ASVAB: Electricity Review

Electric Charge
Much like gravity, electricity is an everyday observable phenomenon which is very complex, but may be understood as a set of behaviors. A. the gravitational force exists between objects with mass, the electric force exists between objects with electrical charge. In all atoms, the protons have a positive charge, while the electrons have a negative charge. An imbalance of electrons and protons in an object results in a net charge.  Unlike gravity, which only pulls, electrical forces can push objects apart as well as pulling them together.
Similar electric charges repel each other. Opposite charges attract each other.



Current
Electrons (and electrical charge with it) move through conductive materials by switching quickly from one atom to another. This electrical flow can manipulate energy like mechanical systems.


The term for the rate at which the charge flows through a conductive material is current. Because each electron carries a specific charge, current can be thought of as the number of electrons passing a point in a length of time. Current is measured in Amperes
(A), each unit of which is approximately 6.24 × 1018 electrons per


Electric current carries energy much like moving balls carry energy.



Voltage
Voltage is the potential for electric work. Voltage is the push behind electrical work. Voltage is similar to gravitational potential energy.


Anything used to generate a voltage, such as a battery or a generator, is called a voltage source. Voltage is conveniently measured in
Volts (V).

Resistance
Resistance is the amount of pressure to slow electrical current. Electrical resistance is much like friction, resisting flow and dissipating energy.


Different objects have different resistances. A resistor is an electrical component designed to have a specific resistance, measured in
Ohms (Ω).

Basic Circuits
A circuit is a closed loop through which current can flow. A simple circuit contains a voltage source and a resistor. The current flows from the positive side of the voltage source through the resistor to the negative side of the voltage source.


If we plot the voltage of a simple circuit, the similarities to gravitational potential energy appear.


If we consider the circuit to be a track, the electrons would be balls, the voltage source would be a powered lift, and the resistor would be a sticky section of the track. The lift raises the balls, increasing their potential energy. This potential energy is expended as the balls roll down the sticky section of the track.



Ohm's Law
A principle called Ohm's Law explains the relationship between the voltage, current, and resistance. The voltage drop over a resistance is equal to the amount of current times the resistance:


We can gain a better understanding of this equation by looking at a reference simple circuit and then changing one variable at a time to examine the results.



A series circuit is a circuit with two or more resistors on the same path. The same current runs through both resistors. However, the total voltage drop splits between the resistors. The resistors in series can be added together to make an equivalent basic circuit.




A parallel circuit is a circuit with two or more resistors on different, parallel paths. Unlike the series circuit, the current splits between the different paths in a parallel circuit. Resistors in parallel can be reduced to an equivalent circuit, but not by simply adding the resistances. The inverse of the equivalent resistance of parallel resistors is equal to the sum of the inverses of the resistance of each leg of the parallel circuit.  In equation form that means:


Or when solved for equivalent







Electrical power, or the energy output over time, is equal to the current resulting from a voltage source times the voltage of that source:


Thanks to Ohm's Law, we can write this relation in two other ways:




For instance, if a circuit is composed of a 9 Volt battery and a 3 Ohm resistor, the power output of the battery will be:



Up until this point, current has been assumed to flow in one direction. One directional flow is called Direct Current (DC). However, there is another type of electric current:
Alternating Current (AC). Many circuits use AC power sources, in which the current flips back and forth rapidly between directions.


Capacitors are electrical components which store voltage. Capacitors are made from two conductive surfaces separated from each other by a space and/or insulation. Capacitors resist changes to voltage. Capacitors don't stop AC circuits (although they do affect the current flow), but they do stop DC circuits, acting as open circuits.



Inductors are electrical components which effectively store current. Inductors use the relationship between electricity and magnetism to resist changes in current by running the current through coils of wire. Inductors don't stop DC circuits, but they do resist AC circuits as AC circuits utilize changing currents.



Diodes
Diodes are electrical components which limit the flow of electricity to one direction. If current flows through a diode in the intended direction, the diode will allow the flow.
However, a diode will stop current if it runs the wrong way.



Magnetism
Magnetism is an attraction between opposite poles of magnetic materials and a repulsion between similar poles of magnetic materials. Magnetism can be natural or induced with the use of electric currents. Magnets almost always exist with two polar sides: north and south. A magnetic force exists between two poles on objects. Different poles attract each other. Like poles repel each other.