR 1 = 10. 0 Ωand a resistor ofR 2 = 6. 0 Ω.
The left side of the circuit (in red) is at a potential of 12V. There is a voltage drop acrossR 1 (from red to orange)