# Important Equations and Quantities

Table: Units used in Electricity and Magnetism
Units
Quantity
Symbol
Unit
S.I. Units
Direction
current
I
A (ampere or amp)
A
yes
resistance
R
$\Omega$ (Ohm)
${{kg\cdot m^{2}} \over {A^{2}\cdot s^{2}}}$
potential difference
V
V (volt)
${{kg\cdot m^{2}} \over {A\cdot s^{3}}}$ or $kg\cdot m^{2}\cdot A^{-1}\cdot s^{-3}$ :
* REVIEW:
* A circuit is an unbroken loop of conductive material that allows electrons to flow through continuously without beginning or end.
* If a circuit is "broken," that means it's conductive elements no longer form a complete path, and continuous electron flow cannot occur in it.
* The location of a break in a circuit is irrelevant to its inability to sustain continuous electron flow. Any break, anywhere in a circuit prevents electron flow throughout the circuit.
* REVIEW:
* Electrons can be motivated to flow through a conductor by the same force manifested in static electricity.
* Voltage is the measure of specific potential energy (potential energy per unit charge) between two locations. In layman's terms, it is the measure of "push" available to motivate electrons.
* Voltage, as an expression of potential energy, is always relative between two locations, or points. Sometimes it is called a voltage "drop."
* When a voltage source is connected to a circuit, the voltage will cause a uniform flow of electrons through that circuit called a current.
* In a single (one loop) circuit, the amount of current at any point is the same as the amount of current at any other point.
* If a circuit containing a voltage source is broken, the full voltage of that source will appear across the points of the break.
* The +/- orientation a voltage drop is called the polarity. It is also relative between two points.
* REVIEW:
* Resistance is the measure of opposition to electric current.
* A short circuit is an electric circuit offering little or no resistance to the flow of electrons. Short circuits are dangerous with high voltage power sources because the high currents encountered can cause large amounts of heat energy to be released.
* An open circuit is one where the continuity has been broken by an interruption in the path for electrons to flow.
* A closed circuit is one that is complete, with good continuity throughout.
* A device designed to open or close a circuit under controlled conditions is called a switch.
* The terms "open" and "closed" refer to switches as well as entire circuits. An open switch is one without continuity: electrons cannot flow through it. A closed switch is one that provides a direct (low resistance) path for electrons to flow through.
* REVIEW:
* Connecting wires in a circuit are assumed to have zero resistance unless otherwise stated.
* Wires in a circuit can be shortened or lengthened without impacting the circuit's function - all that matters is that the components are attached to one another in the same sequence.
* Points directly connected together in a circuit by zero resistance (wire) are considered to be electrically common.
* Electrically common points, with zero resistance between them, will have zero voltage dropped between them, regardless of the magnitude of current (ideally).
* The voltage or resistance readings referenced between sets of electrically common points will be the same.
* These rules apply to ideal conditions, where connecting wires are assumed to possess absolutely zero resistance. In real life this will probably not be the case, but wire resistances should be low enough so that the general principles stated here still hold.
* REVIEW:
* Power is additive in any configuration of resistive circuit: $P_{Total}=P_{1}+P_{2}+...P_{n}$
* REVIEW:
* When electrons flow through a conductor, a magnetic field will be produced around that conductor.
* The left-hand rule states that the magnetic flux lines produced by a current-carrying wire will be oriented the same direction as the curled fingers of a person's left hand (in the "hitchhiking" position), with the thumb pointing in the direction of electron flow.
* The magnetic field force produced by a current-carrying wire can be greatly increased by shaping the wire into a coil instead of a straight line. If wound in a coil shape, the magnetic field will be oriented along the axis of the coil's length.
* The magnetic field force produced by an electromagnet (called the magnetomotive force, or mmf), is proportional to the product (multiplication) of the current through the electromagnet and the number of complete coil "turns" formed by the wire.