Now that we have learned about the early theories of light, we also want to learn about the early theories
of two other very important quantities of electrodynamics.
These are the charge and the electric field.
It was already 550 B.C. that Thalia's conducted a very interesting experiment or just a general observation.
So he was rubbing a piece of ember with a thorough.
And what he observed is that when he took a feather, that this feather was attracted to the piece of
amber.
So what was happening here by today's understanding is that initially the amber, the feather and the
fur are all electrically neutral.
So they have protons and electrons and on average, all of these charges will cancel out.
However, when we wrapped this Amber with the piece of fur, the fur transferred some of its electrons
to the amber.
So this is why now we have an excess amount of electrons here, which is why this piece of amber is
electrically charged and therefore it attracts this feather.
And some fun facts that you may not know, or at least I didn't know before preparing this course is
that the Greek word for Amber is actually electron.
So the word electron or electronic comes from Amber.
So the theory for this experiment was derived in in the 70s and 80s, but actually already Rubinson
and 1769 analyzed his experiments where he took two charges or two objects that were charged and investigated.
The force between these two objects and its dependence on the distance are.
And he found out that it's scaled with one over R to the power of two point 006.
So this already gave rise to that's probably of one over our square law.
And then Coulomb was the first really famous person to formulate this law, that the force is characterized
by the product of the two charges and one over distance square dependence.
And that's the orientation for the forces is always oppositely.
If you if you have two charges and that these two forces would lead to an attractive interaction if
the charges are oppositely and that they will have a repulsive shape if the charges are of equal sine
and the, prefectly here is this number and it can be calculated as one over four PI times Epsilon zero.
Now, more elaborate way of understanding what is happening here was developed in the eighteen thirties
by Faraday, so he considered lines of force.
So he again considered to charges or two charged objects and investigated on the force.
And on the previous slide we had to Cullum's law, which basically analyzed the force independence of
the distance.
But here you can see also that he analyzed this in terms of another quantity, which is e the electric
field.
So the electric field times, that charge gives rise to the force.
And these lines of forces are actually really helpful because they allow to understand what would happen
if you would bring an additional charge into the system.
For example, here you can see in the background I have in this bright color here, the electric field
lines for a simple radial symmetric charge.
So these will be radial arrows pointing away from the charge.
And if you have reversed a sign of the charged and you will also reverse the sign of the electric field.
However, in total, this system, consisting out of two oppositely charged charges, will give rise
to such an electric field profile.
So I basically took these two arrows here, for example, and you are in a way, interpolated the electric
field between them.
So you when you and, you know, the electric field in the vicinity of this charge and in this charge,
you can guess what's the total electric field will look like.
So this shape of these lines really allows us to understand what would happen if we would put another
charge, for example, here.
And also this electric field was quite useful to explain earlier experiments or even devices.
For example, in the 1748, there was already the very first capacitor that's in fact relies on storing
charge and generating electric fields.
So there was already quite a lot known about charges, electric fields and also the coolant law.
And this all in the end was condensed in the Maxwell's equations.
And similarly, there was already quite a lot early knowledge about magnetism.
And this is what we want to discuss in the next lecture.