# Atoms

Matter is made of atoms. So, also the materials used in electric and electronic components consist of atoms. Atoms have a nucleus containing protons and neutrons. Around each nucleus, one or more electrons are orbiting. We assume that you are familiar with these concepts.

Objects have mass because protons, neutrons and electrons have mass. The unit of mass is the kilogram, abbreviated with the symbol kg. The mass of a proton and a neutron is almost the same and equal to 1.67 x 10-27 kg. Electrons are much lighter and have a mass of 9.11 x 10-31 kg. So, all these particles have mass, and they all contribute to the total mass of the objects we know. This makes the concept of mass easy to understand.

## Electric charge

Besides mass, protons and electrons however also have electric charge. For this physical quantity the symbol Q is used. Neutrons do not have electric charge. The unit of charge is the coulomb, abbreviated with the symbol C. The electric charge of one single proton is 1.60 x 10-19 C. The electric charge of one single electron is -1.60 x 10-19 C. Note the minus sign, and the conclusion that a proton and an electron have equal but opposite charge. This is an important difference with mass. Mass is always positive (with the exception perhaps of mass in the world of theoretical physics). Gravity never repels. Gravity always attracts. But electric charges can repel and attract. As you probably remember from high school physics: charges with equal sign repel, charges with opposite sign attract.

Now, since most atoms have the same number of protons and electrons, their total net charge is zero. As a result, most daily objects we now have no net electric charge. Or their net charge is so small that we are not able to perceive it.

Anyway, why is this important? Because these electrons will start to move in electric and electronic circuits. In that case, we talk about electric currents.

Go to 1.2 Electric Current