# What does a resistor do?

OK, very basic question here.

I read lots of books, searched quite a bit, and every description I read seemed to talk about the flow of electrons and right away go too deep in theory for me to grasp the basic principle of their use.

I understand a resistor limits the "flow", so that an LED doesn't blow up for example. But I fail to understand exactly what a resistor does to current and voltage...

Do resistors affect both current and voltage? In what manner?

• It sounds a bit abstract, but resistance affects charge. And in doing so indirectly affects both voltage and current. – Ignacio Vazquez-Abrams Oct 29 '14 at 17:45
• Have you looked at Ohm's law? E=I*R. This says that if you keep the resistance constant, current through is proportional to the voltage across; if you keep the voltage across constant, current through is inversely proportional to the resistance. – DoxyLover Oct 29 '14 at 17:50
• LEDs are semiconductors and do not obay ohm's law. Think about them as having zero resistance but still having a voltage drop. – HL-SDK Oct 29 '14 at 18:08
• In this case, you have the resistor in series with another load, not by itself across the power supply. An LED is a non-linear device that does not have a constant resistance. Instead, it has a (relatively) fixed voltage drop, regardless of current (kind of a reverse resistor). In this case, subtract the LED's voltage drop from the source voltage and use the resulting voltage with the resistor to control the current. – DoxyLover Oct 29 '14 at 18:09
• For example, you have supply voltage (Vs) = 5V and the LED drop voltage (Vd) = 1V. You want 10mA of current. Solve R = E/I = 4/0.01 = 400 ohms. (Just edited - had 5 instead of 4V) – DoxyLover Oct 29 '14 at 18:12

Electric flow is the motion of electrical charges through a material. Resistance is the physical obstruction of these moving charges.

A certain amount of energy is required to keep these charges in motion, and since the energy drop is proportional to the amount of charge kept in motion, this results in a voltage drop across the material since electromotive force (in volts) is energy (in joules) per charge (in coulombs).

Since it is a physical obstruction, it also restricts the rate at which charges can move across a given point per unit time. This results in a maximum current, since current (in amperes) is charges (in coulombs) per unit time (in seconds).

And as it turns out, if you apply more or less electromotive force across the same resistance, the current increases or decreases exactly linearly. This gives rise to Ohm's Law, which states that electromotive force is proportional to the product of current and resistance, that is, $E = IR$.

• Is the physical obstruction relative of constant ? Ie.: If I switch the power source from 5V to 9V, will I still measure the same numbers after a resistance ? ( Given that both power sources are under the maximal load that a resistor can take ) – FMaz008 Oct 29 '14 at 18:16
• The increased electromotive force will allow more charges to flow per unit time, i.e. the current will increase. – Ignacio Vazquez-Abrams Oct 29 '14 at 18:18
• I think this is a good answer. I think it also helps to think about what causes this physical obstruction to the flow of current. When free electrons experiencing an electromotive force collide with atoms, this is resistance. And it also makes since that this energy is converted to heat, as heat is just motion, in the form of molecular vibrations. So when those electrons collide with atoms, they cause those atoms to vibrate. – krb686 Oct 29 '14 at 18:22
• So the water pipe (resistance) is the same diameter, but the water is coming faster, so more water will go through the pipe. If both power sources are 1A, would the restriction lead to an increased charge too ? – FMaz008 Oct 29 '14 at 18:24
• Using the water pipe analogy, resistance equates inversely to the pipe diameter. A larger pipe diameter is less resistance, and vice versa. Voltage then is water pressure. So it requires a higher water pressure to force the same amount of water through a smaller diameter pipe. In other words, it requires higher voltage to force the same amount of current through a larger resistor. – krb686 Oct 29 '14 at 18:25

It can be helpful to think of voltage as the pressure or force that is propelling the electrons through the pipe that is the wire. Current is the number or amount of electrons passing a given point at any one time. Resistors do just what their name says; they resist. You can use them to limit either current or voltage, depending upon whether they are wired in series (one after the other), or parallel (sharing the same connection points, side-by-side. Think of electrons as ping pong balls passing through a tube, push one in and the ones already inside push one out the other end.Doubling the length of the tube (series wiring a resistor) increases the force needed to push it through, so it limits voltage. However, if you put the tubes side-by-side, then the same number of balls have to go through twice as many paths, limiting how many can go at once, and thus limiting current. I know this is grossly oversimplified and does not account for all situations, but it can give your mind's eye a visual representation of the theory of electron flow and how resistors can affect such.

• Even if just a mental model, this is the clearest and most practical answer. In all the the many layman explanations I've read on resistors, I've never seen someone explicitly lay out the parallel vs. series concept. Is it then accurate that a single resistor (hence no parallel or serial) inhibits both voltage and current, and if so, in what proportion? – N8allan Nov 22 '16 at 2:52

Hopefully this is simple enough:

Voltage arises from the potential energy in separation of charges (one node is positive with less electrons, one node is negative with more electrons). Think about it like having a bowling ball (charge) on the ground, versus at the top of a ladder. The ball at the top of the ladder has more potential energy, more voltage.

Current arises from the "flow" of charge.

Resistors let you choose how much current flows for a given voltage since you can think of wires as having no resistance (simplified).

In short: Resistors limit the flow of electrons, reducing current. Voltage comes about by the potential energy difference across the resistor.

• "Voltage comes about by the potential energy difference across the resistor."... I was OK until that line :-S – FMaz008 Oct 29 '14 at 18:08
• Think about the gravity analogy. Something that is on top of a hill has more gravitational potential energy than something at the bottom of the hill, you know? – HL-SDK Oct 29 '14 at 18:10

The mathematical answer is that a resistor is a two-terminal electric device which obeys, or you could say enforces, Ohm's law: V=IR.

V is the voltage between the two terminals, I is the current flowing from one terminal to the other (through the resistor) and R is the value known as resistance. For an ideal resistor, R is a constant and does not depend on V, I, or anything else. Another way to describe Ohm's law is to say that the voltage across a resistor and the current through it are proportional. The constant of proportionality is R, the resistance.

A fundamental consequence of physics is that resistors convert electric potential energy into heat. So they tend to get warm when current flows through them. Real resistors have maximum allowable power dissipation, and also, they may have R which depends on temperature slightly, and other shortcomings from the ideal.

As far as how resistors are made, well, real resistors are constructed from materials which have a conductivity somewhere in between insulators (dielectric materials) and conductors (such as copper wire). If you can determine the path current takes through the resistor, making that path longer increases the resistance. Making the cross-section wider decreases the resistance.

As far as what makes materials good conductors... Well, generally good conductors have mobile electrons at the molecular level. Good insulators do not. Good resistors are somewhere in between.