Download How Batteries Work

Rīgas Tehniska Universitāte
Daugavpils filiāle
Viktorija AGAFONOVA
TEKSTU KRĀJUMS
angļu valodā
Metodiskie norādījumi
Elektrozinību profila studentiem
Daugavpils 2008
Contents
1. How batteries work ..................................................................................................................... 3
2. How flash memory works ........................................................................................................... 6
3. Electric field ............................................................................................................................... 8
4. Electrical conductor .................................................................................................................. 10
5. Electric current .......................................................................................................................... 12
2
How Batteries Work
Batteries are all over the place - in our cars, our PCs, laptops, portable MP3 players and cell
phones. A battery is essentially a can full of chemicals that produce electrons. Chemical
reactions that produce electrons are called electrochemical reactions. In this article, you'll learn
all about batteries -- the basic concept at work, the actual chemistry going on inside a battery,
rechargeable versions, what the future holds for batteries and possible power sources that could
replace them.
If you look at any battery, you'll notice that it has two terminals. One terminal is marked (+), or
positive, while the other is marked (-), or negative. In an AA, C or D cell (normal flashlight
batteries), the ends of the battery are the terminals. In a large car battery, there are two heavy
lead posts that act as the terminals.
Electrons collect on the negative terminal of the battery. If you connect a wire between the
negative and positive terminals, the electrons will flow from the negative to the positive terminal
as fast as they can (and wear out the battery very quickly -- this also tends to be dangerous,
especially with large batteries, so it is not something you want to be doing). Normally, you
connect some type of load to the battery using the wire. The load might be something like a light
bulb, a motor or an electronic circuit like a radio.
Inside the battery itself, a chemical reaction produces the electrons. The speed of electron
production by this chemical reaction (the battery's internal resistance) controls how many
electrons can flow between the terminals. Electrons flow from the battery into a wire, and must
travel from the negative to the positive terminal for the chemical reaction to take place. That is
why a battery can sit on a shelf for a year and still have plenty of power -- unless electrons are
flowing from the negative to the positive terminal, the chemical reaction does not take place.
Once you connect a wire, the reaction starts. The ability to harness this sort of reaction started
with the voltaic pile.
Battery History
The first battery was created by Alessandro Volta in 1800. To create his battery, he made a stack
by alternating layers of zinc, blotting paper soaked in salt water, and silver. This arrangement
was known as a voltaic pile. The top and bottom layers of the pile must be different metals. If
you attach a wire to the top and bottom of the pile, you can measure a voltage and a current from
the pile. The pile can be stacked as high as you like, and each layer will increase the voltage by a
fixed amount.
In the 1800s, before the invention of the electrical generator (the generator was not invented and
perfected until the 1870s), the Daniell cell was extremely common for operating telegraphs and
doorbells. The Daniell cell is also known by three other names:
 Crowfoot cell (because of the typical shape of the zinc electrode)
 Gravity cell (because gravity keeps the two sulfates separated)
 Wet cell (because it uses liquids for the electrolytes, as opposed to the modern dry cell)
The Daniell cell is a wet cell consisting of copper and zinc plates and copper and zinc sulfates.
To make the Daniell cell, the copper plate is placed at the bottom of a glass jar. Copper sulfate
solution is poured over the plate to half-fill the jar. Then a zinc plate is hung in the jar and a zinc
sulfate solution is poured very carefully into the jar. Copper sulfate is denser than zinc sulfate, so
the zinc sulfate "floats" on top of the copper sulfate. Obviously, this arrangement does not work
very well in a flashlight, but it works fine for stationary applications.
3
Battery Reactions and Chemistry
In any battery, an electrochemical reaction occurs like the ones described on the previous page.
This reaction moves electrons from one pole to the other. The actual metals and electrolytes used
control the voltage of the battery -- each different reaction has a characteristic voltage. For
example, here's what happens in one cell of a car's lead-acid battery:




The cell has one plate made of lead and another plate made of lead dioxide, with a strong
sulfuric acid electrolyte in which the plates are immersed.
Lead combines with SO4 (sulfate) to create PbSO4 (lead sulfate), plus one electron.
Lead dioxide, hydrogen ions and SO4 ions, plus electrons from the lead plate, create
PbSO4 and water on the lead dioxide plate.
As the battery discharges, both plates build up PbSO4 and water builds up in the acid.
The characteristic voltage is about 2 volts per cell, so by combining six cells you get a
12-volt battery.
A lead-acid battery has a nice feature -- the reaction is completely reversible. If you apply
current to the battery at the right voltage, lead and lead dioxide form again on the plates so you
can reuse the battery over and over. In a zinc-carbon battery, there is no easy way to reverse the
reaction because there is no easy way to get hydrogen gas back into the electrolyte.
Modern Battery Chemistry
Modern batteries use a variety of chemicals to power their reactions. Typical battery chemistries
include:










Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon chemistry
is used in all inexpensive AA, C and D dry-cell batteries. The electrodes are zinc and
carbon, with an acidic paste between them that serves as the electrolyte.
Alkaline battery - Alkaline chemistry is used in common Duracell and Energizer
batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte.
Lithium-iodide battery - Lithium-iodide chemistry is used in pacemakers and hearing
aides because of their long life.
Lead-acid battery - Lead-acid chemistry is used in automobiles, the electrodes are made
of lead and lead-oxide with a strong acidic electrolyte (rechargeable).
Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, with
potassium-hydroxide as the electrolyte (rechargeable).
Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium
because it does not suffer from the memory effect that nickel-cadmiums do
(rechargeable).
Lithium-ion battery - With a very good power-to-weight ratio, this is often found in
high-end laptop computers and cell phones (rechargeable).
Zinc-air battery - This battery is lightweight and rechargeable.
Zinc-mercury oxide battery - This is often used in hearing-aids.
Silver-zinc battery - This is used in aeronautical applications because the power-toweight ratio is good.
Rechargeable Batteries
With the rise in portable devices such as laptops, cell phones, MP3 players and cordless power
tools, the need for rechargeable batteries has grown substantially in recent years. The concept of
the rechargeable battery has been around since 1859, when French physicist Gaston Plante
4
invented the lead acid cell, which would later become the world's first rechargeable battery. That
same chemistry is still used in today's car battery.
The basic idea behind the rechargeable battery is simple: when electrical energy is applied to the
battery, the electron flow from negative to positive that occurs during discharge is reversed and
power is restored. This requires an adapter in the case of devices with built-in batteries or for
standard nickel-cadmium or nickel-metal hydride batteries, the most common multi-use
rechargeable batteries used today in your remote control, flashlight or digital camera.
Car batteries are one of the oldest kinds of rechargeable batteries and in fact, the electric car
predates its gas-powered cousin. In a standard car, there is a single lead-acid SLI battery that
supplies power to the starter, lights and ignition system. The battery charger in this case is the
alternator, a clever device that converts gasoline power to electrical energy and distributes it
where needed. In electric and hybrid cars, traction batteries are used to power the vehicle down
the road. Traction batteries come in many varieties, from lead acid, to nickel-cadmium, nickel
metal hydride and lithium ion.
The recharging rate has improved substantially over the years and is broken down into three
categories:



Slow: 14-16 hours
Quick: 3-6 hours
Fast: Less than one hour
The rate of charge is determined by how much electrical current is allowed into the battery by
the charger. Some batteries can handle higher voltage in a shorter amount of time without
overheating, while others need a lesser voltage applied over a longer period of time. The quicker
the rate of charge, the more chance there is of over charging, which can ruin a battery's chance of
holding its charge. The key in avoiding an over charge is the ability to dissipate the charging
current once maximum power has been reached. Most chargers have built-in voltage regulators
do this, allowing you to safely leave your cell phone or computer plugged in overnight.
The speed and effectiveness of the charge depends largely on the quality of the charger itself.
Chargers vary in performance based on the price tag and like most products you get what you
pay for. Chargers are generally designed for specific cell chemistries, although newer universal
chargers have sensors built in that identify the cell type and react appropriately. There are also
smart chargers that use a microprocessor to monitor temperature, voltage and state of charge,
which is the percentage of power available compared to its full capacity.
One common problem in nickel-cadmium rechargeable batteries is something known as the
memory effect. This is when the battery is continually recharged before it has discharged more
than 50 percent of its power, causing it to essentially forget that it could fully discharge to begin
with. Memory effect is caused by the formation of hard-to-dissolve cadmium crystals deep
within the battery. Cadmium crystals are an unavoidable by-product of discharge; the trick is to
keep them small enough to be reformed as cadmium during the charging process. When a battery
is not fully discharged, the crystals deep within the battery are not affected by the influx of
electrical current, so they are not reformed as cadmium and can grow into the troublesome larger
cadmium crystals. The battery will still function normally, but is maxed out at 50 percent. The
memory effect can be avoided by fully cycling the battery once every two to three weeks by
allowing it to discharge completely, and then fully recharge.
5
It's important to remember that no battery, rechargeable or otherwise, will last forever. All
batteries suffer from aging cells and the longer they are used, the less capacity they ultimately
will have. Rechargeable batteries are still a great way to save money and reduce waste.
How Flash Memory Works
We store and transfer all kinds of files on our computers -- digital photographs, music files, word
processing documents, PDFs and countless other forms of media. But sometimes your
computer's hard drive isn't exactly where you want your information. Whether you want to make
backup copies of files that live off of your systems or if you worry about your security, portable
storage devices that use a type of electronic memory called flash memory may be the right
solution.
Electronic memory comes in a variety of forms to serve a variety of purposes. Flash memory is
used for easy and fast information storage in computers, digital cameras and home video game
consoles. It is used more like a hard drive than as RAM. In fact, flash memory is known as a
solid state storage device, meaning there are no moving parts -- everything is electronic instead
of mechanical.
Here are a few examples of flash memory:






Your computer's BIOS chip
CompactFlash (most often found in digital cameras)
SmartMedia (most often found in digital cameras)
Memory Stick (most often found in digital cameras)
PCMCIA Type I and Type II memory cards (used as solid-state disks in laptops)
Memory cards for video game consoles
Flash Memory: Tunneling and Erasing
Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge,
usually 10 to 13 volts, is applied to the floating gate. The charge comes from the column, or
bitline, enters the floating gate and drains to a ground.
This charge causes the floating-gate transistor to act like an electron gun. The excited electrons
are pushed through and trapped on other side of the thin oxide layer, giving it a negative charge.
These negatively charged electrons act as a barrier between the control gate and the floating gate.
A special device called a cell sensor monitors the level of the charge passing through the
floating gate. If the flow through the gate is above the 50 percent threshold, it has a value of 1.
When the charge passing through drops below the 50-percent threshold, the value changes to 0.
A blank EEPROM has all of the gates fully open, giving each cell a value of 1.
The electrons in the cells of a flash-memory chip can be returned to normal ("1") by the
application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to
apply the electric field either to the entire chip or to predetermined sections known as blocks.
This erases the targeted area of the chip, which can then be rewritten. Flash memory works much
faster than traditional EEPROMs because instead of erasing one byte at a time, it erases a block
or the entire chip, and then rewrites it.
6
You may think that your car radio has flash memory, since you're able to program the presets
and the radio remembers them. But it's actually using flash RAM. The difference is that flash
RAM has to have some power to maintain its contents, while flash memory will maintain its data
without any external source of power. Even though you've turned the power off, the car radio is
pulling a tiny amount of current to preserve the data in the flash RAM. That is why the radio will
lose its presets if your car battery dies or the wires are disconnected.
Removable Flash Memory Cards
While your computer's BIOS chip is the most common form of Flash memory, removable solidstate storage devices are also popular. SmartMedia and CompactFlash cards are both wellknown, especially as "electronic film" for digital cameras. Other removable flash-memory
products include Sony's Memory Stick, PCMCIA memory cards, and memory cards for video
game systems. We'll focus on SmartMedia and CompactFlash, but the essential idea is the same
for all of these products -- every one of them is simply a form of flash memory.
There are a few reasons to use flash memory instead of a hard disk:



It has no moving parts, so it's noiseless.
It allows faster access.
It's smaller in size and lighter.
So why don't we just use flash memory for everything? Because the cost per megabyte for a hard
disk is drastically cheaper, and the capacity is substantially more.
The solid-state floppy-disk card (SSFDC), better known as SmartMedia, was originally
developed by Toshiba. SmartMedia cards are available in capacities ranging from 2 MB to 128
MB. The card itself is quite small, approximately 45 mm long, 37 mm wide and less than 1 mm
thick.
As shown below, SmartMedia cards are extremely simple. A plane electrode is connected to the
flash-memory chip by bonding wires. The flash-memory chip, plane electrode and bonding
wires are embedded in a resin using a technique called over-molded thin package (OMTP).
This allows everything to be integrated into a single package without the need for soldering.
The OMTP module is glued to a base card to create the actual card. Power and data is carried by
the electrode to the Flash-memory chip when the card is inserted into a device. A notched corner
indicates the power requirements of the SmartMedia card. Looking at the card with the electrode
facing up, if the notch is on the left side, the card needs 5 volts. If the notch is on the right side, it
requires 3.3 volts.
SmartMedia cards erase, write and read memory in small blocks (256- or 512-byte increments).
This approach means that they are capable of fast, reliable performance while allowing you to
specify which data you wish to keep.They are less rugged than other forms of removable solidstate storage, so you should be very careful when handling and storing them. Because of newer,
smaller cards with bigger storage capacities, such as xD-Picture Cards and Secure Digital cards,
Toshiba has essentially discontinued the production of SmartMedia cards, so they're now
difficult to find.
CompactFlash cards were developed by Sandisk in 1994, and they're different from
SmartMedia cards in two important ways:
7


They're thicker.
They utilize a controller chip.
CompactFlash consists of a small circuit board with flash-memory chips and a dedicated
controller chip, all encased in a rugged shell that is thicker than a SmartMedia card.
CompactFlash cards are 43 mm wide and 36 mm long, and come in two thicknesses: Type I
cards are 3.3 mm thick, and Type II cards are 5.5 mm thick.
CompactFlash cards support dual voltage and will operate at either 3.3 volts or 5 volts.
The increased thickness of the card allows for greater storage capacity than SmartMedia cards.
CompactFlash sizes range from 8 MB to as much as 100GB. The onboard controller can increase
performance, particularly in devices that have slow processors. The case and controller chip add
size, weight and complexity to the CompactFlash card when compared to the SmartMedia card.
Electric field
In physics, the space surrounding an electric charge or in the presence of a time-varying
magnetic field has a property called an electric field (that can also be equated to electric flux
density). This electric field exerts a force on other electrically charged objects. The concept of an
electric field was introduced by Michael Faraday.
The electric field is a vector field with SI units of newtons per coulomb (N C−1) or, equivalently,
volts per meter (V m−1). The strength of the field at a given point is defined as the force that
would be exerted on a positive test charge of +1 coulomb placed at that point; the direction of the
field is given by the direction of that force. Electric fields contain electrical energy with energy
density proportional to the square of the field intensity. The electric field is to charge as
gravitational acceleration is to mass and force density is to volume.
A moving charge has not just an electric field but also a magnetic field, and in general the
electric and magnetic fields are not completely separate phenomena; what one observer
perceives as an electric field, another observer in a different frame of reference perceives as a
mixture of electric and magnetic fields. For this reason, one speaks of "electromagnetism" or
"electromagnetic fields." In quantum mechanics, disturbances in the electromagnetic fields are
called photons, and the energy of photons is quantized.
Definition
A stationary charged particle in an electric field experiences a force proportional to its charge
given by the equation
where the magnetic flux density is given by
and where
is the Coulomb force.
Electric charge is a characteristic of some subatomic particles, and is quantized when expressed
as a multiple of the so-called elementary charge e. Electrons by convention have a charge of -1,
while protons have the opposite charge of +1. Quarks have a fractional charge of −1/3 or +2/3.
The antiparticle equivalents of these have the opposite charge. There are other charged particles.
In general, same-sign charged particles repel one another, while different-sign charged particles
attract. This is expressed quantitatively in Coulomb's law, which states the magnitude of the
8
repelling force is proportional to the product of the two charges, and weakens proportionately to
the square of the distance.
The electric charge of a macroscopic object is the sum of the electric charges of its constituent
particles. Often, the net electric charge is zero, since naturally the number of electrons in every
atom is equal to the number of the protons, so their charges cancel out. Situations in which the
net charge is non-zero are often referred to as static electricity. Furthermore, even when the net
charge is zero, it can be distributed non-uniformly (e.g., due to an external electric field), and
then the material is said to be polarized, and the charge related to the polarization is known as
bound charge (while the excess charge brought from outside is called free charge). An ordered
motion of charged particles in a particular direction (in metals, these are the electrons) is known
as electric current. The discrete nature of electric charge was proposed by Michael Faraday in his
electrolysis experiments, then directly demonstrated by Robert Millikan in his oil-drop
experiment.
The SI unit for quantity of electricity or electric charge is the coulomb, which represents
approximately 1.60 × 1019 elementary charges (the charge on a single electron or proton). The
coulomb is defined as the quantity of charge that has passed through the cross-section of an
electrical conductor carrying one ampere within one second. The symbol Q is often used to
denote a quantity of electricity or charge. The quantity of electric charge can be directly
measured with an electrometer, or indirectly measured with a ballistic galvanometer.
Formally, a measure of charge should be a multiple of the elementary charge e (charge is
quantized), but since it is an average, macroscopic quantity, many orders of magnitude larger
than a single elementary charge, it can effectively take on any real value. Furthermore, in some
contexts it is meaningful to speak of fractions of a charge; e.g. in the charging of a capacitor.
If the charged particle can be considered a point charge, the electric field is defined as the force it
experiences per unit charge:
where
is the electric force experienced by the particle
q is its charge
is the electric field wherein the particle is located
Taken literally, this equation only defines the electric field at the places where there are
stationary charges present to experience it. Furthermore, the force exerted by another charge q
will alter the source distribution, which means the electric field in the presence of q differs from
itself in the absence of q. However, the electric field of a given source distribution remains
defined in the absence of any charges with which to interact. This is achieved by measuring the
force exerted on successively smaller test charges placed in the vicinity of the source
distribution. By this process, the electric field created by a given source distribution is defined as
the limit as the test charge approaches zero of the force per unit charge exerted thereupon.
This allows the electric field to be dependent on the source distribution alone.
As is clear from the definition, the direction of the electric field is the same as the direction of
the force it would exert on a positively-charged particle, and opposite the direction of the force
on a negatively-charged particle. Since like charges repel and opposites attract (as quantified
below), the electric field tends to point away from positive charges and towards negative
charges.
is the vacuum permittivity.
Coulomb's law is actually a special case of Gauss's Law, a more fundamental description of the
relationship between the distribution of electric charge in space and the resulting electric field.
Gauss's law is one of Maxwell's equations, a set of four laws governing electromagnetics.
9
Time-varying fields
Charges do not only produce electric fields. As they move, they generate magnetic fields, and if
the magnetic field changes, it generates electric fields. A changing magnetic field gives rise to an
electric field,
which yields Faraday's law of induction,
where
indicates the curl of the electric field,
represents the vector rate of decrease of magnetic field with time.
This means that a magnetic field changing in time produces a curled electric field, possibly also
changing in time. The situation in which electric or magnetic fields change in time is no longer
electrostatics, but rather electrodynamics or electromagnetics.
Energy in the electric field
The electric field stores energy. The energy density of the electric field is given by
where
is the permittivity of the medium in which the field exists
is the electric field vector.
The total energy stored in the electric field in a given volume V is therefore
where
dV is the differential volume element.
Electrical conductor
In science and engineering, a conductor is a material which contains movable electric charges. In
metallic conductors, such as copper or aluminium, the movable charged particles are electrons
(See electrical conduction). Positive charges may also be mobile in the form of atoms in a lattice
missing electrons (called "holes") or ions, such as in the electrolyte of a battery.
The following applies to direct current only. When the direction of voltage/current alternates,
other effects (inductive and capacitive reactance) come into play also.
All conductors contain electric charges which will move when an electric potential difference
(measured in volts) is applied across separate points on the material. This flow of charge
(measured in amperes) is what is meant by electric current. In most materials, the rate of current
is proportional to the voltage (Ohm's law,) provided the temperature remains constant and the
material remains in the same shape and state. The ratio between the voltage and the current is
called the resistance (measured in ohms) of the object between the points where the voltage was
applied. The resistance across a standard mass (and shape) of a material at a given temperature is
called the resistivity of the material. The inverse of resistance and resistivity is conductance and
conductivity. Some good examples of conductors are metal.
Most familiar conductors are metallic. Copper is the most common material for electrical wiring
(silver is the best but expensive), and gold for high-quality surface-to-surface contacts. However,
10
there are also many non-metallic conductors, including graphite, solutions of salts, and all
plasmas.
Non-conducting materials lack mobile charges, and so resist the flow of electric current,
generating heat. In fact, all materials offer some resistance and warm up when a current flows.
Thus, proper design of an electrical conductor takes into account the temperature that the
conductor needs to be able to endure without damage, as well as the quantity of electrical
current. The motion of charges also creates an electromagnetic field around the conductor that
exerts a mechanical radial squeezing force on the conductor. A conductor of a given material and
volume (length x cross-sectional area) has no real limit to the current it can carry without being
destroyed as long as the heat generated by the resistive loss is removed and the conductor can
withstand the radial forces. This effect is especially critical in printed circuits, where conductors
are relatively small and close together, and inside an enclosure: the heat produced, if not properly
removed, can cause fusing (melting) of the tracks.
Since all conductors have some resistance, and all insulators will carry some current, there is no
theoretical dividing line between conductors and insulators. However, there is a large gap
between the conductance of materials that will carry a useful current at working voltages and
those that will carry a negligible current for the purpose in hand, so the categories of insulator
and conductor do have practical utility.
Thermal and electrical conductivity often go together (for instance, most metals are both
electrical and thermal conductors). However, some materials are practical electrical conductors
without being a good thermal conductor.
In power engineering, an electrical wire is a length of metal, usually surrounded by an insulating
sheath, that is used to conduct electricity.
Conductor materials
Of the metals commonly used for conductors, copper has a high conductivity. Silver is more
conductive, but due to cost it is not practical in most cases. However, it is used in specialized
equipment, such as satellites, and as a thin plating to mitigate skin effect losses at high
frequencies. Because of its ease of connection by soldering or clamping, copper is still the most
common choice for most light-gauge wires. Aluminum has been used as a conductor in housing
applications for cost reasons. It is actually more conductive than copper when compared by unit
weight, but it has technical problems related to heat and its coefficient of thermal expansion,
which tends to loosen connections over time.
Conductor voltage
The voltage on a conductor is determined by the connected circuitry and has nothing to do with
the conductor itself. Conductors are usually surrounded by and/or supported by insulators and
the insulation determines the maximum voltage that can be applied to any given conductor.
Voltage of a conductor "V" is given by
V = IR
where
I is the current, measured in amperes
V is the potential difference measured in volts
R is the resistance measured in ohms
11
Electric Current
If the two requirements of an electric circuit are met, then charge will flow
through the external circuit. It is said that there is a current - a flow of charge.
Using the word current in this context is to simply use it to say that something
is happening in the wires - charge is moving. Yet current is a physical quantity
which can be measured and expressed numerically. As a physical quantity,
current is the rate at which charge flows past a point on a circuit. As depicted in
the diagram below, the current in a circuit can be determined if the quantity of
charge Q passing through a cross section of a wire in a time t can be measured.
The current is simply the ratio of the quantity of charge and time.
Current is a rate quantity. There are several rate quantities in physics. For instance,
velocity is a rate quantity - the rate at which an object changes its position.
Mathematically, velocity is the position change per time ratio. Acceleration is a rate
quantity - the rate at which an object changes its velocity. Mathematically, acceleration is
the velocity change per time ratio. And power is a rate quantity - the rate at which work is
done on an object. Mathematically, power is the work per time ratio. In every case of a
rate quantity, the mathematical equation involves some quantity over time. Thus, current
as a rate quantity would be expressed mathematically as
Note that the equation above uses the symbol I to represent the quantity current.
As is the usual case, when a quantity is introduced in The Physics Classroom, the standard
metric unit used to express that quantity are introduced as well. The standard metric unit
for current is the ampere. Ampere is often shortened to Amp
and is abbreviated by the unit symbol A. A current of 1
ampere means that there is 1 coulomb of charge passing
through a cross section of a wire every 1 second.
Conventional Current Direction
The particles which carry charge through wires in a circuit
are mobile electrons. The electric field direction within a
circuit is by definition the direction which positive test
charges are pushed. Thus, these negatively charged electrons
move in the direction opposite the electric field. But while
electrons are the charge carriers in metal wires, the charge
carriers in other circuits can be positive charges, negative
charges or both. In fact, the charge carriers in semiconductors, street lamps and fluorescent
lamps are simultaneously both positive and negative charges traveling in opposite
directions.
Ben Franklin, who conducted extensive scientific studies in both static and current
electricity, envisioned positive charges as the carriers of charge. As such, an early
12
convention for the direction of an electric current was
established to be in the direction which positive
charges would move. The convention has stuck and is
still used today. The direction of an electric current
is by convention the direction in which a positive
charge would move. Thus, the current in the external
circuit is directed away from the positive terminal and
toward the negative terminal of the battery. Electrons would actually move through the
wires in the opposite direction. Knowing that the actual charge
carriers in wires are negatively charged electrons may make this
convention seem a bit odd and outdated. Nonetheless, it is the
convention which is used world wide and one that a student of
physics can easily become accustomed to.
Current versus Drift Speed
Current has to do with the number of coulombs of charge which
pass a point in the circuit per unit of time. Because of its
definition, it is often confused with the quantity drift speed. Drift speed refers to the
average distance traveled by a charge carrier per unit of time. Like the speed of any object,
the drift speed of an electron moving through a wire is the distance to time ratio. The path
of a typical electron through a wire could be described as a rather chaotic, zigzag path
characterized by collisions with fixed atoms. Each collision results in a change in direction
of the electron. Yet because of collisions with atoms in the solid network of the metal
conductor, there are two steps backwards for every three steps forward. With an electric
potential established across the two ends of the circuit, the electron continues to migrate
forward. Progress is always made towards the positive terminal. Yet the overall affect of
the countless collisions and the high between-collision speeds is that the overall drift speed
of an electron in a circuit is abnormally low. A typical drift speed might be 1 meter per
hour. That is slow!
One might then ask: How can there by a current on the order of 1 or 2 ampere in a circuit
if the drift speed is only about 1 meter per hour? The answer is: there are many, many
charge carriers moving at once throughout the whole length of the circuit. Current is the
rate at which charge crosses a point on a circuit. A high current is the result of several
coulombs of charge crossing over a cross section of a wire on a circuit. If the charge
carriers are densely packed into the wire, then there does not have to be a high speed to
have a high current. That is, the charge carriers do not have to travel a long distance in a
second, there just has to be a lot of them passing through the cross section. Current does
not have to do with how far charges move in a second but rather with how many charges
pass through a cross section of wire on a circuit.
To illustrate how densely packed the charge carriers are, we will consider a typical wire
found in household lighting circuits - a 14-gauge copper wire. In a 0.01 cm-long (very
thin) cross-sectional slice of this wire, there would be as many as 3.51 x 1020 copper
atoms. Each copper atom has 29 electrons; it would be unlikely that even the 11 valence
electrons would be in motion as charge carriers at once. If we assume that each copper
atom contributes just a single electron, then there would be as much as 56 coulombs of
charge within a thin 0.01-cm length of the wire. With that much mobile charge within such
a small space, a small drift speed could lead to a very large current.
To further illustrate this distinction between drift speed and current, consider this racing
analogy. Suppose that there was a very large turtle race with millions and millions of
turtles on a very wide race track. Turtles do not move very fast - they have a very low drift
13
speed. Suppose that the race was rather short - say 1 meter in length - and that a large
percentage of the turtles reached the finish line at the same time - 30 minutes after the start
of the race. In such a case, the current would be very large - with millions of turtles
passing a point in a short amount of time. In this analogy, speed has to do with how far the
turtles move in a certain amount of time; and current has to do with how many turtles cross
the finish line in a certain amount of time.
The Nature of Charge Flow
Once it has been established that the average drift speed of an electron is very, very slow,
the question soon arises: Why does the light in a room or in a flashlight light immediately
after the switched is turned on? Wouldn't there be a noticeable time delay before a charge
carrier moves from the switch to the light bulb filament? The answer is NO! and the
explanation of why reveals a significant amount about the nature of charge flow in a
circuit.
As mentioned above, charge carriers in the wires of electric circuits are electrons. These
electrons are simply supplied by the atoms of copper (or whatever material the wire is
made of) within the metal wire. Once the switch is turned to on, the circuit is closed and
there is an electric potential difference is established across the two ends of the external
circuit. The electric field signal travels at nearly the speed of light to all mobile electrons
within the circuit, ordering them to begin marching. As the signal is received, the electrons
begin moving along a zigzag path in their usual direction. Thus, the flipping of the switch
causes an immediate response throughout every part of the circuit, setting charge carriers
everywhere in motion in the same net direction. While the actual motion of charge carriers
occurs with a slow speed, the signal which informs them to start moving travels at a
fraction of the speed of light.
The electrons which light the bulb in a flashlight do not have to first travel from the switch
through 10 cm of wire to the filament. Rather, the electrons which light the bulb
immediately after the switch is turned to on are the electrons which are present in the
filament itself. As the switch is flipped, all mobile electrons everywhere begin marching;
and it is the mobile electrons present in the filament whose motion are immediately
responsible for the lighting of its bulb. As those electrons leave the filament, new electrons
enter and become the ones which are responsible for lighting the bulb. The electrons are
moving together much like the water in the pipes of a home move. When a faucet is turned
on, it is the water in the faucet which emerges from the spigot. One does not have to wait a
noticeable time for water from the entry point to your home to travel through the pipes to
the spigot. The pipes are already filled with water and water everywhere within the water
circuit is set in motion at the same time.
The picture of charge flow being developed here is a picture in which charge carriers are
like soldiers marching along together, everywhere at the same rate. Their marching begins
immediately in response to the establishment of an electric potential across the two ends of
the circuit. There is no place in the electrical circuit where charge carriers become
consumed or used up. While the energy possessed by the charge may be used up (or a
better way of putting this is to say that the electric energy is transformed to other forms of
energy), the charge carriers themselves do not disintegrate, disappear or otherwise become
removed from the circuit. And there is no place in the circuit where charge carriers begin
to pile up or accumulate. The rate at which charge enters the external circuit on one end is
the same as the rate at which charge exits the external circuit on the other end. Current the rate of charge flow - is everywhere the same. Charge flow is like the movement of
soldiers marching in step together, everywhere at the same rate.
14
15