Alexander Fufaev
My name is Alexander FufaeV and here I will explain the following topic:

Electric Current: The Most Basic Explanation From a Physics Point of View


Formula: Electric current (definition)
Electric current between plus and minus pole Hover the image!
What do the formula symbols mean?

Electric current

Electric current is the amount of charge \(Q\) that crosses the cross-sectional area of a wire within a time period \(t\). Electric current is therefore charge per time.

Electric charge

This is the amount of charge that has passed through the cross-sectional area of a wire within the time \(t\).

If the individual (positively) charged particles carry the charge \(q\) and \(N\) particles have passed through the cross-sectional area in the time \(t\), then the charge passed through: \[ Q ~=~ N \, q \]


Time period during which the charges were counted through a cross-sectional area.
Table of contents
  1. Formula
  2. Video
  3. Electric charges as a condition for the current Here, positive and negative charges are briefly explained because they are critical to understanding electric current.
  4. Current as motion of charges Here it is explained how the electric current is generated and how it is physically described.


This lesson is also available as a YouTube video: Electric Current explained simply in 7 minutes!

You have surely seen the indication of an electric current on your charger cable: For example an input current of 0.5 Ampere. Let's try to understand what this value physically means.

Electric charges as a condition for the current

Electric positive / negative charge - attraction and repulsion Hover the image!
Charges of the same sign repel each other. Charges of different signs attract each other.

In our universe there are electrically charged particles that interact with each other. Some are positive (+), others are negatively (-) electrically charged.

Electrons and protons, of which atoms are made up, are for example such electrically charged particles. Electrons are negatively charged. Protons are positively charged. When you bring two positively charged particles closely together, they repel each other. Two negatively charged particles also repel each other. When you bring a positive and a negative particle together, they attract each other. Through such repulsion and attraction, charged particles interact with each other.

When do charges attract and when do they repel each other?

Charges (+ and +) or (- and -) of the same polarity repel each other. Charges (+ and -) of different polarity attract each other.

Experiments have also shown that these charged particles do not all attract or repel each other equally strongly! There are charged particles that attract each other more than others. To quantify this difference in attraction and repulsion, we say that charged particles carry different charges \(q\). The charge is measured in the unit Coulomb and is abbreviated with the letter \(\text{C}\).

The greater the charge \(q\) of a charged particle (e.g. \(q = 1\,\text{C}, 2\,\text{C}, 3\,\text{C}\)), the more it is repelled or attracted by other electrically charged particles.

In order to distinguish positive and negative particles, we define:

  • We assign a negative sign to negatively charged particles. For example: \(q = -1\,\text{C}\), \(-2\,\text{C}\), \(-3\,\text{C}\).

  • And positively charged particles we assign a positive sign. For example: \(q = +1\,\text{C}\), \(+2\,\text{C}\), \(+3\,\text{C}\), whereby we can of course leave out the plus.

Current as motion of charges

What do electric charges have to do with electric current? Let's do a little thought experiment with what we know so far. We'll take two boxes, with a hole that can be opened and closed. First, we close both holes. In one box, we put many positive particles. Each of them carries the same charge \(q\) and for fun it is one Coulomb: \(q = 1 \, \text{C}\).

In the other box we place many negative particles, each with a negative charge: \(q = -1 \, \text{C}\). The two boxes now form a large electric charge. We can also say that the negative box forms a negative pole and the positive box a positive pole. The two charged boxes will now of course attract each other. To prevent this, we fix the two boxes so that they cannot move towards each other.

Electric current between plus and minus pole Hover the image!
Positive charges from the open box (positive pole) are attracted by the closed box (negative pole) and thus generate an electric current.

Next, we connect the two boxes with a conductive wire, for example with a copper wire. By "conductive" we mean that charges can move through this wire.

What happens to the positive charges in the positive box when we open it? The positive charges can now move to the negative box along the wire. Why are they doing this? Well, because they are attracted by the negative charges. This movement of the charges is an electric current and it is in the direction of the negative pole. Qualitatively speaking, the current becomes larger when more charges pass through the wire. But how do we quantify the current now? Let's abbreviate the electric current through the wire with the letter \(I\). If we cut the wire, we would get a round circular area. This circular area is called theem>cross-sectional area of the wire. Then we concentrate on a cross-sectional area of the wire. The positive particles will of course pass through this area.

To quantify the current, we simply count how many charges \(q\), pass through a cross-sectional area of the wire within a given period of time \(t\).

Formula anchor

Since the electric current \(I\) is the amount of charge \(Q\) that passes through a cross-sectional area of the wire per time \(t\), we must divide \(Q\) by \(t\) to obtain the electric current:

Formula anchor

Unit of electric current:
The charge \(Q\) has the unit \(\mathrm{C}\) (Coulomb). The time \(t\) has the unit \(\mathrm{s}\) (second). So the electric current \(I\) has the unit \( \mathrm{C}/\mathrm{s}\) (Coulomb per second) or short: \(\mathrm{A}\) (Ampere).

When does the current become as large as possible?

The current \(I\) is greater the more charge \(Q\) passes through the cross-sectional area of the wire within a certain time \(t\).

Example: Calculate current

A charge of 0.5 Coulomb passes through the cross-sectional area of a wire within 10 seconds. So here is the time: \(t = 10 \, \text{s}\). And the charge \(Q\) that has crossed the wire in that time: \(Q=0.5 \, \text{C}\). If you divide the charge by time, you get the charge per time, that is, the electric current:

Formula anchor
Is one ampere a large current?

In our thought experiment, we assigned a random value of one Coulomb to the positive particles. But in reality the particles carry a much charge. Usually the negative electrons are responsible for the electric current in conducting wires. They carry a tiny negative charge, namely:

Formula anchor

This is a very small value with many zeros:

Formula anchor

For the electrons to generate a current of \(I = 1 \, \text{A}\), 6250 QUADRILLION of electrons per second must pass through the cross-sectional area of the wire! This number is unimaginably large! So one ampere is quite a large electric current if we illustrate it with the number of electrons.

So, to sum up: The current \(I\) describes how much charge per second is transported through a cross-sectional area. In order for the current to be generated at all, charged particles must be set into directional motion. This directional motion occurs when positively and negatively charged particles are separated and then attract each other. The separation of charges is characterized by the electric voltage.