Electric Current

To put it simply, electric current is the flow of charge. It is defined as the total charge which pass a point in the circuit each second and has the units of Amperes [A] or Coulombs per second [C/s] by definition.

Where I is the current in Amperes, Q is the charge in Coulombs and t is the time taken in seconds for the charge to pass a point.

Now for the somewhat confusing part unfortunately, we have the concepts of electron current and conventional current which assume the direction that the current flows through the circuit. Before it was known that electricity was carried by electrons, Benjamin Franklin believed that positive charges were flowing through the circuit from the positive to the negative terminal. Due to this we now have two ways to represent the direction of the current, the path which electrons flow, which is from the negative terminal to the positive terminal and conventional current which flows the other way.

Conventional current flow is the standard conventional direction that Electronic Engineers follow, however, this aspect has no effects on any observed behaviour so it is best to think of current as flowing this way as illustrated below:

A very important point to always remember at this point is that current flows in loops! There is always a return path!

Charge vs Time

Current can also be derived from a charge vs time graph where the gradient of the graph reflects the current at that point in time.

Instantaneous current is expressed as the derivative of the charge function with respect to time:

Note: We express time varying currents with a lowercase i

Current vs Time

We can also determine the total charge which has passed a point from a current vs time graph where the area under the graph represents the charge.

Instantaneous charge is expressed as the integral of the current function with respect to time:

Examples:

1. What is the current if 5C of charge passed a point in 4s?

2. In the charge vs time graph shown earlier, what is the current between the following times:

a) 0-2s

b) 2-4s

From an understanding point of view a horizontal line of 2C from 2s to 4s simply means no additional charges have accumulated over time at the location of measurement which means zero current flow, i.e. 0A which agrees with our answer.

c) 4-6s

Note that in the above a negative current means that the current is travelling in the opposite direction.

Ready for the next module? Return to the course home.