What Is Current? Current Explained

Current is measured in a unit called Amperes (shortened to Amps and written 'A')

Current is represented with an uppercase or lowercase "I" in equations or on circuits.

Current is the number of electrons moving past a single point in a circuit per second. 1A of current means 1 Coulomb (some unknown number of coulombs is written "Q" in equations) of electrons passes a point every second. 1 Coulomb is just a number and it equals \(6.2415091 \times 10^{18}\).

The fact above is written algebraically as: \(I = \frac{Q}{t}\) That's read as "the current equals the coulombs of electrons divided by the number of seconds"

There are two different ways to show the direction of current. "Electron flow" and "Conventional flow". "Electron flow" is the direction that electrons actually move in the circuit however "Conventional flow" is the opposite and is the one we use for historical reasons. Even though it is "wrong" we still get all the correct answers. In "conventional flow" we show positive charges going from high voltages to low voltages or leaving the positive end of a battery and going to the negative end. Whenever I or anybody else mentions "current" we mean "Conventional flow" current.

We measure current with a device called an Ammeter which has the following symbol:

An ammeter must be connected in series with whatever you want to measure the current of so that the current flows through it. We'll cover more on what "in series" means in later sections.

Assemble a circuit to measure the current through a resistor connected to a battery

To measure the current flowing through the resistor R1 we assemble the circuit as follows: As you can see all of the current that flows through R1 will also flow through the ammeter and no current flows through the ammeter that didn't also flow through the resistor.

An Ideal ammeter has zero resistance. This is of course impossible but it is the goal in ammeter design. This is so that adding the ammeter into the circuit doesn't change the current in that part of the circuit. If the ammeter had a large resistance it would lower the current from its "normal" value when we tried to measure it.