Alternating current (AC) is the kind of current that reverses its direction along a wire several times a second, in regular cycles. Here are examples of what common conventional AC looks like:
Other common names for AC: "standard house current".
Direct Current (DC) is the kind of current that travels along a wire in ONE direction ONLY. It is usually a steady and constant at all times. A good example of how DC compares to AC: look at the "black line" in the diagram above compared to the green or red AC "sine-wave".
Other common names for DC: "battery current", "flatline current".
This also travels in only one direction, too, but looks like AC with its bottom-half "flipped" upward above the "black line" in the diagram above. It is neither flat NOR constant, as it too pulsates like AC, however in only one direction.
A solenoid is a coil of wire in the form of several thousand windings and loops of wire. It is mostly characterized as a single "helix" of wire wound either clockwise or counterclockwise.
It is similar to, or the same as a solenoid. It is common to use the two terms interchangeably. However, an electromagnet also refers to a solenoid that has its poles brought close together.
Helmholtz DIPOLE COIL/LOOP:
A Helmholtz Dipole is a solenoid that has usually has fewer windings, but very large radius. Below are examples of what such a dipole looks like. These are used to show repulsion and attraction between two solenoids.
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