During other half, it travels other direction. One complete revolution of the coil is called a cycle. The number of such cycles in a second is called the frequency of the voltage or current. In this case alternating current. They are tiny particles forming atoms.
Electrons have a negative charge.
Protons and neutrons are made from smaller particles called quarks . So, protons are positive. In an atom protons and neutrons form nucleus, the center of an atom. The nucleus has a positive charge. The negative electrons orbit around it.