Like
people, the most interesting charge is the charge that is in motion, moving
about rather than sitting still.
Charge in motion is referred to
as current.
Current
can exist in many different physical forms because there are many different
physical situations in which charge can flow.
The most common manifestation
of charge in motion is the movement of electrons in a wire such as the wire
leading to the computer that is running this lesson. That's one form of current.
Here's a simulation that lets you see how charge flows around an electrical
circuit through several elements. Click the green button to see that.
However, ions in water carry
charge and a current can flow in water with ions in solution. Standing in
distilled (ion-free) water near an electrical outlet in your bathroom is nowhere
near as dangerous as standing in tap water with some ionic content, but do not
try the experiment.
Charged particles moving in a
vacuum are another manifestation of current, and you experience that every time
you watch television or look at a computer screen. Charged particles fly
from an electron gun at the back of a picture tube or a monitor tube, strike the
screen and you see the light emitted from the screen as a picture.
Even ink-jet printers charge
the ink blobs in the jet, and the jet is an example of a current!
Goals For This Lesson
There
are lots of different forms of current, and you need to understand current - the
flow of charge if you want to understand the electrical devices you use.
Objectives for this system include the following:
For yourself
To develop a mental model that helps you picture and understand current in
an electrical circuit.
In an electrical circuit
Be able to define and measure currents in any element.
Be able to use units of current correctly.
Current - continued
There are a
number of different ways of thinking about current. In different
situations you might want to use different ways of thinking about current to
help you figure out what's going on.
Water flowing in a pipe is
analogous to current. The water flows in the interior of the pipe, and
current actually flows through the empty spaces between atoms in a wire, but the
analogy can be useful. It helps if you have some sort of analogy that lets
you use something you already know about to help you think about new things like
current.
Current is an information
carrying signal. There will be times when you don't care so much about the
charge that's being transported as current flows but you will care about the
information that is being sent using current.
There will be times when the
charge that is being transported is what is important, and there are times when
you will have to think in a backwards sort of way about that.
Semiconductor engineers do this all the time when they talk about holes moving
in semiconductors. They have invented a concept that is based on missing
electrons and the spaces they should occupy in an atomic lattice, and they work
with ideas of missing electrons - holes - that move about in semiconductors.
Most of
the time, when you are dealing with current you are dealing with electrons
moving through metallic wires of electronic devices. At this point, we
will begin discussing electron flow through wires.
Current
usually flows through wires, and electrical engineers usually idealize the
situation. The figure below shows a wire carrying current, and the
idealized representation we use - the arrow that points in the direction the
current flows. Note that the current in the idealization is symbolized by
an arrow along the idealized wire, and the arrow points in the direction that
positive charge flows.
In fact, electrons are flowing the opposite way,
but we imagine current as a flow of positive charge.
We
want to emphasize the concept of current as a through variable. Whenever
we speak of current we specify the area that it flows through. The figure below
shows a current flowing through a rectangular cross section wire.
If we imagine the wire split in the middle (along
the divider shown) then the current is split between these areas. If the total
current is twelve amperes, then six amperes will probably flow through each half
of the rectangular wire. That's shown below.
Later, when we consider electrical elements - like
resistors - we will want to consider elements in parallel, and you will need to
understand this situation. If the two halves of the conductor above are
considered to be resistors, then they are in parallel in the picture above.
We could connect something at either end of the conductors and current would
split entering the parallel conductors, and could come together when exiting the
parallel conductors.
Current - continued
Current
is charge in motion. To be more precise, consider the situation below.
If we imagine "slicing" the wire, we can then count the rate at which charge
flows through the slice. That's shown with the slice and arrow below.
Hopefully, it is clear that the flow rate of charge through the slice is
measured in couloumbs/second. However, couloumbs/second has another name,
amperes. Current is usually measured in amperes (really coulombs/second!).
So, to measure the current passing through the wire, you can "sit" on the dark
gray slice and watch charge (coulombs) move past the slice, count the coulombs
that pass in a give amount of time, then divide the number of coulombs by the
time interval to compute the current.
Problems
1.
Now, here's a question for you. Let's imagine that you have a wire, and
you somehow observe that 2 coulombs passes through the wire in one second.
Click on the button you think gives the value of the current.
Now, if you reallly understand what current is you can turn this around.
In the problems above you were given the charge passing through a wire in a
given amount of time. Turning that around we can ask a different
question. If we have a constant current, I, flowing through a wire,
then we can compute how much charge flows through the wire in some given
time interval. Say we have the following situation:
I = Current = 3.2 amperes
Time interval = 15
seconds.
Then we would know that the amount of charge that flowed through the wire in
the 15 second time interval would be:
Total charge = 3.2
amperes x 15 seconds
= (3.2 coul/sec) x 15 sec
= 48 couloumbs
For yourself
