How do Lemon Batteries & LEDs Work! By: Branch Education
When life gives you lemons, why not learn about electricity!
in the previous episode we built a lemon battery
using those lemons that life gave you, and now let's go deep under the rind and
explore how this lemon battery produces electricity
and then how we can use that electricity to generate light.
So, let's dive right in.
To start, electricity is a flow of electrons and as in any battery,
this flow is caused by the interaction between two materials,
one that wants to lose electrons, and one that wants to gain electrons.
In the case of our lemon battery, the nails, but more specifically
the protective anti-rust coating of zinc on the nails, wants to lose electrons.
And then the acidic juices in the lemon, specifically the Hydrogen ions, or H+,
wants to gain electrons. So this chemical reaction is between the Zinc
and the acidic lemon juice, and when a path is made available electrons
flow out of the zinc coated nails
and to the hydrogen ions in the lemon juice.
As a result of this interaction the zinc dissolves
off the nail and becomes a positively charged zinc ions
and the hydrogen ions become hydrogen gas and leave the lemon.
So, let’s see it happen.
We’re gonna drop a bunch of zinc coated nails into a cup of lemon juice.
Uhh- this is going to take some time, so let’s fast forward.
Throughout the course of several hours, you can see hydrogen gas bubbles forming
and the lemon juice turning a sickly green color as a result of the dissolved zinc.
Also take a look at the line formed from the nail only being part way submerged
and how the zinc has been dissolved.
This is evidence of the electrons moving from zinc
to the H+ ions in the acidic lemon juice.
So then, now that we have a reaction between zinc and hydrogen,
what help does the copper wire and pennies provide?
And why would electrons take the longer path
instead of just interacting with the lemon juice next to the nail?
To answer this let’s rewind and zoom in.
When the zinc loses its electron it becomes positively charged,
falls off the nail, and dissolves into the lemon juice.
This positively charged zinc repels other hydrogen ions,
so the zinc ion forms a kind of traffic jam
and has to move out of the way before further
hydrogen ions can react with the zinc.
So by adding the wire and copper pennies,
we provide the electrons a secondary path to get to the hydrogen ions.
It’s like having two doors, an entrance and an exit,
to a busy building instead of just one.
Zinc takes one path, while electrons go all the way around, along the wire,
through the copper pennies, and through the lemon via a separate path.
Though, let’s stop here and clear up a misconception.
This animation is accurate that there is a flow of electrons,
but it is wrong because in reality, the electrons travel
at an unbelievably slow pace of 33 nanometers per second.
It would take electrons almost a whole week to travel
from the bottom of this LED up to the top.
That being said, once the LED is connected, the flow of electrons along the
entire path starts up near instantaneously.
To say it differently, when a zinc atom loses 2 electrons,
those electrons push into the next adjacent electrons and so on,
thereby starting up a flow until 2 electrons on the other side
join 2 hydrogen atoms as we discussed earlier.
The propagation of this force from the electrons
and starting up a flow of electrons happens at around the speed of light.
Electrons may move unbelievably slow themselves,
however the force that propagates is near instantaneous.
Also, let's be real, it's not just two electrons that are passing by this cross section of wire.
The number is closer to 9 quadrillion electrons per second,
which is 1.4 milliAmps of current.
Let's move on and explore how an LED uses electricity to produce light,
and why we have to use 3 lemons batteries in series instead of just one.
We’re going to have to use an analogy to help
with this explanation as it’s a little complex.
Let's imagine in an LED there is a road.
And electrons are balls moving along the road.
In an LED this road is composed of two special materials
next to each other, named N and P, and they form a one-way road.
Electrons can only move in this direction.
Additionally, this road is not flat, but rather P is uphill from N
and thus, the electrons don’t move from N to P,
at least without extra energy, ya know, because it’s uphill.
So, to fix this let’s give some energy to the electrons.
This energy comes from the zinc pushing away the electrons,
and the hydrogen wanting the electrons.
In our analogy, let’s represent this energy by the height of the electron.
Technically this height is called electrical potential energy,
or voltage, but let’s stick with the analogy.
With the energy from the chemical reaction between the nails and lemon,
we raise the height of the electron,
and it can then move along the one-way road from N to P- easy enough- right?
Well… no… one issue is that N is a special material as we mentioned,
and with that it has special properties.
Specifically, it can only allow its electrons to be certain heights,
and nowhere in between.
That means we can't just raise the electron’s height up a little,
no- the special material doesn't allow that.
It’s like an elevator- you can only stop at floor 1 or floor 2,
but not at floor 1.25.
In essence, we must raise electrons to at least this height, or none at all.
And that's why 3 lemon batteries in series are required.
Each lemon battery has the potential to raise
the electron a certain height, and it takes 3 lemon batteries to
get to the height, or energy level that is acceptable to material N.
Now that the electrons are at the higher energy level,
they can flow from the N side to the P side
and make their way through the copper
and to the hydrogen ions in the lemon.
But now, since N has a much higher height than P,
you can see there is a big drop for electrons
from their height in material N to their height in material P.
As a result, the electron drops from this height down to here
and when it does, the difference in energy
level is released as photons, or light.
And that's where the light is coming from:
the zinc/hydrogen interaction brings the electrons
to a higher electrical potential,
essentially raising the electrons up to a higher energy level in N,
and when the electrons move to P,
they drop in energy and this energy is released as light.
The technical term for this is electroluminescence.
Without material N being picky about the energy level of its electrons,
and both materials N and P making a one-way road,
there wouldn’t be a drop-in energy resulting in the production of light.
That's what makes an LED special!
Also, the color of the light is dependent on the height of the drop.
Blue light, which has more energy per photon than red light
has a larger drop between N and P
And this is related to why efficient blue LEDs were so difficult to discover.
Furthermore, the intensity of the light is dependent
on how many electrons are flowing
through the LED. The more electrons, or current, passing from N to P,
the more photons generated and the brighter the light from the LED is.
My Super lemon battery and LED had around 1.4 milliAmps running through it,
which equates to around 9 with 15 zeros after it
or 9 quadrillion electrons dropping from N to P per second,
and around that many photons emitted as well.
I’m excited to hear what questions you have.
I included a set of frequently asked questions and answers in the description.
But you should ask them in the comments as well
I love this activity. You may think it's fun and simplistic,
as it just makes a small amount of light.
But, LEDs are everywhere! They can be found in lightbulbs
Smartphones, TVs, computer monitors,
and this 8 by 8 LED cube I built a few years back,
and they all function using the principles that we discussed earlier.
Also LEDs demonstrate one of the core interactions between N and P materials
which is at the center of microchips.
The combination of N and P materials is so amazing that many Nobel prizes
have been awarded to people for their discoveries and research into these materials.
In fact, in 1921, Albert Einstein was awarded the Nobel prize
for his understanding of topics related to discrete energy levels,
very similar to what we discussed here. Pretty cool, no?
That about wraps it up.
Thanks for watching, don't forget to like,
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If you have any questions post them in the comments below.
This episode branches to how to build Lemon Batteries,
solar cells, understanding current and voltage,
and understanding electricity.
Also remember to think about the conceptual simplicity,
and structural complexity
throughout the world around you.