Direct current (DC) circuits basically consist of a loop of conducting wire (like copper) through which an electric current flows. An electric current consists of a flow of electric charges, analogous to the flow of water (water molecules) in a river. In addition to the copper wire in a circuit there usually are components such as resistors which restrict the flow of electric charge, similar to the way rocks and debris in a river restrict the flow of the river water. For a clear picture of an electric circuit go to an informative website with interactive pictures that will show you quickly what is going on in an electric circuit: http://230nsc1.phy-astr.gsu.edu/hbase/hframe.html then click on "Electricity
and Magnetism" button, You can click anyplace
on the interactive "
Now that we have an idea about electric charge and how they experience a force when placed in an electric field (SEE THE PAGE ON ELECTRIC CHARGE) we can discuss DC circuits (covered in Chapter 25 Current, Resistance, and Electromotive Force and in Chapter 26 DC Circuits - see below).
Electron trajectories
in a conductor are shown in the diagrams below. If there is no electric
field inside a conducting material electrons move randomly. If a field
is present the electric force
The
motion of a ball If the pegs were Electric charges move through a material in a similar way. If the charge has its motion greatly restricted we say the material through which it is moving has a large electrical RESISTANCE. The resistance of a material depends on the arrangement, or spacing, of the atoms or molecules in the material. The motion of the atoms affects the electrical resistance of the material. If the atoms or molecules vibrate a lot, in general, they will increase the resistance of the material. And since we know that as the temperature rises molecules vibrate more and more, we conclude that the electrical resistance of a material is increased as the temperature rises. The amount of electrical resistance caused by a temperature increase is described by factor (a) called the temperature coefficient of resistivity (resistivity is closely related to resistance). Other factors that
affect the resistance of an electrical conductor are its length and cross-sectional
area. The longer the conductor the greater the resistance, and the greater
the cross-section area through which the charges flow the smaller the
resistance of the conductor. We can formulate these ideas in an equation
relating the resistance, the resistivity, the length, and the area of
a conductor. Since the resistance is proportional to the length and inversely
proportional to the area we can say that The dependence of
resistance (
where T is the Celsius
temperature, To is the reference temperature (usually room temperature
or 20 degrees C), Ro is the resistance at the reference temperature, and
The current through a cross-section area A is the net rate (dQ/dt) at which charge passes through the area. If the moving charges are positive the drift velocity is in the same direction as the field, as shown.
Current through a
conductor flows from a higher electric potential (or voltage) to a lower
electric potential; just as water flows from a higher level of potential
energy to a lower level of potential energy, The greater the electric
potential between the ends of a conductor, the greater the current through
the conductor. The difference in voltage levels is often referred to as
the voltage. If the current in a conductor is proportional the the potential
difference (or voltage) driving the current through the conductor we say
that Ohm's Law is obeyed:
Below is a simple circuit that has an external resistor R and a 12 Volt source with an internal resistance r. The diagram following shows the electric potential (or voltage) drops in this circuit.
The circuit above is redrawn below to show the voltage (or electric potential) rises and drops:
We will see later
that the electric potential (V, a scalar quantity with units of Volts)
is defined as the potential energy per charge:
For a clear picture of an electric circuit go to the informative website with interactive pictures that will show you quickly what is going on in an electric circuit: http://230nsc1.phy-astr.gsu.edu/hbase/hframe.html then click on "Electricity
and Magnetism" button,
In the figures (26.1) below: (a) Three
(b) Three
Figures (c) and (d) show combinations of series and parallel resistors.
One way to calculate the current in each resistor (to see whether the component will burn out from too much current, for example) is to first find the total resistance of the circuit by simplifying the resistance of parallel combinations, as indicated. Once the total current is know the current in the individual branches can be calculated. Figure 26.3
The difference between an ammeter (measures current passing through a circuit) and a voltmeter (measures voltage between two points in a circuit) is in the path of the current through the meter. (a) (b)
In household circuits (actually alternating current circuits) the appliances are connected in parallel between the hot and neutral power lines as shown below. Why not series connections?
Household appliances and tools usually have a third wire called a "ground" wire to provide a current path, bypassing your body, in the event of a wiring failure as shown on the right above.
©2009 J. F. Becker |
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