27.0 - Introduction
Electric circuits include components such as batteries, resistors and capacitors.
These basic elements can be combined in a myriad of ways. How these
components function, by themselves and in combination, defines the fundamental
operation of electric circuits. In this chapter, we examine direct current electric
circuits, circuits in which the current always flows in the same direction.
In this chapter, you will learn how to analyze direct current circuits. On the right is a
simulation in which you can build your own electric circuits. Initially, the circuit
consists of a battery, a light bulb and wires that connect these components. The
simulation also contains additional wire segments and light bulbs.
You can place light bulbs in various places in the circuit. Pay special attention to the
brightness of the light bulbs: The brighter the light, the more power the circuit is
supplying to it. You can also use two devices in the control panel to study the circuit.
One, a voltmeter, measures the potential difference across components, while the
other, an ammeter, measures the current flowing through the wire at any location in
the circuit.
In this simulation, the battery and wires effectively have zero resistance. The simulation includes a total of five light bulbs, each with a
resistance of 50 ohms. You add and remove light bulbs and wires by dragging them; the components will snap into place. Only one light bulb
can be placed on each wire segment.
The purpose of this simulation is for you to experiment with the electric components in a circuit. One way to start is by assessing the circuit in
its initial state using the voltmeter and ammeter. How does the potential difference across the battery compare with the potential difference
across the light bulb? What about the current? Is it the same everywhere or does it differ from place to place?
Now add another light bulb above the first one in the circuit: Snap in two wire segments in a vertical orientation, and then put a segment
containing a light bulb between them. How does the potential difference across the battery now compare to the potential difference across each
light bulb? Is the current still the same everywhere? This time you should find that the current can differ by location.
You can also use the voltmeter to confirm Ohm’s law. Since you are told the bulb’s resistance (50 ohms) and can measure the potential
difference across it using the voltmeter, you can use the law to calculate the current flowing through the wire segment containing the bulb. You
then can verify your calculation using the ammeter.
You are probably thinking that this introduction has asked you to answer a lot of questions! If you cannot answer all the questions above, that is
fine. This chapter is dedicated to preparing you to address them.
27.1 - Electric circuits
In this section, we provide an overview of the components of a basic circuit. All these
components merit more discussion, but here we want to provide some context on how
they function together in a circuit.
A flashlight provides an example of a circuit. The flashlight we show contains two
batteries, a switch, a light bulb, and some metal wires that connect these components.
In the flashlight, the batteries are the source of the energy that causes the net motion of
charge in the circuit. The ends of a battery are at different electric potentials. There is a
potential difference across the two terminals that are at opposite ends of the battery. A
typical potential difference for a battery like those shown is 1.5 volts. The terminal with
the greater electric potential is marked with a plus (+) sign, and the terminal with the
lower electric potential is marked with a negative (í) sign. Putting together two batteries
as shown creates a potential difference across the two batteries of approximately 3.0
volts. The batteries are used to create an electric field that causes a net flow of
electrons: a current.
Current will only flow when there is a complete path: a loop, as opposed to a dead end.
(Circuit comes from the Latin word circumire, to go around). When the switch is in the “off” position, it creates a gap in the circuit, and current
cannot flow. When the switch is pushed to “on”, the gap is closed and a current can flow.
We have highlighted the circuit inside a flashlight in Concept 2. Let’s trace the direction of conventional current around the circuit. A positive
charge starts at the positive terminal of the battery on the right, and moves through a coiled wire called the filament in the light bulb. It exits the
filament and moves through the wire that contains the switch. The charge then flows through the batteries and starts its round trip over again.
There is resistance inside the batteries, in the wires and in the light bulb. The resistance inside the batteries and in the wires is minor compared
to that of the filament in the light bulb, and it is often reasonable to ignore these minor resistances. The light bulb is the major source of
resistance, and it supplies what is called the load resistance of the circuit. The resistance of this component and the potential difference across
the batteries determine the amount of current in the circuit.
The flashlight creates light when current flows through the filament. Modern day filaments often are made of very thin tungsten wires. As a
Electric circuit
Set of electric components connected
by wires