Introduction
Electricity is all around us--powering technology
like our cell phones, computers, lights, soldering irons, and air conditioners.
It's tough to escape it in our modern world. Even when you try to escape
electricity, it's still at work throughout nature, from the lightning in a
thunderstorm to the synapses inside our body. But what exactly is electricity?
This is a very complicated question, and as you dig deeper and ask more
questions, there really is not a definitive answer, only abstract
representations of how electricity interacts with our surroundings.
Electricity is briefly defined as the flow
of electric charge, but there's so much behind that simple statement.
Where do the charges come from? How do we move them? Where do they move to? How
does an electric charge cause mechanical motion or make things light up? So
many questions! To begin to explain what electricity is we need to zoom way in,
beyond the matter and molecules, to the atoms that make up everything we
interact with in life.
Going Atomic
To understand the fundamentals of electricity, we
need to begin by focusing in on atoms, one of the basic building blocks of life
and matter. Atoms exist in over a hundred different forms as chemical elements
like hydrogen, carbon, oxygen, and copper. Atoms of many types can combine to
make molecules, which build the matter we can physically see and touch.
Atoms are tiny, stretching at a max to
about 300 picometers long (that's 3x10-10 or 0.0000000003 meters).
A copper penny (if it actually were made of 100% copper) would have 3.2x1022 atoms
(32,000,000,000,000,000,000,000 atoms) of copper inside it.
Even the atom isn't small enough to explain the
workings of electricity. We need to dive down one more level and look in on the
building blocks of atoms: protons, neutrons, and electrons.
Building Blocks of Atoms
An atom is built with a combination of three
distinct particles: electrons, protons, and neutrons. Each atom has a center
nucleus, where the protons and neutrons are densely packed together.
Surrounding the nucleus are a group of orbiting electrons.
A very simple atom model. It's not to scale but helpful for
understanding how an atom is built. A core nucleus of protons and neutrons is
surrounded by orbiting electrons.
Every atom must have at least one proton in it. The
number of protons in an atom is important, because it defines what chemical
element the atom represents. For example, an atom with just one proton is
hydrogen, an atom with 29 protons is copper, and an atom with 94 protons is
plutonium. This count of protons is called the atom's atomic number.
The proton's nucleus-partner, neutrons, serve an
important purpose; they keep the protons in the nucleus and determine the
isotope of an atom. They're not critical to our understanding of electricity,
so let's not worry about them for this tutorial.
Electrons are critical to the workings of
electricity (notice a common theme in their names?) In its most stable,
balanced state, an atom will have the same number of electrons as protons. As
in the Bohr atom model below, a nucleus with 29
protons (making it a copper atom) is surrounded by an equal number of
electrons.
As our understanding of atoms has evolved, so too has our method for
modeling them. The Bohr model is a very useful atom model as we explore
electricity.
The atom's electrons aren't all forever bound to
the atom. The electrons on the outer orbit of the atom are called valence
electrons. With enough outside force, a valence electron can escape orbit of
the atom and become free. Free electrons allow us to move
charge, which is what electricity is all about. Speaking of charge...
Flowing Charges
As we mentioned at the beginning of this tutorial,
electricity is defined as the flow of electric charge. Charge is
a property of matter--just like mass, volume, or density. It is measurable.
Just as you can quantify how much mass something has, you can measure how much
charge it has. The key concept with charge is that it can come in two
types: positive (+) or negative (-).
In order to move charge we need charge
carriers, and that's where our knowledge of atomic particles--specifically
electrons and protons--comes in handy. Electrons always carry a negative
charge, while protons are always positively charged. Neutrons (true to their
name) are neutral, they have no charge. Both electrons and protons carry the
same amount of charge, just a different type.
A lithium atom (3 protons) model with the charges labeled.
The charge of electrons and protons is important,
because it provides us the means to exert a force on them. Electrostatic force!
Electrostatic Force
Electrostatic force (also called Coulomb's
law) is a force that operates between charges. It states that
charges of the same type repel each other, while charges of opposite types are
attracted together. Opposites attract, and likes repel.
The amount of force acting on two
charges depends on how far they are from each other. The closer two charges
get, the greater the force (either pushing together, or pulling away) becomes.
Thanks to electrostatic force, electrons will push
away other electrons and be attracted to protons. This force is part of the
"glue" that holds atoms together, but it's also the tool we need to
make electrons (and charges) flow!
Making Charges Flow
We now have all the tools to make charges
flow. Electrons in atoms can act as our charge carrier,
because every electron carries a negative charge. If we can free an electron
from an atom and force it to move, we can create electricity.
Consider the atomic model of a copper atom, one of
the preferred elemental sources for charge flow. In its balanced state, copper
has 29 protons in its nucleus and an equal number of electrons orbiting around
it. Electrons orbit at varying distances from the nucleus of the atom. Electrons
closer to the nucleus feel a much stronger attraction to the center than those
in distant orbits. The outermost electrons of an atom are called the valence
electrons, these require the least amount of force to be freed from an
atom.
This is a copper atom diagram: 29 protons in the nucleus, surrounded by
bands of circling electrons. Electrons closer to the nucleus are hard to remove
while the valence (outer ring) electron requires relatively little energy to be
ejected from the atom.
Using enough electrostatic force on the valence
electron--either pushing it with another negative charge or attracting it with
a positive charge--we can eject the electron from orbit around the atom
creating a free electron.
Now consider a copper wire: matter filled with
countless copper atoms. As our free electron is floating in a
space between atoms, it's pulled and prodded by surrounding charges in that
space. In this chaos the free electron eventually finds a new atom to latch on
to; in doing so, the negative charge of that electron ejects another valence
electron from the atom. Now a new electron is drifting through free space
looking to do the same thing. This chain effect can continue on and on to
create a flow of electrons called electric current.
A very simplified model of charges flowing through atoms to make
current.
Conductivity
Some elemental types of atoms are better than
others at releasing their electrons. To get the best possible electron flow we
want to use atoms which don't hold very tightly to their valence electrons. An
element's conductivity measures how tightly bound an electron is to an atom.
Elements with high conductivity, which have very
mobile electrons, are called conductors. These are the types of
materials we want to use to make wires and other components which aid in
electron flow. Metals like copper, silver, and gold are usually our top choices
for good conductors.
Elements with low conductivity are called insulators.
Insulators serve a very important purpose: they prevent the flow of electrons.
Popular insulators include glass, rubber, plastic, and air.
Static or Current Electricity
Before we get much further, let's discuss the two
forms electricity can take: static or current. In working with electronics,
current electricity will be much more common, but static electricity is
important to understand as well.
Static Electricity
Static electricity exists when there is a build-up
of opposite charges on objects separated by an insulator. Static (as in
"at rest") electricity exists until the two groups of opposite
charges can find a path between each other to balance the system out.
When the charges do find a means of equalizing,
a static discharge occurs. The attraction of the charges
becomes so great that they can flow through even the best of insulators (air, glass,
plastic, rubber, etc.). Static discharges can be harmful depending on what
medium the charges travel through and to what surfaces the charges are
transferring. Charges equalizing through an air gap can result in a visible
shock as the traveling electrons collide with electrons in the air, which
become excited and release energy in the form of light.
Spark gap igniters are used to create a
controlled static discharge. Opposite charges build up on each of the
conductors until their attraction is so great charges can flow through the air.
One of the most dramatic examples of static
discharge is lightning. When a cloud system gathers enough charge
relative to either another group of clouds or the earth's ground, the charges
will try to equalize. As the cloud discharges, massive quantities of positive
(or sometimes negative) charges run through the air from ground to cloud
causing the visible effect we're all familiar with.
Static electricity also familiarly exists when we
rub balloons on our head to make our hair stand up, or when we shuffle
on the floor with fuzzy slippers and shock the family cat
(accidentally, of course). In each case, friction from rubbing different types
of materials transfers electrons. The object losing electrons becomes
positively charged, while the object gaining electrons becomes negatively
charged. The two objects become attracted to each other until they can find a
way to equalize.
Working with electronics, we generally don't have
to deal with static electricity. When we do, we're usually trying to protect
our sensitive electronic components from being subjected to a static discharge.
Preventative measures against static electricity include wearing ESD
(electrostatic discharge) wrist straps, or adding special components in
circuits to protect against very high spikes of charge.
Current Electricity
Current electricity is the form of electricity
which makes all of our electronic gizmos possible. This form of electricity
exists when charges are able to constantly flow. As opposed to
static electricity where charges gather and remain at rest, current electricity
is dynamic, charges are always on the move. We'll be focusing on this form of
electricity throughout the rest of the tutorial.
Circuits
In order to flow, current electricity requires
a circuit: a closed, never-ending loop of
conductive material. A circuit could be as simple as a conductive wire
connected end-to-end, but useful circuits usually contain a mix of wire and
other components which control the flow of electricity. The only rule when it
comes to making circuits is they can't have any insulating gaps in
them.
If you have a wire full of copper atoms and want to
induce a flow of electrons through it, all free electrons need
somewhere to flow in the same general direction. Copper is a great conductor,
perfect for making charges flow. If a circuit of copper wire is broken, the
charges can't flow through the air, which will also prevent any of the charges
toward the middle from going anywhere.
On the other hand, if the wire were connected
end-to-end, the electrons all have a neighboring atom and can all flow in the
same general direction.
We now understand how electrons
can flow, but how do we get them flowing in the first place? Then, once the
electrons are flowing, how do they produce the energy required to illuminate
light bulbs or spin motors? For that, we need to understand electric fields.
Electric Fields
We have a handle on how electrons flow through
matter to create electricity. That's all there is to electricity. Well, almost
all. Now we need a source to induce the flow of electrons. Most often that
source of electron flow will come from an electric field.
What's a Field?
A field is a tool we use to model
physical interactions which don't involve any observable contact.
Fields can't be seen as they don't have a physical appearance, but the effect
they have is very real.
We're all subconsciously familiar with one field in
particular: Earth's gravitational field, the effect of a
massive body attracting other bodies. Earth's gravitational field can be
modeled with a set of vectors all pointing into the center of the planet;
regardless of where you are on the surface, you'll feel the force pushing you
towards it.
The strength or intensity of fields isn't uniform
at all points in the field. The further you are from the source of the field
the less effect the field has. The magnitude of Earth's gravitational field
decreases as you get further away from the center of the planet.
As we go on to explore electric fields in
particular remember how Earth's gravitational field works, both fields share
many similarities. Gravitational fields exert a force on objects of mass, and
electric fields exert a force on objects of charge.
Electric Fields
Electric fields (e-fields) are an important tool in
understanding how electricity begins and continues to flow. Electric
fields describe the pulling or pushing force in a space between charges.
Compared to Earth's gravitational field, electric fields have one major
difference: while Earth's field generally only attracts other objects of mass
(since everything is so significantly less massive), electric
fields push charges away just as often as they attract them.
The direction of electric fields is always defined
as the direction a positive test charge would move if it was
dropped in the field. The test charge has to be infinitely small, to keep its
charge from influencing the field.
We can begin by constructing electric fields for
solitary positive and negative charges. If you dropped a positive test charge
near a negative charge, the test charge would be attracted towards the negative charge.
So, for a single, negative charge we draw our electric field arrows pointing
inward at all directions. That same test charge dropped near
another positive charge would result in an outward repulsion,
which means we draw arrows going out of the positive charge.
The electric fields of single charges. A negative charge has an inward
electric field because it attracts positive charges. The positive charge has an
outward electric field, pushing away like charges.
Groups of electric charges can be combined to make
more complete electric fields.
The uniform e-field above points away from the
positive charges, towards the negatives. Imagine a tiny positive test charge
dropped in the e-field; it should follow the direction of the arrows. As we've
seen, electricity usually involves the flow of electrons--negative
charges--which flow against electric fields.
Electric fields provide us with the pushing force
we need to induce current flow. An electric field in a circuit is like an
electron pump: a large source of negative charges that can propel electrons,
which will flow through the circuit towards the positive lump of charges.
Electric Potential (Energy)
When we harness electricity to power our circuits,
gizmos, and gadgets, we're really transforming energy. Electronic circuits must
be able to store energy and transfer it to other forms like heat, light, or
motion. The stored energy of a circuit is called electric potential energy.
Energy? Potential Energy?
To understand potential energy we need to
understand energy in general. Energy is defined as the ability of an object to
do work on another object, which means moving that object some
distance. Energy comes in many forms, some we can see (like
mechanical) and others we can't (like chemical or electrical). Regardless of
what form it's in, energy exists in one of two states: kinetic or
potential.
An object has kinetic energy when
it's in motion. The amount of kinetic energy an object has depends on its mass
and speed. Potential energy, on the other hand, is a stored
energy when an object is at rest. It describes how much work the
object could do if set into motion. It's an energy we can generally control.
When an object is set into motion, its potential energy transforms into kinetic
energy.
Let's go back to using gravity as an example. A
bowling ball sitting motionless at the top of Khalifa
tower has a lot of potential (stored) energy. Once dropped, the
ball--pulled by the gravitational field--accelerates towards the ground. As the
ball accelerates, potential energy is converted into kinetic energy (the energy
from motion). Eventually all of the ball's energy is converted from potential
to kinetic, and then passed on to whatever it hits. When the ball is on the
ground, it has a very low potential energy.
Electric Potential Energy
Just like mass in a gravitational field has
gravitational potential energy, charges in an electric field have an electric
potential energy. A charge's electric potential energy describes how much
stored energy it has, when set into motion by an electrostatic force, that
energy can become kinetic, and the charge can do work.
Like a bowling ball sitting at the top of a tower,
a positive charge in close proximity to another positive charge has a high
potential energy; left free to move, the charge would be repelled away from the
like charge. A positive test charge placed near a negative charge would have
low potential energy, analogous to the bowling ball on the ground.
To instill anything with potential energy, we have
to do work by moving it over a distance. In the case of the
bowling ball, the work comes from carrying it up 163 floors, against the field
of gravity. Similarly, work must be done to push a positive charge against the
arrows of an electric field (either towards another positive charge, or away
from a negative charge). The further up the field the charge goes, the more
work you have to do. Likewise, if you try to pull a negative charge away from
a positive charge--against an electric field--you have to do work.
For any charge located in an electric field its
electric potential energy depends on the type (positive or negative), amount of
charge, and its position in the field. Electric potential energy is measured in
units of joules (J).
Electric Potential
Electric potential builds upon electric
potential energy to help define how much energy is
stored in electric fields. It's another concept which helps us model the
behavior of electric fields. Electric potential is not the
same thing as electric potential energy!
At any point in an electric field the electric
potential is the amount of electric potential energy divided by the
amount of charge at that point. It takes the charge quantity out of
the equation and leaves us with an idea of how much potential energy specific
areas of the electric field may provide. Electric potential comes in units of
joules per coulomb (J/C), which we define as a volt (V).
In any electric field there are two points of
electric potential that are of significant interest to us. There's a point of
high potential, where a positive charge would have the highest possible
potential energy, and there's a point of low potential, where a charge would
have the lowest possible potential energy.
One of the most common terms we discuss in
evaluating electricity is voltage. A voltage is the difference in
potential between two points in an electric field. Voltage gives us an idea of
just how much pushing force an electric field has.
With potential and potential energy under our belt
we have all of the ingredients necessary to make current electricity. Let's do
it!
Electricity in Action!
After studying particle physics, field theory, and
potential energy, we now know enough to make electricity flow. Let's make a
circuit!
First we will review the ingredients we need to
make electricity:
- The definition of electricity is the flow
of charge. Usually our charges will be carried by free-flowing
electrons.
- Negatively-charged electrons are
loosely held to atoms of conductive materials. With a little push we can
free electrons from atoms and get them to flow in a generally uniform
direction.
- A closed circuit of
conductive material provides a path for electrons to continuously flow.
- The charges are propelled by an electric
field. We need a source of electric potential (voltage), which pushes
electrons from a point of low potential energy to higher potential energy.
A Short Circuit
Batteries are common energy sources which convert
chemical energy to electrical energy. They have two terminals, which connect to
the rest of the circuit. On one terminal there are an excess of negative
charges, while all of the positive charges coalesce on the other. This is an
electric potential difference just waiting to act!
If we connected our wire full of conductive copper
atoms to the battery, that electric field will influence the negatively-charged
free electrons in the copper atoms. Simultaneously pushed by the negative
terminal and pulled by the positive terminal, the electrons in the copper will
move from atom to atom creating the flow of charge we know as electricity.
After a second of the current flow, the electrons
have actually moved very little--fractions of a centimeter.
However, the energy produced by the current flow is huge,
especially since there's nothing in this circuit to slow down the flow or
consume the energy. Connecting a pure conductor directly across an energy
source is a bad idea. Energy moves very quickly through the system
and is transformed into heat in the wire, which may quickly turn into melting
wire or fire.
Illuminating a Light Bulb
Instead of wasting all that energy, not to mention destroying
the battery and wire, let's build a circuit that does something useful!
Generally an electric circuit will transfer electric energy into some other
form--light, heat, motion, etc. If we connect a light bulb to the battery with
wires in between, we have a simple, functional circuit.
Schematic: A battery (left) connecting to a lightbulb (right), the
circuit is completed when the switch (top) closes. With the circuit closed,
electrons can flow, pushed from the negative terminal of the battery through
the lightbulb, to the positive terminal.
While the electrons move at a snails pace, the
electric field affects the entire circuit almost instantly (we're talking speed
of light fast). Electrons throughout the circuit, whether at the lowest
potential, highest potential, or right next to the light bulb, are influenced
by the electric field. When the switch closes and the electrons are subjected
to the electric field, all electrons in the circuit start flowing at seemingly
the same time. Those charges nearest the light bulb will take one step through
the circuit and start transforming energy from electrical to light (or heat).
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