Un combo important: électrons, conductions et tension (document en anglais)

An important combo: Electrons,

conductors, and voltage

Say that you have a wire (a conductor), and you attach one of its ends to the

positive terminal of a battery and the other end of the wire to the negative terminal

of the battery. Electrons then flow through the wire from the negative to

the positive terminal. This flow of electrons is referred to as an electric current.

When you combine electrons, a conductor, and voltage you create an electric

current in a form that you can use.

To help you picture how conductors and voltage affect the flow of electric

current in a wire, think of how water pressure and pipe diameter affect the

flow of water through a pipe. Here’s how this analogy works:

_ Increasing water pressure causes more water to flow through the pipe.

This is analogous to increasing voltage, which causes more electrons to

flow, producing greater electric current.

_ Using a larger diameter pipe allows more water to flow through the pipe

for a given amount of pressure. This is analogous to using wire with a

larger diameter, which allows more electrons to flow for a given voltage,

producing greater electric current.

Where Do You Get Electricity?

Electricity is created when voltage pulls an electric current through a conductor.

But when you sit down and run a wire between a switch and a light, just

where do you get the juice (the electricity) to power that light?

12 Part I: Getting Started in Electronics

Conventional current versus real current

Early experimenters believed that electric current

was the flow of positive charges. So they

described electric current as the flow of a positive

charge from positive to negative voltage.

Much later, experimenters discovered electrons

and determined that the flow of electrons in

wires goes from negative to positive voltage.

The original convention is still with us today, —

so the standard is to depict the direction of electric

current in diagrams with an arrow that

points opposite to the direction that electrons

actually flow. Conventional current is the flow

of a positive charge from positive to negative

voltage and is just the reverse of real current.

There are many different sources of electricity — everything from the old

walking-across-a-carpet-and-touching-a-doorknob kind to solar power. But to

make your life simple, this book takes a look at the three sources that you’re

likely to use for electronics projects: batteries, your wall outlet, and solar cells.

They just keep on going: Batteries

A battery uses a process called electrochemical reaction to produce a positive

voltage at one terminal and a negative voltage at the other terminal. The battery

creates these charges by placing two different metals in a certain type of

chemical. Because this isn’t a chemistry book, we don’t get into the guts of a

battery here — but trust us, this is essentially what goes on.

Batteries have two terminals (a terminal is just a fancy word for a piece of

metal to which you can hook up wires). You often use batteries to supply

electricity to devices that are portable, such as a flashlight. In a flashlight,

the bulb has two wires running to the battery, one to each terminal. What

happens next? Something like this:

_ Voltage pulls electrons through the wire from the negative terminal of

the battery to the positive terminal.

_ The electrons moving through the wire pass through the wire filament in

the light bulb, causing the bulb to light up.

Because the electrons move in only one direction, from the negative terminal

through the wires to the positive terminal, the electric current generated by a

battery is called direct current, or DC. This is in contrast to alternating current

(AC) which is discussed in the following section, “Garden-Variety Electrical

Outlets.”

The wires on a battery must connect to both terminals. This setup allows

electrons to flow from one terminal of the battery, through the bulb, and all

the way to the other terminal. If the electrons can’t complete this kind of loop

between negative and positive, electrons don’t flow.

Garden-variety electrical outlets

When you plug a light into an electrical outlet in your wall, you’re using electricity

that originated at a generating plant. That plant may be located at a dam

or come from another power source, such as nuclear power. Or it may be fired

by coal or natural gas. Because

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