Download current in a metal wire

CURRENT IN A METAL WIRE
A solid conductive metal contains a large population of
mobile, or free, electrons. These electrons are bound to the
metal lattice but not to any individual atom. Even with no
external electric field applied, these electrons move about
randomly due to thermal energy but, on average, there is
zero net current within the metal. Given an imaginary plane
through which the wire passes, the number of electrons
moving from one side to the other in any period of time is on
average equal to the number passing in the opposite
direction.
A typical metal wire for electrical conduction is the stranded copper wire.
When a metal wire is connected across the two terminals of a DC voltage source such
as a battery, the source places an electric field across the conductor. The moment
contact is made, the free electrons of the conductor are forced to drift toward the
positive terminal under the influence of this field. The free electrons are therefore the
current carrier in a typical solid conductor. For an electric current of 1 ampere, 1
coulomb of electric charge (which consists of about 6.242 × 1018 electrons) drifts every
second through any imaginary plane through which the conductor passes.
The current I in amperes can be calculated with the following equation:
where
is the electric charge in coulombs (ampere seconds)
is the time in seconds
It follows that:
and
More generally, electric current can be represented as the time rate of change of
charge, or
.
Current density
Main article: Current density
Current density is a measure of the density of electrical current. It is defined as a
vector whose magnitude is the electric current per cross-sectional area. In SI units, the
current density is measured in amperes per square meter.
The drift speed of electric charges
The mobile charged particles within a conductor move constantly in random directions,
like the particles of a gas. In order for there to be a net flow of charge, the particles
must also move together with an average drift rate. Electrons are the charge carriers
in metals and they follow an erratic path, bouncing from atom to atom, but generally
drifting in the direction of the electric field. The speed at which they drift can be
calculated from the equation:
where
is the electric current
is number of charged particles per unit volume
is the cross-sectional area of the conductor
is the drift velocity, and
is the charge on each particle.
Electric currents in solids typically flow very slowly. For example, in a copper wire of
cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of
the order of a millimetre per second. To take a different example, in the near-vacuum
inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at
about a tenth of the speed of light.
Any accelerating electric charge, and therefore any changing electric current, gives rise
to an electromagnetic wave that propagates at very high speed outside the surface of
the conductor. This speed is usually a significant fraction of the speed of light, as can
be deduced from Maxwell's Equations, and is therefore many times faster than the drift
velocity of the electrons. For example, in AC power lines, the waves of electromagnetic
energy propagate through the space between the wires, moving from a source to a
distant load, even though the electrons in the wires only move back and forth over a
tiny distance.
The ratio of the speed of the electromagnetic wave to the speed of light in free space is
called the velocity factor, and depends on the electromagnetic properties of the
conductor and the insulating materials surrounding it, and on their shape and size.
The nature of these three velocities can be illustrated by an analogy with the three
similar velocities associated with gases. The low drift velocity of charge carriers is
analogous to air motion; in other words, winds. The high speed of electromagnetic
waves is roughly analogous to the speed of sound in a gas; while the random motion
of charges is analogous to heat - the thermal velocity of randomly vibrating gas
particles.
Ohm's law
Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be the
applied voltage divided by resistance:
where
I is the current, measured in amperes
V is the potential difference measured in volts
R is the resistance measured in ohms
Conventional current
Conventional current was defined early in the history of electrical science as a flow of
positive charge. In solid metals, like wires, the positive charge carriers are immobile,
and only the negatively charged electrons flow. Because the electron carries negative
charge, the electron current is in the direction opposite to that of conventional (or
electric) current.
Diagram showing conventional current notation. Electric charge moves from the
positive side of the power source to the negative.
In other conductive materials, the electric current is due to the flow of charged
particles in both directions at the same time. Electric currents in electrolytes are flows
of electrically charged atoms (ions), which exist in both positive and negative varieties.
For example, an electrochemical cell may be constructed with salt water (a solution of
sodium chloride) on one side of a membrane and pure water on the other. The
membrane lets the positive sodium ions pass, but not the negative chloride ions, so a
net current results. Electric currents in plasma are flows of electrons as well as positive
and negative ions. In ice and in certain solid electrolytes, flowing protons constitute
the electric current. To simplify this situation, the original definition of conventional
current still stands.
There are also materials where the electric current is due to the flow of electrons and
yet it is conceptually easier to think of the current as due to the flow of positive "holes"
(the spots that should have an electron to make the conductor neutral). This is the
case in a p-type semiconductor.
Examples
Natural examples include lightning and the solar wind, the source of the polar auroras
(the aurora borealis and aurora australis). The artificial form of electric current is the
flow of conduction electrons in metal wires, such as the overhead power lines that
deliver electrical energy across long distances and the smaller wires within electrical
and electronic equipment. In electronics, other forms of electric current include the
flow of electrons through resistors or through the vacuum in a vacuum tube, the flow
of ions inside a battery, and the flow of holes within a semiconductor.
According to Ampère's law, an electric current produces a magnetic field.
Electromagnetism
Electric current produces a magnetic field. The magnetic field can be visualized as a
pattern of circular field lines surrounding the wire.
Electric current can be directly measured with a galvanometer, but this method
involves breaking the circuit, which is sometimes inconvenient. Current can also be
measured without breaking the circuit by detecting the magnetic field associated with
the current. Devices used for this include Hall effect sensors, current clamps, current
transformers, and Rogowski coils.
Reference direction
When solving electrical circuits, the actual direction of current through a specific circuit
element is usually unknown. Consequently, each circuit element is assigned a current
variable with an arbitrarily chosen reference direction. When the circuit is solved, the
circuit element currents may have positive or negative values. A negative value means
that the actual direction of current through that circuit element is opposite that of the
chosen reference direction.
Electrical safety
The most obvious hazard is electrical shock, where a current passes through part of
the body. It is the amount of current passing through the body that determines the
effect, and this depends on the nature of the contact, the condition of the body part,
the current path through the body and the voltage of the source. While a very small
amount can cause a slight tingle, too much can cause severe burns if it passes through
the skin or even cardiac arrest if enough passes through the heart. The effect also
varies considerably from individual to individual. (For approximate figures see Shock
Effects under electric shock.)
Due to this and the fact that passing current cannot be easily predicted in most
practical circumstances, any supply of over 50 volts should be considered a possible
source of dangerous electric shock. In particular, note that 110 volts (a minimum
voltage at which AC mains power is distributed in much of the Americas, and 4 other
countries, mostly in Asia) can certainly cause a lethal amount of current to pass
through the body.
Electric arcs, which can occur with supplies of any voltage (for example, a typical arc
welding machine has a voltage between the electrodes of just a few tens of volts), are
very hot and emit ultra-violet (UV) and infra-red radiation (IR). Proximity to an electric
arc can therefore cause severe thermal burns, and UV is damaging to unprotected eyes
and skin.
Accidental electric heating can also be dangerous. An overloaded power cable is a
frequent cause of fire. A battery as small as an AA cell placed in a pocket with metal
coins can lead to a short circuit heating the battery and the coins which may inflict
burns. NiCad, NiMh cells, and lithium batteries are particularly risky because they can
deliver a very high current due to their low internal resistance.
VOCABULARY
Ampere, in practice often shortened to amp, (symbol: A) is a unit of electric current,
or amount of electric charge per second. The ampere is an SI base unit, and is named
after André-Marie Ampère, one of the main discoverers of electromagnetism.
Electrons: fundamental subatomic particle that carries a negative electric charge. It is
a spin ½ lepton that participates in electromagnetic interactions, and its mass is
approximately 1 / 1836 of that of the proton. Together with atomic nuclei, which
consist of protons and neutrons, electrons make up atoms. The electron(s) interaction
with electron(s) of adjacent nuclei is the main cause of chemical bonding.
Electric charge is a fundamental conserved property of some subatomic particles,
which determines their electromagnetic interaction. Electrically charged matter is
influenced by, and produces, electromagnetic fields. The interaction between a moving
charge and an electromagnetic field is the source of the electromagnetic force, which is
one of the four fundamental forces.