in this video we're introducing the breadboard and we'll illustrate how to set up a circuit
on the breadboard and then we're going to work an example that i would call a concept
circuit it's a circuit with three light bulbs in it and there's some really tricky arguments that
have to be made for how the brightnesses of those light bulbs will change when we modify the circuit
so the breadboard is the thing you see on the left here and this is a rapid prototyping device
for electronics so all the typical electronic components like resistors and capacitors and
inductors they all have wires that will clamp firmly into these little terminals so you can
quickly set up a circuit and then test the properties of it to make sure it's doing what
you want before you start soldering things on the breadboard i've highlighted some strips in yellow
and what i'm doing here is illustrating holes that are actually electrically connected to each other
with a continuous piece of metal on the back side of the breadboard so over here by the blue line
all of those holes on that vertical strip are electrically connected to each other
by a continuous metal strip next to the red line
same deal but those are isolated from the ones that are next to the blue line and then i get to
my horizontal strips so each of these groups of five is a little horizontal strip that's
electrically connected they are not connected across the gap in the middle of the breadboard
so each of those grips is just five connected holes i highlighted four of those groups of five
those groups are not connected to each other they're only connected horizontally across
those sets of five holes each what you see on the right is a container of miscellaneous jumper wires
so this allows you to connect these metal strips to each other and that way you can spread your
circuit out and organize it the way you need it on the breadboard so this is the concept circuit
that we're going to tackle we're actually going to wire this on the breadboard and every one of
the resistors here is actually a light bulb that i took out of a set of christmas lights i can see
that all the current in the circuit passes through r1 and then i end up with this parallel branching
where r2 and r3 are wired in parallel i can tell for sure they're in parallel i know the potential
is the same at the high side of r2 and r3 because they're both connected directly with wire to this
upper node so i could call that v for now and the potential with the low side of r2 and r3 is the
same because they're connected to this bottom node directly by wire and then i notice that
that node is connected to the negative terminal of the battery so i could just call that zero
so definitely i have the same potential difference across r2 and r3 so those are wired in parallel
so i have two questions about the circuit first how should the brightnesses of the bulbs compare
in the original circuit so one of the key points here is if the amount of current
going through the bulb is greater certainly the brightness will be greater but i can
also say if the voltage across a bulb is greater well that would make the current
greater so the bulb would be brighter and we're going to use both types of reasoning
as we answer these sorts of questions so there's a couple ways of answering the first question
the simplest way would be to say well all the current in the circuit passes through r1
i'll call that i and then when i hit this node and i pass through r2 and r3
that current gets split into i2 and i3 so only part of the total current is going through each of
those light bulbs r2 and r3 so they must be dimmer than r1 so we have the r1 is the brightest r2 and
r3 are dimmer since the current has been split and in fact you can say r2 and r3 are equally bright
because they have the same voltage across them and therefore the same current through them
all right question two asks what happens to all the brightnesses if we remove r3 from the
circuit well clearly r3 turns off because we removed it from the circuit but what happens
to everything else and the key here is to think of the simplified circuit you would get by combining
the two resistors that are in parallel and i'm just going to replace that with req so this is
really our original circuit just writing down the equivalent resistance of these two parallel
resistors r2 and r3 and i know something about the comparison between the equivalent
parallel resistance in the individual resistors in the combination the equivalent resistance
is less than the resistance of either one of the participant resistors so less than the resistance
of a single light bulb so then i do a comparison what happens if i just pluck r3 out of the circuit
well the total resistance of this circuit is bigger than the total resistance of the
original circuit because r2 is bigger than req so our total is bigger what does that mean about
the total current in the circuit well i'm thinking about ohm's law here so i total
is v over r total and because i have a bigger total resistance i total goes down since all
the current in the circuit travels through r1 i just figured out that r1 has a smaller current
going through it thus it's going to get dimmer i can also conclude from ohm's law v equals ir
that if i have a smaller amount of current going through r1 that means there's a smaller voltage
drop across it so v1 got smaller but my two voltages across these resistors must equal
the battery voltage when i add them up if v1 got smaller than v2 got bigger
and if the voltage across r2 got bigger that bulb is going to shine brighter
alright so this is the classic mental gymnastics of a concept circuit okay now let's take a look at
how it actually turned out in the lab okay here's the setup for our first concept circuit i've got a
power source over here that i'm going to set to 3 volts dc out of the high side of that power source
i'm connected to the red strip and i'm going to use that as my plus terminal for any circuits
that i put on the breadboard and then connected to the blue strip i have a jumper wire going to
the ground of the power source on the right i have a selection of jumper wires that i'm going to use
to just kind of spread things out in a convenient way and then our resistors here are just christmas
lights and what we're doing is looking for qualitative changes in the circuit based on
removing one of these christmas lights i'm going to start by jumping to one of the horizontal
strips with this orange jumper so i'm plugging that in to the high side the plus terminal
the red strip and then i want to get all the current to flow through this light bulb so i'm
going to plug it between two of the horizontal strips so the leads coming out of that light
bulb one is in strip number six and one is in strip number seven it's a really common mistake
when students are first learning the breadboard to plug both leads of a circuit element into the same
horizontal strip but that would just connect those two leads with the metal strip and no current will
flow through the circuit element so you've got to jump between two different strips to get current
to flow all right then i'm going to jump down to a new horizontal strip just to give myself
some space here so i went from the negative lead on this first light bulb down to this new strip
it's like strip number 13. now i've got to set up my two lights in parallel off of this new strip
so i'm gonna put one lead of each of these lights in that strip number 13 and i'll just bridge them
over to strip 15 and i'm going to spread them out a little just to make it easier to observe them
so we've got at least a little separation between the bulbs and then i need to get the low side of
these back to ground and so i'm going to have to jump down to a new strip just to give myself
some space and then i'll get myself back to ground again that's the blue vertical strip
so i've turned on the power source at three and a half volts and on camera it probably
just looks like a bright white blob well r1 that first christmas light is glowing very brightly
the other two bulbs you can actually detect a dim glow on them just barely glowing red in
the filament so i'll try to get some up close photographs of this so it's easier to do the
comparison but real quick what the experiment is is to remove r3 one of the parallel lights
and then see what happens to the other two so i'm just going to pull it out real quick
and perhaps that's visible on camera what happened is the very brightly glowing r1
just got dimmer and then the second one r2 that was in the parallel combination just got brighter
so if it wasn't clear from the lab video this is how it turned out in the initial state this
bulb r1 was shining very brightly and if i look at this picture i can barely see anything happening
with the other two resistors now in real life i was able to see the filament on these glowing
just barely it was just barely a visible glow so definitely dimmer than r1 hard to say if they were
equally bright in their dimness but qualitatively this is consistent with what we expected
and then i look at the final state and here's r1 it's definitely gotten dimmer as we predicted and
r2 has definitely gotten brighter so that's it for our introduction to the breadboard and our first
concept circuit if you find the physics content on zach's lab helpful click on the zaxlab logo
on the right to browse playlists and subscribe to the channel i produce over 100 new videos
per month and subscribing is the easiest way to find new content thanks for watching you