Here's a physics-based intro to the EE concepts you're trying to understand.
Your questions are answered at the bottom.
Everything derives from the flow of "charge"
Electronics, as its root word electron denotes, is very much a study of the flow of electrons in a particular system.
Electrons are the fundamental "carriers" of charge in a typical circuit; i.e., they are how charge gets "moved around" in most circuits.
We adopt a signing convention saying that electrons have a "negative" charge. Moreover, an electron represents the smallest unit of charge at the atomic (classical physics) scale. This is called the "elementary" charge and sits at \$-1.602{\times}{10}^{-19}\$ Coulombs.
Conversely, protons have a "positive" signed charge of \$+1.602{\times}{10}^{-19}\$ Coulombs.
However, protons cannot move around so easily because they are typically bound to neutrons within the atomic nuclei by the nuclear strong force. It takes far more energy to remove protons from atomic nuclei (the basis for nuclear fission technology, by the way) than to remove electrons.
On the other hand, we can dislodge electrons from their atoms pretty easily. In fact, solar cells are based entirely on the photoelectric effect (one of Einstein's seminal discoveries) because "photons" (particles of light) dislodge "electrons" from their atoms.
Electric fields
All charges exert an electric field "indefinitely" into space. This is the theoretical model.
A field is simply a function that produces a vector quantity at every point (a quantity containing both magnitude and direction... to quote Despicable Me).
An electron creates an electric field where the vector at each point in the field points towards the electron (direction) with a magnitude corresponding to Coulomb's law:
$$\lvert \vec E\rvert~~=~~\underbrace{\frac{1}{4\pi\epsilon_{0}}}_{
\begin{array}{c}
\text{constant} \\
\text{factor}
\end{array}
}~~\underbrace{\frac{\lvert q\rvert}{r^{2}}}_{
\begin{array}{c}
\text{focus on} \\
\text{this part}
\end{array}
}$$
The directions can be visualized as:
These directions and magnitudes are determined based on the force (direction and magnitude) that would be exerted upon a positive test charge. In other words, the field lines represent the direction and magnitude a test positive charge would experience.
A negative charge would experience a force of the same magnitude in the opposing direction.
By this convention, when an electron is near an electron or a proton near a proton, they will repulse.
Superposition: collections of charges
If you sum up all the electric fields exerted individually by all charges in a region on a particular point, you get the total electric field at that point exerted by all the charges.
This follows the same principle of superposition used to solve kinematics problems with multiple forces acting on a singular object.
Positive charge is the absence of electrons; negative charge is the surplus of electrons
This specifically applies to electronics where we are dealing with charge flow through solid materials.
To re-iterate: electronics is the study of the flow of electrons as charge carriers; protons are not the primary charge carriers.
Again: for circuits, electrons move, protons do not.
However, a "virtual" positive charge can be created by the absence of electrons in a region of a circuit because that region has more net protons than electrons.
Recall Dalton's valence electron model where protons and neutrons occupy a small nucleus surrounded by orbiting electrons.
The electrons that are the farthest away from the nucleus in the outermost "valence" shell have the weakest attraction to the nucleus based on Coulomb's law which indicates that electric field strength is inversely proportional to the square of the distance.
By accumulating charge e.g. on a plate or some other material (say, by rubbing them vigorously together like in the good ol' days), we can generate an electric field. If we place electrons in this field, the electrons will macroscopically move in a direction opposite the electric field lines.
Note: as quantum mechanics and Brownian motion will describe, the actual trajectory of an individual electron is quite random. However, all electrons will exhibit a macroscopic "average" movement based on the force indicated by the electric field.
Thus, we can accurately calculate how a macroscopic sample of electrons will respond to an electric field.
Electric potential
Recall the equation based on Coulomb's law indicating the magnitude of force \$\lvert \vec E\rvert\$ exerted on a positive test charge:
$$\lvert \vec E\rvert = \frac{1}{4\pi\epsilon_{0}} \frac{\lvert q\rvert}{r^{2}}$$
From this equation, we see as \$r \to 0\$, \$\lvert \vec E\rvert \to \infty\$. That is, the magnitude of force exerted on a positive test charge becomes larger the closer we get to the origin of the electric field.
Said in the opposite, as \$r \to \infty\$, \$\lvert \vec E\rvert \to 0\$: as you get infinitely far away from the origin of an electric field, the field strength tends to zero.
Now, consider the analogy of a planet. As the total cumulative mass of the planet increases, so does its gravity. The superposition of the gravitational pulls of all the matter contained in the planet's mass produces gravitational attraction.
Aside: the mass of your body exerts a force on the planet, but the mass of the planet so far exceeds your body's mass \$\left(M_{\text{planet}} \gg m_{\text{you}}\right)\$ that your gravitational attraction is eclipsed by the planet's pull.
Recall from kinematics that gravitational potential is the amount of potential an object has owing to its distance from the planet's gravitational center. The planet's gravitational center can be treated as a point gravity source.
Similarly, we define electric potential as how much energy is required to move a positive test charge \$q\$ from infinitely far away to a specific point.
In the case of gravitational potential, we assume that the gravity field is zero infinitely far away from the planet.
If we have a mass \$m\$ that starts infinitely far away, the planet's gravitational field \$\vec g_{\text{planet}}\$ does work to pull the mass closer. Therefore, the gravitational field "loses potential" as a mass approaches the planet. Meanwhile, the mass accelerates and gains kinetic energy.
Similarly, if we have a positive test charge that starts from infinitely far away from a source charge \$q_{\text{source}}\$ which generates an electric field \$\vec E_{\text{source}}\$, the electric potential at a point is how much energy would be required to move the test charge to some distance \$r\$ from the source charge.
This results in:
- Negative charges gain electric potential when moving in the direction of the electric field \$\vec E\$ and away from a positive source charge.
- Negative charges lose electric potential when moving opposite the direction of electric field \$\vec E\$ and towards a positive source charge.
- Conversely, positive charges lose electric potential when moving in the direction of the electric field \$\vec E\$ and away from a positive source charge.
- Positive charges gain electric potential when moving in the direction opposite the electric field \$\vec E\$ and towards a positive source charge.
Electric potential in conductors
Consider the model of conductors or transition metals like copper or gold having a "sea of electrons". This "sea" is composed of valence electrons which are more loosely coupled and sort of "shared" amongst multiple atoms.
If we apply an electric field to these "loose" electrons, they are inclined, on a macroscopic average, to move in a specific direction over time.
Remember, electrons travel in the direction opposite the electric field.
Similarly, placing a length of wire conductor near a positive charge will cause a charge gradient across the length of wire.
The charge at any point on the wire can be calculated using its distance from the source charge and known attributes of the material used in the wire.
Positive charge owing to the absence of electrons will appear farther away from the positive source charge, while negative charge owing to the collection and surplus of electrons will form closer to the source charge.
Because of the electric field, a "potential difference" will appear between two points on the conductor. This is how an electric field generates voltage in a circuit.
Voltage is defined as electric potential difference between two points in an electric field.
Eventually, the charge distribution along the length of wire will reach "equilibrium" with the electric field. This doesn't mean charge stops moving (remember Brownian motion); only that the "net" or "average" movement of charge approaches zero.
Non-ideal batteries
Let's make up a galvanic or voltaic cell power source.
This cell is powered by the electrochemical redox reaction of Zinc and Copper rods in an aqueous solution of ammonium nitrate salt \$\left(\text{NH}_{4}\right)\left(\text{NO}_{3}\right)\$.
Ammonium nitrate is an ionically bonded salt that dissolves in water into its constituent ions \$\text{NH}_{4}^{+}\$ and \$\text{NO}_{3}^{-}\$.
Useful terminology:
- cation: a positively charged ion
- anion: a negatively charged ion
- cathode: the cations accumulate at the cathode
- anode: the anions accumulate at the anode
Useful mnemonic: "anion" is "an ion" is "A Negative ion"
If we examine the reaction for the Zinc-Copper galvanic cell above:
$$\text{Zn}\left(\text{NO}_{3}\right)_{2}~~+~~\text{Cu}^{2+} \quad\longrightarrow\quad \text{Zn}^{2+}~~+~~\text{Cu}\left(\text{NO}_{3}\right)_{2}$$
The movement of cations \$\text{Zn}^{2+}\$ and \$\text{Cu}^{2+}\$ is the flow of positive charge in the form of ions. This movement goes towards the cathode.
Note: Earlier we said that positive charge is the "absence" of electrons. Cations (positive ions) are positive because stripping away electrons results in a net positive atomic charge owing to the protons in the nucleus. These cations are mobile in the galvanic cell's solution, but as you can see, the ions do not travel through the conductive bridge connecting the two sides of the cell. That is, only electrons move through the conductor.
Based on the fact that positive cations move and accumulate towards the cathode, we label it negative (positive charges are attracted to negative).
Conversely, because electrons move towards and accumulate at the anode, we label it positive (negative charges are attracted to positive).
Remember how you learned that current flows from \$\textbf{+}\$ to \$\textbf{-}\$? This is because conventional current follows the flow of positive charge and cations, not negative charge.
This is because current is defined as the flow of virtual positive charge through a cross-sectional area. Electrons always flow opposite to current by convention.
What makes this galvanic cell non-ideal is that eventually the chemical process generating the electric field through the conductor and causing electrons and charge to flow will come to equilibrium.
This is because ion buildup at the anode and cathode will prevent the reaction from proceeding any further.
On the other hand, an "ideal" power source will never lose electric field strength.
Ideal voltage sources are like magic escalators
Let's return to the analogy of gravitational potential.
Assume you're on a hill and you have some arbitrary path down the hill constructed with cardboard walls. Let's say you roll a tennis ball down this path with cardboard walls. The tennis ball will follow the path.
In circuits, the conductor forms the path.
Now let's say you have an escalator at the bottom of the hill. Like a Rube Goldberg machine, the escalator scoops up tennis balls you roll down the path, then drops them off at the start of the path at the top of the hill.
The escalator is your ideal power source.
Now, let's say you almost completely saturate the entire path (escalator included) with tennis balls. Just a long line of tennis balls.
Because we didn't completely saturate the path, there are still gaps and spaces for the tennis balls to move.
A tennis ball that is carried up the escalator bumps into another ball, which bumps into another ball which... goes on and on.
The tennis balls going down the path on the hill gain energy owing to the potential difference in gravity. They bounce into each other until finally, another ball is loaded onto the escalator.
Let's call the tennis balls our electrons. If we follow the flow of electrons down the hill, through our fake cardboard "circuit", then up the magical escalator "power source", we notice something:
The "gaps" between tennis balls are moving in the exact opposite direction of the tennis balls (back up the hill and down the escalator) and they are moving much faster. The balls are naturally moving from high potential to low potential, but at a relatively slow speed. Then they are moved back to a high potential using the escalator.
The bottom of the escalator is effectively the negative terminal of a battery, or the cathode in the galvanic cell we were discussing earlier.
The top of the escalator is effectively the positive terminal of a battery, or the anode in a galvanic cell. The positive terminal has a higher electric potential.
Current
Okay, so the direction that positive charge flows in is the direction of electrical current.
What is current?
By definition, it is: the amount of charge that passes through a cross-sectional area per second (units: Coulombs per second). It is directly proportional to the area of the cross section of the wire/conducting material and the current density. Current density is the amount of charge flowing through a unit of area (units: Coulombs per meter-squared).
Here's another way to think of it:
If you have a tennis ball launcher spitting positively charged balls through a doorway, the number of balls it gets through the door per second determines its "current".
How fast those balls are moving (or how much kinetic energy they have when they hit a wall) is the "voltage".
Conservation of charge and voltage
This is a fundamental principle.
Think of it like this: there are a fixed number of electrons and protons. In an electrical circuit, matter is neither created nor destroyed... so the charge always stays the same. In the tennis-ball escalator example, the balls were just going in a loop. The number of balls remained fixed.
In other words, charge does not "dissipate". You never lose charge.
What happens is that charge loses potential. Ideal voltage sources give charge its electric potential back.
Voltage sources do NOT create charge. They generate electric potential.
Current flowing in and out of nodes, resistance
Let's take that conservation of charge principle. A similar analogy can be applied to the flow of water.
If we have a river system down a mountain that branches, each branch is analogous to an electric "node".
/ BRANCH A
/
/
MAIN ---
\
\
\ BRANCH B
-> downhill
The amount of water that flows into a branch must be equal to the amount of water flowing out of the branch by the conservation principle: water (charge) is neither created nor destroyed.
However, the amount of water that flows down a particular branch is dependent on how much "resistance" that branch puts up.
For example, if Branch A is extremely narrow, Branch B is extremely wide, and both branches are the same depth, then Branch B naturally has the larger cross-sectional area.
This means Branch B puts up less resistance and a larger volume of water can flow through it in a single unit of time.
This describes Kirchoff's Current Law.
You're still here? Awesome!
1. What happens to the rest of the current not used?
Because of the conservation principle, all charge into a node must flow out. There is no "unused" current because current isn't used. There is no change in current in a single series circuit.
However, different amounts of current can flow down different branches in an electrical node in a parallel circuit depending on the resistances of the different branches.
2. Does the LED use all the current?
Technically, the LED and resistor(s) don't "use" current, because there is no drop in current (the amount of charge passing through the LED or resistor(s) in a unit of time). This is because of the conservation of charge applied to a series circuit: there is no loss in charge throughout the circuit, hence no drop in current.
The amount of current (charge) is determined by the behavior of the LED and resistor(s) as described by their i-v curves
3. Why does the LED "drop voltage" by a certain amount?
Here's a basic LED circuit.
An LED has an activation voltage, usually around ~1.8 to 3.3 V. If you do not meet the activation voltage, practically no current will flow. Refer to the LED i-v curves linked below.
If you attempt to push current in the direction opposite the LEDs polarity, you will be operating the LED in a "reverse-bias" mode in which almost no current passes through. The normal operating mode of an LED is forward-bias mode. Beyond a certain point in reverse-bias mode, the LED "breaks down". Check out the i-v graph of a diode.
LEDs are actually PN junctions (p-doped and n-doped silicon squashed together). Based on Fermi levels of the doped silicon (which is contingent on the electron band-gaps of the doped material) the electrons require a very specific amount of activation energy to jump to another energy level. They then radiate their energy as a photon with a very specific wavelength/frequency as they jump back down to a lower level.
This accounts for the high efficiency (well over 90% of energy dissipated by an LED is converted to light, not heat) of LEDs compared to filament and CFL bulbs.
This is also why LED lighting seems so "artificial": natural light contains a relatively homogeneous mix of a broad spectrum of frequencies; LEDs emit combinations of very specific frequencies of light.
The energy levels also explain why the voltage drop across an LED (or other diodes) is effectively "fixed" even as more current goes through it. Examine the i-v curve for an LED or other diode: beyond the activation voltage, the current increases a LOT for a small increase in voltage. In essence, the LED will attempt to let as much current flow through it as it possibly can, until it physically deteriorates.
This is also why you use an inline current-limiting resistor to limit the current flow through a diode / LED to a specific rated milliamp based on the LED spec.
3(b). And what happens to the rest of the components in series, does the voltage drop for every component, up until there is nothing left?
Yep, Kirchoff's voltage law is that the sum of all voltage drops in a loop around a circuit is zero. In a simple series circuit, there is only one loop.
4. Do you choose your resistor to the extent of "using up all the current/voltage" before it gets to the end of the circuit?
No. You choose your resistor based on the LED current rating (say 30 mA = 0.03 A) and Ohm's law as described in the LED circuit article.
Your voltage will get used up. Your current remains the same throughout a single series circuit.
5. Why does a battery go into a dead short if you connect the terminals directly, but if you add a light bulb (resistor), it does not?
I'm not sure what you mean by "dead short".
Connecting the terminals of a battery together results in a large current discharged at the voltage of the battery. That voltage is dissipated through the battery's internal resistance and the conductor wire in the form of heat -- because even conductors have some resistance.
This is why shorted batteries get super hot. That heat can adversely affect a chemical cell's composition until it blows up.
6. Why are resistors necessary?
Here's the rhetoric: imagine there's this amazing concert. All your favorite bands are going to be there. It's going to be a smashing good time.
Let's say the event organizers have no concept of reality. So they make the entry fee to this amazing concert almost completely free. They put it in an extremely accessible area. In fact, they're so disorganized, they don't even care if they oversell and there aren't enough seats for everyone who buys tickets.
Oh, and this is in NYC.
Pretty quickly, this amazing concert turns into a total disaster. People are sitting on each other, spilling beer everywhere; fights are breaking out, the restrooms are jammed, the groupies are freaking everyone out, and you can barely hear the music above all the commotion.
Think of your LED as that amazing concert. And think of how messed up your LED is going to be if you don't have more resistance there to prevent EVERYONE and their moms from showing up to the concert.
In this dumb example, "resistance" translates into "cost of entry". By simple economic principles, raising the cost of the concert decreases the number of people who will attend.
Similarly, raising the resistance in a circuit prevents charge (and subsequently current) from going through. This means your LED (concert) doesn't get completely wrecked by all the people (charge).
Yeah, electrical engineering is a real party.
