Shock Value

Electric Current
What is an Electric Circuit?

Electric potential is the amount of electric potential energy per unit of charge that would be possessed by a charged object if placed within an electric field at a given location. The concept of potential is a location-dependent quality � it expresses the quantity of potential energy per charge basis such that it is independent on the amount of charge actually present on the object possessing the electric potential. Electric potential difference is simply the difference in electric potential between two different locations within an electric field.

To illustrate the concept of electric potential difference and the nature of an electric circuit, consider the following situation. Suppose that there are two metal plates oriented parallel to each other and each being charged with an opposite type of charge - one being positive and the other negative. The arrangement of charged plates would create an electric field in the region between the plates which is directed away from the positive plate and towards the negative plate. A positive test charge placed between the plates would move away from the positive plate and toward the negative plate. This movement of positive test charge from the positive plate to the negative plate would occur without the need of energy input in the form of work; it would occur naturally and thus lower the potential energy of the charge. The positive plate would be the high potential location and the negative plate would be the low potential location. There would be a difference in electric potential between the two locations.

Now suppose that the two oppositely charged particles are connected by a metal wire. What would happen? The wire serves as a sort of charge pipe through which charge can flow. Over the course of time, one could think of positive charges moving from the positive plate through the wire to the negative plate. That is, positive charge would naturally move in the direction of the electric field which has been created by the arrangement of the two oppositely charged plates. As a positive charge leaves the upper plate, the plate would become less positively charged. As a positive charge reaches the negative plate, the plate would become less negatively charged. Over the course of time, the amount of positive and negative charge on the two plates would slowly diminish. Since the electric field depends upon the amount of charge present on the object creating the electric field, the electric field created by the two plates would gradually diminish in strength over the course of time. Eventually, the electric field between the two plates would become so small that there would be no observable movement of charge between the two plates. The plates would ultimately lose their charge and reach the same electric potential. In the absence of electric potential difference there would be no charge flow.

The above illustration comes close to demonstrating the meaning of an electric circuit. However, to be a true circuit, charges must continually flow through a complete loop, returning to their original position and cycling through again. If there is a means of moving positive charge from the negative plate back up to the positive plate, then the movement of positive charge downward through the charge pipe would occur continuously. In such a case, a circuit or loop would be established.

A common lab activity that illustrates the necessity of a complete loop utilizes a battery pack, a light bulb, and some connecting wires. The activity involves observing the affect of connecting and disconnecting wire in a simple arrangement of the battery pack, light bulb and wires. When all connections are made to the battery pack, the light bulb lights. In fact, the lighting of the bulb occurs immediately after the final connection is made. There is no perceivable time delay between when the last connection is made and when the light bulb is perceived to light up.

The fact that the light bulb lights and remains lit is evidence that charge is flowing through the light bulb filament and that an electric circuit has been established. A circuit is simply a closed loop through which charges can continuously move. To demonstrate that charges are not only moving through the light bulb filament but also through the wires connecting the battery pack and the light bulb, a variation of the above activity is made. A compass is placed beneath the wire at any location such that its needle is placed in alignment with the wire. Once the final connection is made to the battery pack, the light bulb lights and the compass needle deflects. The needle serves a detector of moving charges within the wire. When it deflects, charges are moving through the wire. And if the wire is disconnected at the battery pack, the light bulb is no longer lit and the compass needle returns to its original orientation. When the light bulb lights, charge is moving through the electrochemical cells of the battery, the wires and the light bulb filaments; the compass needle detects the movement of this charge. It can be said that there is a current - a flow of charge within a circuit.

The electric circuit demonstrated by the combination of battery, light bulb and wires consists of two distinct pats: the internal circuit and the external circuit. The part of the circuit containing electrochemical cells of the battery is the internal circuit.

Requirements of a Circuit

Suppose that you were given a small light bulb, an electrochemical cell and a bare copper wire and were asked to find the four different arrangements of the three items that would result in the formation of a an electric circuit that would light the bulb.

One way to do this is by connecting the wire to the negative terminal of the cell and the light bulb, while the lightbulb is connected to the positive terminal of the cell, noting that the light bulb is part of the loop.

Light Bulb Anatomy

A light bulb is a relatively simple device consisting of a filament resting upon or somehow attached to two wires. The wires and filament are conducting materials which allow charge to flow through them. One wire is connected to the ribbed sides of the bulb and the other is connected to the bottom of the base of the bulb. The ribbed edge and the bottom base are separated by an insulating material which prevents the direct flow of charge between the bottom base and the ribbed edge. The only pathway by which charge can make it from the ribbed edge to the bottom base or vice versa is the pathway which includes the wires and the filament. Charge can either enter the ribbed edge, make the pathway through the filament and exit out the bottom base; or it can enter the bottom base, make the pathway through the filament and exit out the ribbed edge. As such, there are two possible entry points and two corresponding exit points.

The Requirement of a Closed Conducting Path

There are two requirements which must be met to establish an electric circuit. The first is clearly demonstrated by the above activity. There must be a closed conducting path which extends from the positive terminal to the negative terminal. It is not enough that there is a closed connecting loop; the loop itself must extend from the positive terminal to the negative terminal of the electrochemical cell. An electric circuit is like a water circuit at a water park. The flow of charge through the wires is similar to the flow of water through the pipes and along the slides of the water park. If a pipe gets plugged or broken such that water cannot make a complete path through the circuit, then the flow of water will soon cease. In an electric circuit, all connections must be made and made by conducting materials capable of carrying charge. Metallic materials are conductors and can be inserted into the circuit to successfully light the bulb. There must be a closed conducting loop from the positive to the negative terminal in order to establish a circuit and to have a current.

The Requirement of an Energy Supply

The second requirement of an electric circuit that is common is that there must be an electric potential difference across the two ends of the circuit. This is most commonly established by the use of an electrochemical cell, a pack of cells, or some other energy source. It is essential that there is some source of energy capable of increasing the electric potential energy of a charge as it moves from the low energy terminal to the high energy terminal. As applied to electric circuits, the movement of a positive test charge through the cell from the low energy terminal to the high energy terminal is a movement against the electric field. This movement of charge demands that work be done on it in order to lift it up to the higher energy terminal. An electrochemical cell serves the useful role of supplying the energy to do work on the charge in order to pump it or move it through the cell from the negative to the positive terminal. By doing so, the cell establishes an electric potential difference across the two ends of the electric circuit.