Charge quantisation is intrinsically linked to the conservation of charge, which states that the total electric charge in an isolated system remains constant. Charge quantisation means that charge can only exist in discrete amounts, equal to multiples of the elementary charge (approximately 1.6 x 10^-19 coulombs). This discreetness ensures that in any physical process, charges cannot be created or destroyed but only transferred or transformed. This concept thereby upholds the principle of charge conservation in all physical processes, as the total amount of charge (in terms of multiples of the elementary charge) remains unchanged.

Electric current generally requires a potential difference to exist in a circuit. The potential difference, or voltage, creates an electric field in the conductor, which exerts a force on the charge carriers (such as electrons), causing them to move and create a current. However, there are exceptional cases, such as in superconductors, where current can flow without a potential difference. In these materials, once the current is established, it can continue flowing indefinitely without any external voltage, due to the absence of electrical resistance. But in standard conductors, a potential difference is necessary to maintain the flow of electric current.

Metals are excellent conductors of electricity primarily due to the presence of free electrons. In metallic bonds, electrons are not bound to any specific atom but are free to move throughout the entire metal lattice. This sea of delocalised electrons allows metals to easily conduct electric current. When a potential difference is applied across a metal, these free electrons drift towards the positive terminal, facilitating the flow of electric charge. Additionally, the closely packed atoms in a metal allow these electrons to move efficiently, contributing to the high conductivity of metals. The structure and bonding in metals are thus key to their ability to conduct electricity effectively.

The free movement of electrons in conductors is fundamental for the existence of electric current. In conductors, such as metals, the outermost electrons of the atoms are loosely bound, allowing them to move freely through the material. This mobility is crucial for conducting electricity. When a potential difference is applied across such a material, these free electrons are driven towards the positive terminal, creating a flow of charge, or electric current. Without these free electrons, the material would not be able to conduct electricity, as there would be no charge carriers available to move and carry the current.

The direction of electric current being opposite to electron flow is a historical convention. When the nature of electricity was first being explored, scientists didn't know that electric current in metals was due to the flow of electrons. They defined the direction of current as flowing from the positive to the negative terminal. Later, when electron theory developed, it was discovered that electrons, being negatively charged, actually flow from the negative to the positive terminal. However, the original convention for current direction was retained. Therefore, in a conductor, the electric current is considered to flow in the opposite direction to the actual flow of electrons.