Příprava vystoupení

Hlavní část přípravy vystoupení proběhla distančně, studenti v mezinárodních dvojicích diskutovali připravované pokusy, sdíleli nápady. Bylo třeba se domluvit na přípravě obdobné sady pomůcek ve Finsku a pak v České republice, protože převoz potřebných pomůcek letecky  by byl v mnoha případech nemožný a ve většině případů komplikovaný. 

Studenti si pokusy vzájemně popisovali, sdíleli fotografie pomůcek i nahrávky pokusů. Výsledkem byla příprava první verze všech pokusů finskými partnery.

Část komunikace je zveřejněna níže:

An induction cooker, as the name suggests, operates on the principle of electrical induction. Beneath the glass plate lies a copper coil through which an alternating electric current passes, generating a rapidly changing magnetic field above the cooker. The cooking vessel placed on the hob must be made of metal. The quickly changing magnetic field induces an electric current in the vessel, which is then transformed into heat.

If a bulb with a copper wire is placed above the cooker, it will flicker occasionally. Placing a pot on the wire, without directly touching the hob, will cause the bulb to glow steadily. This effectively creates a transformer, with the primary coil residing in the induction cooker and the secondary coil in the bulb. The pot's presence is not strictly necessary for the bulb to flicker, but it serves to maintain the cooker's operation.

Elektrony v akci_Modra.mp4

- Electromagnetic induction -

Electromagnetic induction describes the process of generating an electromotive force and an electric current in a conductor in a changing magnetic field. Michael Faraday is thought to be the discoverer of induction and to this day we use Faraday’s law of induction. It states that whenever there is a relative motion between a conductor and a magnetic field or a change in the magnetic field strength through a conductor, an electromagnetic force is induced. If the circuit is complete, induced voltage causes an electric current to flow in a close circuit.

The induction cooker consists of a coil of copper wire beneath the cooking surface. When an electric current flows through this coil, it generates an alternating magnetic field. When we put a suitable cooking vessel on a cooktop, this changing magnetic field induces an electric current in the bottom of the vessel, according to Faraday’s law of induction. The coil in the induction cooker has generally many turns compared to the cooking vessel. It forms a transformer that steps down the voltage and steps up the current. The induced electric current generates resistive heating within the vessel due t the resistance of the material (also called Joule heating). The vessel becomes hot and transfers the heat to its contents by heat conduction.

The cooking vessel need to be made of magnetic materials, for example iron or stainless steel. Non-magnetic materials like aluminium do not work on an induction cooker.

Unlike traditional gas or electric stoves, an induction cooker does not heat the cookware through conduction. It uses electromagnetic induction to directly heat the cooking vessel. Hence the cooktop will heat by heat conduction from the cookware but on its own it has no reason to become hot. It has generally a heat-proof glass-ceramic surface which is not magnetic at all. An electromagnetic induction does not apply to it.

It is possible to use an induction cooker not only for generating heat. Another coil is necessary for that. We have to connect the bulb to the coil to create a closed circuit. Then we place it within the proximity of the cooktop. The electromagnetic field generated by the cooker’s induction coil would induce a small electric current in the circuit of the bulb. If we decrease the intensity of the induction cooker, the bulb will alternately light up and go out. That is because an induction cooker does not adjust the power intensity directly, instead it adjusts the power by controlling the intervals between inductions.

EXPERIMENTS – Electromagnetic induction

Hypothesis: We can cook with objects between the cooktop and the cookware because the cooking surface is not directly heated.

Materials: any banknote, an induction cooker, a suitable cooking vessel

Procedure: First, put the banknote on the induction cooker. Then cover it with the cooking vessel filled with water. Let the water boil. Take out the banknote. Make it is entirely undamaged.

Results:The banknote was indeed completely fine.

Hypothesis: The half we place on the vessel will cook, while the other half, placed on the cooktop, will remain raw.

Materials: an induction cooker, enough eggs, a suitable cooking vessel preferably without edges

Procedure: Place an egg on the induction cooker so one its half is situated on the vessel while the other half is lying directly on the cooktop. Watch what is happening.

Results: The hypothesis was right; only one half of the egg has cooked.

Hypothesis: If we connect a bulb to coil, we will create circuit in which we can generate electric current using an induction cooker.

Materials: an induction cooker, conductive wire, a little bulb, a multimeter

Procedure: First, create a coil with the conductive wire. Then, using the multimeter, measure the voltage on the coil. Fing a bulb which needs approximately as much voltage as you measured. Connect the bulb to the coil. Bring it really close to the induction cooker. Watch if the bulb lights up.

Results: The bulb did light up, even though just a little bit. When we decreased the intensity of the induction cooker, we could see the bulb change from shining to going out.

Lenz's law - script

We know how electromagnetic induction works: if we change the magnetic field around a conductor, it causes the induction of an electric current inside a closed circuit. But this is actually not the entire thing – when the electric current starts flowing through the conductor, it creates its own magnetic field. So let's take a look at how these magnetic fields interact with each other.

We have two rings hanging on strings – one is from aluminium and one from copper. We can see that the magnet doesn't stick to the rings, but take a look at what happens when we take a magnet and move it around the rings.

[Experiment – Lenz´s law and metal strings]

We can see that when we move the magnet closer to the ring, we actually make the ring go away. By moving the magnet around the ring, we caused electromagnetic induction – we induced an electric current inside the ring, and as we said, this induced current created its own magnetic field. This field pushed against the magnet, and so it made the ring go away. We actually know that this happens every time: the magnetic field that was created by the induced electric current always acts against the change that caused the induction. This phenomenon is called Lenz's law.

Let's try to do something similar, but with a little twist. What if instead of using a metal ring, which is basically a wire, we tried to use a massive metal object?

We have a metal cup sitting on a needle so that it can spin. Let's see what happens when we move the magnet above the cup.

[Experiment – Lenz´s law and a spinning cup]

We can see that the cup starts to spin. The principle here is very similar to the last experiment: by moving the magnet, we induce an electric current in the cup. But since the cup is not a wire but a  massive metal object, the electrons inside it don't move in a single loop but in multiple different loops at once. This way of electron movement is called eddy currents. But in this situation, eddy currents actually work the same way as a single loop – they also create a magnet field around the cup, so Lenz's law works here as well.

Let's change the experiment one more time. Now, we are going to drop the magnet through this aluminium pipe to see if Lenz´s law also applies in this situation.

[Experiment – a magnet falling through an aluminium pipe]

We can see that it hes taken the magnet unusually long to fall through the pipe. And this is again due to Lenz's law. The falling magnet induces eddy currents inside the pipe, and as we know, the magnetic field around these currents should push against the change that caused the induction. But this change was the falling magnet, so pushing against the change means pushing the magnet upwards, which makes it slow down.

Hrnicek.MOV
Krouzky_2.MOV
elm. dělo.mp4

Franklin’s Bell Experiment

Hello everybody, my name is Anna, and I'll be showing you an experiment called Franklin's Bell. It's called Franklin's Bell after Benjamin Franklin, who invented and popularized it in the 18th century. He had one of these to tell if a lightning storm was coming near. But probably the most important thing about Franklin’s Bell is that it was the first time that electrical energy was converted into mechanical energy. So, how did Benjamin Franklin use it and how did it work?

He had a lightning rod on his house, which was connected to a bell inside the house. I'm a poor student from the Czech Republic, so instead of the bell, I brought an empty can, which will serve me just the same. (I will place the can on the table.) Right next to that bell, he had another bell (I will place the other can close to the first one) that was connected to the ground. The ground will be demonstrated by a polystyrene foam board, which has the same electrical qualities. (Under one can, I will place the foam board.) In between the bells, there was a metal ball attached to a string. Instead of the metal ball, I'll use a pull tab from a can, which I tie to a thread and hang on a wooden stick. (I will insert the wooden stick through the pull tab of the can.) And that’s the whole setup.

Then, Benjamin Franklin only waited for a storm to come. If big clouds or a storm passed by, the lightning rod would get that positive charge, and the bells would start ringing. Now, since I probably don't have a lightning storm coming up, I'll simulate it with a plastic rod. I’ll electrify the plastic rod with a piece of cloth and bring it close to the side of one can.

How it worked is that as this can become exceedingly positive, the negative charges inside the pull tab would become closer to this side, while the positive charges would be over here. This causes the pull tab to be attracted to the positively charged can. So, the pull tab would swing and hit this can. As it would hit the can, the pull tab would also become positively charged. This means that now it would be repelled, and since this can is connected to the ground or basically has a negative charge, it would quickly swing to it and discharge itself through this can. And then, this process would repeat again and again, making the bells ring.

With this relatively simple experiment, Franklin was able to predict that a storm was coming. Nowadays, we all probably have the weather forecast downloaded on our mobile phones, so the experiment probably has no use for us. But if your phone dies or you simply can't access the internet, it could come in handy. Plus, most forecasts (at least those we have in the Czech Republic) are pretty unreliable, so with this experiment, you'll have 100% accuracy.

A: Greetings everyone! Today, we will be showing you some experiments related to electrostatics. I'm A, and today I will be joined by B. How does that sound?

B: That’s a great idea. Also hello everyone! The first experiment we will be showing you involves the following. Our tools of the trade are quite simple – a plastic stick, a cloth, and a bit of physics magic. Watch closely! (A is holding up the plastic stick and cloth, then the metal can as B speaks of it, showing it to the audience) 

(A places the metal can on the table)

A: So, we start by placing this metal can on a flat surface so it does not roll anywhere. Now, I will rub the stick with the cloth/towel. Then we will bring it closer to the can.(Various responses from the audience.)

B: And the question for you is: What do you think will happen when we bring this charged plastic stick close to the metal can? Any predictions?

(Audience makes a guess)

B: Well let’s find out! (A brings the stick near the can and it starts rolling) 

B: When we rub the stick and the towel, we make some electrons jump from one to another and the stick gets a negative charge. That charge causes the metal can to move its own free electrons and roll.

A: Yeah, but what if we replaced the metal with something different, like this paper roll for example? (A swaps the metal can for a paper roll.)

B: Well, I am not really sure. (To the audience) Do you expect  the experiment will work when with this paper roll instead of the metal can? Will it roll?

(Allow for audience responses.)

A: Only one way to find out! Charge the stick and let's see what happens!

(B rubs the plastic stick, and then brings it close to the paper roll. Surprisingly, the paper roll also starts to roll towards the stick.)

B: Well, that is quite a surprise! The paper roll rolls just as the metal one. Now, why did it move even though it lacks the free electrons like the metal can? 

A:  While there are no free electrons, we can observe an effect called polarization here. The electrons are not free but still can rotate depending on the outside charges. So they still move a bit, that is why we can see the can rolling to the stick.

B: Alright, what experiment did we prepare next?

A: We were supposed to prepare more than one experiment?! I planned on just blowing bubbles for the rest of the show. (pulls out bubble solution)

B: Actually, we could use those bubbles to show something. Make a few here on the table.

(A blows a bubble on the table, while B charges the stick)

B: As you can see, thanks to the polarization we can move the bubble just as we moved the paper roll. 

A: But we already saw that, can we do something else with it?

B: Of course! Would you be so kind and make two bubbles one inside the other?

A: I can at least try, let's see what I do. (starts making the bubbles)

meanwhile B (to the audience): And for you I have another question for you guys. What do you think will happen to the bubble inside when we bring the charge stick closer?

(audience answers)

B: Let’s see. (brings the charged stick closer, and the audience can see the inner bubble does not move)

A: Oh would you look at that. It seems that the inner bubble doesn’t move an inch. That is because the outer bubble is made of conductive material and therefore acts as a Faraday’s cage, shielding the bubble inside from the electric charge.

B: That certainly is interesting but enough bubbles. What do we get next?

A: This conveniently modified bottle and a laser pointer. Any ideas what we can do with it?

B: We could trap the laser beam inside the water stream using total refraction, how does that sound?

A: Great, let's try that. So I will place the pointer here and turn it on, then we let the water flow. Ready?

(B lets the water flow and the water stream lights up with the laser beam in it)

(B puts a finger in the way of the water stream to show that the laser is trapped in it as their finger lights up)

B: As you can see, the laser is trapped. But this experiment is supposed to involve electrons? What do we do about it?

A: Well we can just use the charged stick again, no?

B: Yeah, we should be able to bend the water with it, let’s see if the laser stays inside it.

(A charges the stick and passes it to B, B brings it to the water stream, which bends)

A: It worked! So as you can see not only were we able to trap the laser inside the water but we can also move it as we please, isn’t that amazing?

B: You are right, it truly is, but that is all you will hear from us, thanks for being a wonderful audience and let’s see what experiments did others prepare.

A: Enjoy the rest of the show and goodbye!

(A and B leave the stage)

Listening to Light

Experimental aids:

 

Script:

Have you ever listened to the light?

Do you know how speakers work? The resource of voltage produces current into a cylindrical coil of wire, which is suspended in the circular gap between the poles of a permanent magnet. This coil moves back and forth inside the magnetic field as the current passing through it alternates in direction with the signal applied. The center of the speaker cone is attached to one end, which gets driven back and forth by the moving coil. As the cone moves, it pushes and pulls the surrounding air; by doing so it creates pressure waves in the air, called sound.

As a resource of voltage I am going to use a solar cell. It needs some light to produce voltage. So let´s use this LED which consists of three LEDs of different colors and a flip-flop circuit which makes our LED blink. As you can hear, when you see blinking you can hear banging, when the frequency is so high than we are not able to see blinking we can hear a tone. But if there is no change in light there is no sound because sound is produced through vibrations of a speaker cone.

ovládáme zvuk světlem.mp4

Tools and components:
Tesla Coil for Kids kit (transistor, coil, resistor, capacitor, etc), solder, tin with lead, power supply


Principle:
We relatively slowly charge a capacitor through coil, which does not pass the high-frequency voltage. The moment the capacitor is charged, the transistor turns on. Once the transistor turns on, the capacitor will quickly discharge through the primary coil. The primary coil has only one turn of thick wire. This causes a high voltage to be induced in the secondary coil. The secondary coil in our case has hundreds of turns of thin insulated copper wire therefore generating hundreds times higher voltage. This voltage is so great that it causes a spontaneous discharge in the air at the end of the wire of this secondary coil. Then the discharged capacitor will start to charge again and the whole process is repeated with a given frequency. We then see sparks at the end of the coil. A spark or discharge is basically a charged particle (ion) such as an electron, which our electromagnetic field generated by our coil attracts very quickly. This causes the creation of ionised air, or plasma. At the same time, when the spark appears, we hear a crack. Similar to when we see lightning and then hear thunder during a storm. 

 

video of experiment:

2.  Experiment proves that heat is also produced with the discharge. By putting a piece of paper to the place where the discharges are coming from we can demonstrate
high temperatures caused by discharges in the air.

3. We will show how to generate different sounds and even music using the tesla coil. Sound is in principle vibrating air. Every spark from the tesla coil vibrates the air a little bit.

script PLASMA BALL

Hi, we have prepared for you a couple of experiments with this plasma ball. This is a plasma

ball. How does it work? Normally gas doesn't conduct current. But there's a vacuum in the

plasma ball and it's full of low pressure gas. So there's a discharge and we can see lightning.

What if I touch the plasma ball? What do you think? Please raise your hand.

A. The color of the lightning changes.

B. The lightning will disappear from the plasma ball.

C. The lightning will follow my finger.

D. Nothing will happen.

(experiment)

Lightning follows my finger because it's made up of electrons and they're lazy. They always

want to take the shortest route. They also want out of the plasma ball because there's too

many of them. So my finger is the best way for them to get away. They run a finger along the

surface of my body to the ground.

Can you turn on the light? Yes?

Me too I use a switch to turn the light on.

And can you turn on the fluorescent lamps? Even if it's not plugged into an outlet

I do.

(experiment)

How is that possible? The fluorescent lamp contains mercury vapor. The electrons from the

cathode start to ionize the mercury vapor. Some of the energy is converted into ultraviolent

light, which is invisible to us. That's why all fluorescent lamps are white. It's the special

fluorescent dye on the lamps that makes the light visible. The plasma ball works in a similar

way. The alternating voltage at the centre creates electromagnetic waves, and the arcs of

plasma act as antennae, meaning that the extent of the electromagnetic field surrounding

the ball is significantly larger than the bounds of the glass globe. Bringing the fluorescent

tube near to the plasma ball allows the electrons inside to be accelerated by this field, and

those moving electrons constitute an electric current, which causes the bulb to light up.

We can compete. Who does the light prefer?

(experiment)

Electrons are lazy and take the route with the smaller electrical resistance. That's why she

won. Who wins depends on our electrical resistance; one of us is more electrically resistant

Each gas emits a different light. We can verify this by using discharge lamps with different

gases.

(experiment)

There is a changing electric field around the plasma ball, we can use a jack connected to a

speaker to listen to the changing electrical field (experiment)

plasma koule.mp4