2-2 Electric Current

What is current?

Think about the Niagara River, flowing from Lake Erie to Lake Ontario. (Of course the most famous thing about this river is Niagara Falls.)

As far as rivers go, this one is pretty large. According to the Niagara Peninsula Conservation Authority, 5700 cubic metres of water flows through this river every second; this is the "current." Put another way, imagine a standard-sized basketball court, which has a surface area of 420 m². The volume of water flowing down the Niagara River every second is a box of water covering that court, 13.5 m high -- which is over four stories!

On the other hand, the Rouge River on the east edge of Scarborough is much smaller. It's much narrower, isn't very deep, and doesn't flow very quickly. According to the Toronto and Region Conservation Authority, the average amount of water flowing through the Rouge River at any one time is roughly 1 to 2 cubic metres per second.

So, the Rouge River's current is much less than the Niagara River's current: less water flows through it.

Electric current describes how much electric charge flows past a point every second. Let's zoom-in on a wire and take a look at some charges flowing past.

Here we can see a wire, and a few of the electrons in it. (We won't bother showing the positive charges because they don't move anyway.) The electrons are moving to the right, and there aren't too many of them. Here, the current is low.

In this wire, there are a lot more charges moving to the right. (There could be a few reasons why this would happen, which we will get to later.) Here, the current is high.

Measuring Electric Charge

Individual electrons are extremely small. How small, you ask? One electron has a mass of 0.0000000000000000000000000000009 kg.

So, counting individual electrons is not going to work, because there are too many of them. What we can do is count groups of electrons instead. Every electron is the same, so this method works pretty well.

Mass is measured in kilograms (kg). Length is measured in metres (m). Electric charge is measured in coulombs (C), which is named after the colourfully-named Charles-Augustin de Coulomb, a French scientist from the 1700s.

One coulomb (1 C) is the charge on 6.24 × 10¹⁸ electrons. That's a lot of electrons, because electrons are tiny. That's our "group of electrons" from above.

Measuring Electric Current

Now let's revisit our two wires, and put a line across them. This is where we'll be counting our charges; imagine standing on an overpass over a highway, counting cars that go past you every second.

There are two things we can say about these two wires, both which mean the same thing.

• More electrons move past the line every second in wire #1 than wire #2.

• More coulombs of charge move past the line every second in wire #1 than wire #2.

Remember, since it's awkward to count individual electrons, it's much easier to count the number of coulombs of charge instead.

Quantity of electric charge is given the symbol Q. (It would make sense to use C as the symbol, but that's just not what we use.) So, to say we have a charge of positive four coulombs, we could write that like this: Q = +4 C

Now we can formally define the unit of electric current: the ampere, or amp for short.

If a current of one ampere (1 A) passes through a point in a wire, then one coulomb of charge passes that point every second.

Current is given the symbol I. (Again, it would make sense to use C, but that's what the world uses.) If you write your capital letter i without the "sticks" at the top and bottom, you might want to make sure you do that here, to make sure it doesn't look like a lowercase L or the number 1.

Now we can put this together into an equation which relates charge (Q, measured in coulombs), current (I, measured in amps), and time (t, measured in seconds):

Example: In a wire, a charge of 25 C moves past a point in a time of 3.5 s. What is the current in the wire at that point?

Given information:

Q = 25 C

t = 3.5 s

I = ?

I = Q/t = (25 C)/(3.5 s) = 7.1 A

The video below is the first in the TVO Electricity series that looks at current electricity. The first bit talks about electric circuits, but about halfway through it starts to talk about current and charge itself.

Practice Questions

The Basics

1. In your own words, describe what electric current is.

2. How would you write that you had a charge of (a.) positive 3.5 coulombs, (b.) negative 0.75 coulombs, and (c.) positive 0.0001 coulombs?

Extensions

1. Find the current through a light bulb which has 2.9 C of charge passing through in 8.5 s.

2. How long does it take for 77 C of charge to pass through an electric kettle, if the current through the kettle is 11 A?