Download What is an Electric Circuit?

What is an Electric Circuit?
In Lesson 1, the concept of electric potential difference was discussed. 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 quantity - it expresses the
quantity of potential energy on a 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 being negative.
This arrangement of charged plates would create an electric field in the
region between the plates that 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 towards the negative plate. This movement of a 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 plates 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 charge pipe (wire) to the negative plate. That is, positive
charge would naturally move in the direction of the electric field that had 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
illustrated in the animation at the right. As a positive charge reaches the negative
plate, that 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 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 an electric potential
difference, there will 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 were a means of moving positive charge from the negative plate back up onto the
positive plate, then the movement of positive charge downward through the charge pipe (i.e., the wire) 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 collection of D
cells), a light bulb, and some connecting wires. The activity involves observing the affect of connecting and
disconnecting a wire in a simple arrangement of the battery pack, light bulbs 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.
Electric Current
If the two requirements of an electric circuit are met, then charge will flow through the external circuit. It is said
that there is a current - a flow of charge. Using the word current in this context is to simply use it to say that
something is happening in the wires - charge is moving. Yet current is a physical quantity that can be measured
and expressed numerically. As a physical quantity, current is the rate at which charge flows past a point on a
circuit. As depicted in the diagram below, the current in a circuit can be determined if the quantity of charge Q
passing through a cross section of a wire in a time t can be measured. The current is simply the ratio of the
quantity of charge and time.
Current is a rate quantity. There are several rate quantities in physics. For instance, velocity is a rate quantity the rate at which an object changes its position. Mathematically, velocity is the position change per time ratio.
Acceleration is a rate quantity - the rate at which an object changes its velocity. Mathematically, acceleration is
the velocity change per time ratio. And power is a rate quantity - the rate at which work is done on an object.
Mathematically, power is the work per time ratio. In every case of a rate quantity, the mathematical equation
involves some quantity over time. Thus, current as a rate quantity would be expressed mathematically as
Note that the equation above uses the symbol I to represent the quantity current.
As is the usual case, when a quantity is introduced in The Physics Classroom, the standard metric unit used to
express that quantity is introduced as well. The standard metric unit for current is the ampere. Ampere is often
shortened to Amp and is abbreviated by the unit symbol A. A current of 1 ampere means that there is 1 coulomb
of charge passing through a cross section of a wire every 1 second.
1 ampere = 1 coulomb / 1 second
To test your understanding, determine the current for the following two situations. Note that some extraneous
information is given in each situation. Click the Check Answer button to see if you are correct.
A 2 mm long cross section of wire is isolated and A 1 mm long cross section of wire is isolated and
20 C of charge is determined to pass through it in 2 C of charge is determined to pass through it in
40 s.
0.5 s.
I = _____ Ampere
I = _____ Ampere
Resistance
An electron traveling through the wires and loads of the external circuit encounters
resistance. Resistance is the hindrance to the flow of charge. For an electron, the
journey from terminal to terminal is not a direct route. Rather, it is a zigzag path that
results from countless collisions with fixed atoms within the conducting material.
The electrons encounter resistance - a hindrance to their movement. While the
electric potential difference established between the two terminals encourages the
movement of charge, it is resistance that discourages it. The rate at which charge
flows from terminal to terminal is the result of the combined affect of these two quantities.
Variables Affecting Electrical Resistance
The flow of charge through wires is often compared to the flow of water through pipes. The resistance to the
flow of charge in an electric circuit is analogous to the frictional affects between water and the pipe surfaces as
well as the resistance offered by obstacles that are present in its path. It is this resistance that hinders the water
flow and reduces both its flow rate and its drift speed. Like the resistance to water flow, the total amount of
resistance to charge flow within a wire of an electric circuit is affected by some clearly identifiable variables.
First, the total length of the wires will affect the amount of resistance. The longer the wire, the more resistance
that there will be. There is a direct relationship between the amount of resistance encountered by charge and the
length of wire it must traverse. After all, if resistance occurs as the result of collisions between charge carriers
and the atoms of the wire, then there is likely to be more collisions in a longer wire. More collisions mean more
resistance.
Second, the cross-sectional area of the wires will affect the amount of resistance. Wider wires have a greater
cross-sectional area. Water will flow through a wider pipe at a higher rate than it will flow through a narrow
pipe. This can be attributed to the lower amount of resistance that is present in the wider pipe. In the same
manner, the wider the wire, the less resistance that there will be to the flow of electric charge. When all other
variables are the same, charge will flow at higher rates through wider wires with greater cross-sectional areas
than through thinner wires.
A third variable that is known to affect the resistance to charge flow is the material that a wire is made of. Not
all materials are created equal in terms of their conductive ability. Some materials are better conductors than
others and offer less resistance to the flow of charge. Silver is one of the best conductors but is never used in
wires of household circuits due to its cost. Copper and aluminum are among the least expensive materials with
suitable conducting ability to permit their use in wires of household circuits. The conducting ability of a
material is often indicated by its resistivity. The resistivity of a material is dependent upon the material's
electronic structure and its temperature. For most (but not all) materials, resistivity increases with increasing
temperature. The table below lists resistivity values for various materials at temperatures of 20 degrees Celsius.