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Electric Current

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What is current?

Electric current is the rate at which electric charge passes a point.

I = Q ÷ t

I means current, Q means charge in coulombs, and t means time in seconds.

A useful first picture

Imagine people walking through a doorway. Counting how many people pass the doorway each second tells you the rate of flow. Current does something similar for electric charge.

point − − − − − electron drift conventional current

What is really happening?

In a metal conductor, the individual electrons usually drift rather slowly. What establishes through the circuit very quickly is the electric field that causes electrons throughout the conductor to begin drifting.

So the old picture of electrons racing from a battery all the way to a lamp is too simple. The circuit already contains mobile electrons; the applied electric field gets them moving.

Another way to think of this

Picture a long pipe already full of marbles. Push one marble in at one end and another can move at the far end without the first marble having crossed the entire pipe. Again, it is only an analogy, but it helps explain why electrical effects can appear quickly even though individual electrons drift much more slowly.

Reading the symbols

I is the conventional letter used for electric current. A is the unit symbol for amperes (amps). So I = 2 A reads: “the current is two amperes.”

Q means an amount of electric charge, measured in coulombs (C). t means time, usually measured here in seconds (s).

The ampere

1 ampere = 1 coulomb per second

That sentence is worth remembering. It tells us what an ampere actually is.

Where did the name come from?

The ampere is named for French physicist and mathematician André-Marie Ampère (1775–1836). After the connection between electricity and magnetism became apparent in 1820, Ampère carried out important work describing the forces associated with electric currents.

That is why current is measured in amperes, usually shortened to amps.

Worked example

point 6 C passes in 3 s I = 2 A

Question: Six coulombs pass a point in three seconds. What current is flowing?

I = Q ÷ t = 6 ÷ 3 = 2 A

Answer: 2 amperes.

Try these yourself

One coulomb passes a point each second. What current is flowing? point 1 C passes in 1 s I = ?
Current is charge per time. One coulomb per second is exactly one ampere.
Four coulombs pass a point in two seconds. What is the current? point 4 C passes in 2 s I = ?
I = Q ÷ t = 4 C ÷ 2 s = 2 A.
Which way is conventional current shown on ordinary schematics? electron drift conventional current
Conventional current is defined from positive toward negative. Electron drift in a metal conductor is in the opposite direction.

A little deeper

There are two directions worth knowing. Electron drift in a metal is from negative toward positive. Conventional current is defined in the opposite direction, from positive toward negative. Electrical drawings and calculations normally use conventional current.

That convention was established before electrons were understood, and there is no need to reverse all of electrical engineering because of it. Both descriptions give the same circuit predictions when used consistently.

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