78 lines
2.6 KiB
Markdown
78 lines
2.6 KiB
Markdown
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---
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tags: [physics, electricity]
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---
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# Current
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> Electrical current is the movement of electrons from negatively charged atoms
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> to positively charged atoms when an appropriate external force is applied.
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Current is the flow of electrons. Charge is the quantity that flows.
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> The amount of current is the sum of the charges of the moving electrons past a
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> given point.
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## Why current exists
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Current exists because of the [first law of electrostatics](Coulombs_Laws.md).
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When there is an excess of electrons at one terminal (i.e. negatively charged
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atoms) and a deficiency of electrons at the other terminal (i.e. positively
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charged atoms), a [_difference of potential_](Voltage.md) exists between the two
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terminals.
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When the terminals are connected to each other via a conductor (e.g. copper
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wire) electrons will flow along the conductor. This is provided that there is a
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source to supply electrons at one end and remove them at the other. We call this
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force the **voltage source**.
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_The diagram below illustrates the flow of current where the circles are
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electrons knocking into each other and passing current:_
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> Electrons travel very slowly through a conductor. This is in contrast to their
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> intrinsic motion which of course equal to the speed of light (186, 000 miles
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> per second).
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## Formal expression
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We measure **charge** in Coulombs ($C$). A Coulomb is an aggregate of the charge
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of thousands of electrons because their individual charge is so small. One
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Coulomb is equal to the charge of $6.24 \times 10 ^{18}$ electrons.
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We measure **current** in amps. When one coulomb of charge moves past a point in
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one second it is called an **ampere** (amp) represented as $A$.
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This relationship is captured in the following equation:
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$$
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I = \frac{Q}{t}
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$$
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- $I$ = current measured in amps
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- $Q$ = quantity of electrical charge measured in coulombs
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- $t$ = time
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### Application
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_Calculate the current in amps if 9 coulombs of charge flow past a point in an
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electric circuit in 3 seconds._
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$$
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I = \frac{9}{3} \\
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I = 3 A
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$$
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## Direction of current
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In terms of the physics, current flows due to the movement of charged particles
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from atoms which are negatively charged to atoms which are positively charged.
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However when it comes to the representation of current in electrical circuits
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the convention is to view current as flowing from the positive to the negative
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terminal.
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This is obviously physically inaccurate however it became the convention before
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electrons were properly understood when charge was thought to be a property of
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protons. So we are stuck with this convention in electronics.
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