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

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.

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.

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: Total charge as multiple of individual charges

Formula anchor$$ \begin{align} Q ~=~ N\,q \end{align} $$

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$$ \begin{align} I ~=~ \frac{Q}{t} \end{align} $$

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).

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.

+ Perfect for high school and undergraduate physics students + Contains over 500 illustrated formulas on just 140 pages + Contains tables with examples and measured constants + Easy for everyone because without vectors and integrals