Theme 2: Energy and the Environment Background Material

Electric Circuits

Simple Circuit

Simple circuit. Copyright 1991, The National Academy of Sciences. Reproduced with permission from Electric Circuits, National Sciences Research Center.

In a simple battery/bulb circuit, the electric current in the copper wire is the flow of electrons. However, these electrons are not supplied by the battery but originate inside the copper wire.

The electrons were already in the circuit long before the battery was connected. Batteries and generators do not create these electrons, they merely pump them. The battery causes the electric charges already found in the metal conductor to flow.

When the electrons at one point in the circuit are "pumped," electrons in the entire loop of the circuit are forced to flow, and energy spreads almost instantly through the entire circuit. An electric current is a FLOW OF CHARGE. In a conductor, such as copper wire, the flow of charge is the flow of electrons associated with copper atoms.

In any simple electric circuit, the path of the electric current is a complete loop. It goes through all parts of the circuit including the battery, and including the battery's liquid electrolyte. For example, if there is one Ampere of electric current flowing in the external circuit, then there is also a 1-Amp flow of charge in the electrolyte between the battery's positive and negative plates.

Once again, the battery does not supply charges, it merely pumps them. Whenever electric charge flows into one terminal of a battery, an equal amount of charge must flow through the battery and back out through the other terminal. In a simple battery/bulb circuit, the charges flow around and around the circuit, going through both the battery and the bulb. The battery is a charge pump.

EXAMPLE: A Flashlight
In the electrical circuit found in a flashlight, all of the electrons in the copper circuit are forced to move at one time. As soon as the battery moves the electrons, the distant light bulb lights up. The electrons moving into the bulb's filament are exactly the same as the ones moving out; the bulb doesn't change them or extract stored energy from them. The light bulb slows ALL of the electrons throughout the entire circuit and extracts energy from the whole circuit as it lights up.

Series Circuit
In a series circuit, current has only one path to travel from any given point on the circuit. The current must flow through the wires, batteries and bulbs and back to the starting point.

In this type of circuit there is only one path for the electrons to follow.

Series circuit. Copyright 1991, The National Academy of Sciences. Reproduced with permission from Electric Circuits, National Sciences Research Center.

When batteries are arranged in series, the voltage across the bulb is increased. For example, in a simple flashlight each D-cell can produce approximately 1.5 volts.

Two D-cells in series produce approximately 3 volts. The additional cell will cause the light bulb to glow much brighter than it would with only one battery in the circuit.

If two identical light bulbs are arranged in series, they will burn with uniform brightness. However, neither will be as bright as in the case of having one bulb alone, since the current must travel through the resistors (filaments) of both light bulbs in order to complete the circuit.

Series circuit with two light bulbs. Copyright 1991, The National Academy of Sciences. Reproduced with permission from Electric Circuits, National Sciences Research Center.

Both of the light bulbs will stop working if one of the bulbs burns out or is removed from the receptacle because this creates an open circuit. This is a common problem in many types of Christmas lights.

In any series circuit, the total resistance is equal to the sum of the resistance provided by each light bulb. Bulbs and other electrical devices are considered as loads on a circuit in that they convert current into some other form of energy.

Parallel Circuits

A parallel circuit provides more than one path for the electrons to flow. The batteries shown here are connected in parallel. Each cell can produce 1.5 volts. The total voltage for the parallel circuit remains at 1.5 volts and the lights burn with the same brightness as they would with only one cell. However, these batteries will last much longer because they are sharing the load.
The bulbs shown here are in parallel. The current does not have to pass through one bulb to get to get to the next one in a parallel circuit. If one of the bulbs burns out or is removed from the circuit, the other bulb will not be affected. The current can flow through another branch to the remaining bulb. Each branch connects across the main line and receives the same voltage.	Copyright 1991, The National Academy of Sciences. Reproduced with permission from Electric Circuits, National Sciences Research Center.