Knowledge Base Wiki

Electromagnetism
Electricity · Magnetism · Magnetic permeability
Electrostatics Electric charge • Coulomb's law • Electric field • Electric flux • Gauss's law • Electric potential energy • Electric potential • Electrostatic induction • Electric dipole moment • Polarization density
Magnetostatics Ampère's law • Electric current • Magnetic field • Magnetization • Magnetic flux • Biot–Savart law • Magnetic dipole moment • Gauss's law for magnetism
Electrodynamics Lorentz force law • emf • Electromagnetic induction • Faraday’s law • Lenz's law • Displacement current • Maxwell's equations • EM field • Electromagnetic radiation • Liénard–Wiechert potential • Maxwell tensor • Eddy current
Electrical Network Electrical conduction • Electrical resistance • Capacitance • Inductance • Impedance • Resonant cavities • Waveguides
Covariant formulation Electromagnetic tensor • EM Stress-energy tensor • Four-current • Electromagnetic four-potential

An electric current is a flow of electric charge through a conductor. The equation of current is:^[1]

where

is the current flowing

is the change in electric charge

is the change in time

The SI unit of electric current is the ampere (A). This is equal to one coulomb of charge in one second. Electric current can be found in wires, batteries, and lightning.

In conducting materials, some electrons are very loosely bound to the atoms of the material. When large amounts of these atoms come together, there is a sort of electron cloud that "hovers" near the atoms of the material. If you examine a cross-section of the piece of conducting material, the electrons will move very quickly through it. This motion is caused by temperature, and electrons flowing in one direction tend to equal the electrons flowing from the other direction, so this is not what causes current to flow. Electrons flow from one atom to another, a process has been compared to the passing of water buckets from one person to another in a bucket brigade.^[2]

When an electric field is put on the wire, the electrons respond almost instantly by drifting slightly in the opposite direction of the field. They gain energy from the field, which is lost very quickly when they bump into other electrons in the material. As long as the field is in place, however, the electrons will gain back that energy that they lost, and the process will continue. This "jolt" that electrons receive from the electric field is the source of current, not the overall flow of electrons themselves. From this discussion, we can see two things that current is not:

• It is not an actual "flow" of electrons in the everyday sense of the word: If we examine the speed given to the electrons by the field, it is usually very small, on the order of millimeters per second. It would take half an hour for electrons to cross a 10-foot (3 m) room at this rate. Since a light bulb flips on almost immediately after hitting the switch, something else must be at work.
• It is also not a "domino effect", although this analogy is closer than the flow. Since electrons are so tiny, even when they are moving very quickly they are not being propelled by great force.

When current is flowing in a wire circuit, it accelerates when there is no resistance in the circuit. Resistors are used to increase resistance in the circuit so it slows down the current. The relationship between resistance, current, and voltage (another part of the circuit) is shown by Ohm's law.