Circuits and Terms

A Simple Electric Circuit

Purpose:

To demonstrate what is needed to produce a simple electric circuit.

Materials:

1.5 to 6 volt battery; light bulb and socket; at least two pieces of hookup wire.

Procedure:

1. Attach a piece of hookup wire to the battery and show that the bulb will not light if the wire is attached to either terminal of the light bulb or either terminal of the battery.

2. Attach two pieces of hookup wire to the battery, one to each terminal. Show that
the bulb lights only when one wire is attached to one terminal of the bulb and the other wire to the second terminal of the bulb. Point out that in an electric circuit there must be a path from the source to the appliance, through the appliance, and back to the source. It often helps to point out that the word circuit has the same root as the word circle.

Summing Up:

Because charge is conserved, current must flow equally in all points of the circuit. The source receives as much current at one terminal as leaves from the other.

D.C. Circuits Terms

Terms to define

circuit - a closed conducting path around which charge or matter can flow.

current - the flow of electric charge or matter from one place to another.

conventional current - conventionally taken to mean that positive charge flows from a positive to negative direction; in metals where only electrons flow, this is called negative electron current.

EVERYDAY CONNECTIONS

Why won't your car start if one of the terminals falls off the battery?

Discuss the ways that water currents are similar to electric currents.

Electron Speed

When you turn the ignition key in an automobile, you complete a circuit from the negative battery terminal through the electric starter and back to the positive battery terminal. This is a DC circuit and electrons migrate through the circuit in a direction from the negative battery terminal to the positive terminal. About how long must the key be in the ON position for electrons starting from the negative terminal to reach the positive terminal?
a) A time shorter than that of the human reflex turning a switch on or off
b) 1/4 seconds
c) 4 seconds
d) 4 minutes
e) 4 hours

Answer: Electron Speed
The answer is: e. Although the electrical signal travels through the closed circuit at about the speed of light, the actual speed of electron migration (drift velocity) is much less. Although electrons in the open circuit (key in OFF position) at normal temperatures have an average velocity of some millions of kilometers per hour, they produce no current because they are moving in all possible directions. There is no net flow in any preferred direction. But when the key is turned to the ON
position the circuit is completed and the electric field between the battery terminals is directed through the connecting circuit. It is this electric field that is established int he circuit at about the speed of light.
The electrons all along the circuit continue their random motion, but are also accelerated by the impressed electric field in a direction toward the end of the circuit connected to the positive battery terminal. The accelerated electrons cannot gain appreciable speeds because of collisions with anchored atoms in their paths. These collisions continually interrupt the motion of the electrons so that their net average speed is extremely slow--less than a tiny fraction of a centimeter per second. So some hours are required for the electrons to migrate from one battery terminal through the circuit to the other terminal.

Complete Circuits

A simple electric circuit can be made with a dry cell, a lamp, and some wire. In which of the arrangements shown will be lamp be lit?

ANSWER: COMPLETE CIRCUITS
The answer is: c. A battery is not a source of energy which fills lamp s in a sink-like fashion. There must be a current "passing through" in the circuit. Current does not flow into a lamp, nor does it even flow out of a lamp. Current flows through a lamp, just as it does anywhere else in the circuit, including the battery itself. Investigation of the arrangements shows that only in c is a pathway
provided for the necessary in-to and out-of connection to the lamp.