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Shape and dimensions in living beings

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in real time

ELECTRICITY AND CHEMISTRY

Home-made batteries and electrochemical cells

THE ELECTRIC CIRCUIT

What are the essential parts of an electrical circuit?

Pupils build an electrical circuit with a battery, a light bulb and electrical wires. They motice that for an electrical current to flow, it must travel in a closed loop. The first representation of the circuit will be immediate, realistic, then the schematic, formal representation is introduced


What happens inside an electrical circuit?

The electrical current which make the bulb light is a flow of particles called electrons. The copper wires are full of movable charge, the electrons, but they also contain positive charge within the metal atoms. When the circuit is open, the positive and negative charges are balanced. When the circuit is connected to a battery, it, because of its voltage, creates an imbalance of charges by pumping and pushing electrons along the circuit. The battery is like our heart, that pumps and moves blood inside our body.The voltage of a battery is like the difference in level between the ends of a tube: the water doesn’t flow along the tube if it is in a horizontal position.
The positive charges are represented by red dots on a transparent plastic sheet, the negative ones by green dots on a second sheet (to draw the green dots, align perfectly the second sheet over the first one an draw one green dot on top of every single red dot). When the sheets completely overlap, the dots appear black (balanced charges), if we slide one sheet over the other, the dots only partially overlap and a green and a red region appear (unbalanced charges).
(from http://amasci.com/miscon/whatis.html)



The charges flow only inside wires?

If you connect to a circuit a glass filled with a salt water or lemon juice, the tester detects the passage of current and inside the glass bubbles of gas appear. It happens that the current flows also inside the glass, producing chemical transformations.
A flow of charge always requires a flow of carrier particles. Current in a metal wire is a flow of electrons, but in many other substances both the positive and negative charge can flow. In salt water particles with extra electrons (negative charged) or with a lack of electrons (positive charged) can flow along, and this flow is a genuine electric current.


The current flows or doesn’t flow?

Insert along the circuit pieces of different material (toothpick, plastic ruler, corks, gold rings…) and explore their different behaviour (conductors/not conductors).


Proportionality in electricity: the first Ohm's law

  • Pupils build electrical circuits using wires, the same bulb and different voltage and observe what happens to the bulb light in each case.
  • They keep the voltage the same and connect more than one bulb in series and notice what happens to the bulb light in each case.

From an informal, spontaneous way to express the proportionality:

The harder the push, the faster flows the charge (direct proportionality), the more friction you have, the slower the flow (inverse proportionality)

... to the mathematical law: VOLTAGE/RESISTENCE = CURRENT INTENSITY

Proportionality in electricity: the second Ohm's law

Pupils guess and verify the influence of the conductor length and section on the resistance by using different pieces of graphite

 

How can two light bulbs be connected inside a circuit?

:: How do light bulbs connected in series work?

the bulbs light, but with a dimmer light compared to the case when there is a single bulb; if a bulb is switched off, also the other bulb doesn’t work.

:: ... and if they are connected in parallel?

The brightness of the two bulbs is the same as the light of a single bulb inside the circuit; if a bulb is switched off, the other one keeps lighting.
The connections inside our houses are in parallel; connections in series are used for example to light Christmas tree.


How can two batteries be connected inside a circuit?

:: How do batteries connected in series work? There are two ways to do it:

First case
+ pole of a battery connected with the – pole of the other battery, light bulb connected to the free poles--> the bulb is brighter compared to the case with a single battery

Second case
The same poles of two batteries are connected together (+ with +, - with -), light bulb connected to the free poles--> if the batteries have the same voltage, the bulb doesn’t light, if the batteries have different voltage, for example 4,5 V and 1,5 V, the bulb lights and its brightness is the same the bulb has when it is energized by a 3 V battery (difference between 4,5 and 1,5 voltage).

:: How do batteries connected in parallel work?

same poles of two batteries connected together (+ with +, - with -), light bulb connected to a + pole and to a – pole-->the light bulb lights and its brightness is as it were energized by only one battery (the advantage is that the batteries last longer).

 
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