Why Do You Need To Know About Current?You are at: Basic Concepts - Quantities - CurrentFacts About CurrentCurrentCurrent UnitsMeasuring CurrentProblem & Links to Other Lessons
Like people, the most interesting charge is the charge that is in motion, moving about rather than sitting still.
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 yourselfTo develop a mental model that helps you picture and understand current in an electrical circuit.In an electrical circuitBe able to define and measure currents in any element.
Be able to use units of current correctly.
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.
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 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.
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.
observe charge going through a wire for 4 seconds, and you find that 20
coulombs passes. What is the current?
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
a problem for you. You have a car battery, and you leave on an interior
light. The light draws one ampere from the battery. How many
couloumbs will flow through the light if you leave it on for three hours?
If you think about the problem above you see that a couloumb is a pretty small amount of charge. For example, the car battery can probably supply one ampere for somewhere between 50 and 100 hours. That's a lot of couloumbs! Manufacturers who make car batteries use other units of charge. Since:
Charge = Current x Time Interval
we can use other units. For example,
the time interval can be measured in hours. Then, instead of couloumbs
we would measure charge in units of ampere-hours.
Electrical engineers don't use that unit often, but battery manufacturers
use it all the time. A car battery that can provide one ampere for
80 hours is rated at 80 ampere-hours. Of course, it can also provide
80 amperes for one hour.
another problem for you. How many couloumbs are there in one ampere-hour?
There is one last point you need to know.
The most commonly used unit it the ampere, and it is often referred to as amps. If we have a current of 3.5 amperes, we would say:
I = 3.5 amp or I = 3.5A
When you have a circuit and you want to figure out how the circuit will
behave you will generally try to determine what the currents and voltages
in the circuit are going to be. That prediction is what you want.
However, before you can calculate a current you need to have a precise
definition of what you mean by the current, and that's where you get involved
with polarity. We will introduce the idea of polarity with a question
for you to answer.
Q1. In this circuit, Willy Nilly wants to determine current I5. He has defined that current as shown below at the left. Is that the correct definition of the current? Or is the current definition at the right the correct one? (Note, depending on visual space in your browser, the right one may display below the left one so the buttons are labelled Left/Top and Right/Bottom.)
Let's review things a bit.
Current flows through something. Current can flow through a wire, the normal situation, but it can also flow through an ionic solution, through the ground, and many other things. The important word here is through. Current is a through variable. Current always flows through something.
What this means is that in order to measure current you need to get the current to go through a meter. An ammeter is the type of meter used to measure current. In this section we'll talk about measurement of current and using ammeters.
Here's a diagram of a circuit with an ammeter inserted to measure a current. There are many different currents flowing in this circuit. We are interested in current I?, which flows through element 3. We want to measure current I?. (We call it I? because that's the one we want to know.)
Now, if we want to measure that current, we have to get it to flow through through an ammeter - a device that measures current. The way we do that is to break the circuit between element 3 and element 4 and insert an ammeter in series with element 3. We say that two elements are in series whenever the current that goes through one element is forced to go through a second element. Note that all of the current going through the first element (element 3 here) goes through the second element (the ammeter here). Here's a circuit diagram that shows where the ammeter goes.
The important thing here is to see how the ammeter is inserted so that the current you want to measure is made to flow through the ammeter. When that current flows, the ammeter will measure the current. Here, current I? flows through the ammeter after flowing through element 3 and before flowing into element 4.
Here's a pictorial representation of an analog ammeter. It's typical of ammeters. It has two terminals. They are usually red and black. When you have one red terminal and one black terminal, you can be sure that the ammeter will read a current like the one defined in the picture. When the current, I, shown in the figure, is positive, then the ammeter needle will read upscale indicating the measured current.
Now, here's a representation of a digital ammeter. It's going to have the same kind of terminals. The difference here is that it will give a digital value for the measurement, showing the measurement result with an LED display.
There's nothing very complicated about measuring current. You need
to get the current you want to measure to flow through an ammeter which
will then measure the current. In principle it's pretty simple.
10. Here is the same circuit where we introduced you to the ammeter. We kept the ammeter in the same place.
We placed the ammeter there in order to measure the current through element 3.
There's one other item to consider. Charge comes in discrete packets but it is often useful to assume that it can take on continuous values. That lets us bring all the power of calculus to bear when we discuss current. Current is the flow of charge, and it is thought of in terms of a quantity of charge flowing through an area in some small amount of time. However, we often want to drive that concept to the limit, by imagining a current at an instant. Then, we imagine letting the time interval shrink to zero so that we think of current as a derivative:
i(t) = dQ(t)/dt
We do realize through all of this that charge comes in discrete packets,
and that this limit is ultimately mathematical nonsense. Still, the
charge of an electron is so small that we can think in these terms in most
practical situations. When we consider more complex circuits it will
be helpful if we think, however, in terms of charge that can take on continuous
Using Current - Where Do You Use Current?
You use current every time you use an electrical appliance of any sort.
The most common kind of current you will see will be electrons flowing in a wire. That's what you'll see 99% of the time. However, any time any form of charge flows, that's a current.
Here are a few examples of currents. Current - a flow of charge - is what happens when you have any of the following, and it is not an exclusive list!