Take a look at electricity in action Electromagnetice Waves – Electricity in Action

ELECTRICAL CHARGE

What is electricity? a very important question and the answer to this question leads us to our knowledge of the very fundemental parts of nature as we know it. Our understannding of electrical charge is a fundamental property of matter like mass. But not all matter necessarily has a charge and similarly not all matter has mass. As we know from our school work, one basic unit of matter is the atom. The atom in turn is composed of other particles, a nucleus in the center, surrounded by a cloud of one or more electrons defined not by their absolute position like fixed orbits but rather by the probability of being in a position corresponding to known energy states. Electrons are said to have a negative charge, whereas the tiny nucleus is made up of charged particles called protons which carry a positive charge and neutrons which carry no charge at all. Because opposite charges attract, rather like masses attract, there is a resultant force which binds the electron cloud to the nucleus to form the atom. This is called the weak nuclear force and is one of the fundamental forces of nature.

So an electron is a particle with a very small mass but a relatively large negative charge. A proton is a particle with a positive charge equal in strength to the charge on an electron but with a mass very much larger than the mass of an electron. A neutron has nearly the same mass as a proton but has no charge. Another everyday particle which has no mass and no charge is a photon. Light is composed of photons. Photons have momentum but no mass.

GRAVITATIONAL FIELDS

If you think about gravity we know this force exists as we all feel the effect of our own mass as the influence of the earths gravitational field accelerates us towards the ground we stand on. Unlike charge, mass has no repulsive force and is always attractive drawing masses together. If you drop something from a height if will be accelerated by the gravitational field towards the ground by a force defined by

Newtons Universal Gravitational law

Force = G (m1 x m2) / d x d

d is the distance between masses  (meters)

m₁ represents the mass of one mass

m2 represents the mass of the second mass (the earth for example)

G is the universal gravitational constant; 6.674×10⁻¹¹ N m²

The idea of a field is to describe the effect of a force exerted by some natural phenomena like gravity that can be measured at a distance away from the source causing the field to exist. In a similar way we can talk of an electric field.

What is electricity – ELECTRIC FIELDS

If you take two isolated particle charges the electrostatic force effecting each charge is proportional to the product of the individual charges and inversely proportional to the square of the distance between them. If the charges are both positive or negative the force acts to repel the particles. But if the charges are opposite then the force attracts the particles. This is known as Coulomb’s Law. Notice how similar this force equation is to the gravitational force equation.

Coulomb’s Law

Force = k (q1 x q2) / d x d

d is the distance between particles

q₁ represents the quantity of charge of one particle

q₂ represents the quantity of charge of another particle

k is the proportionality constant; k=9 000 000 000 N.m²/C²

Charge is measured in Coulomb’

So looking at atoms you could reasonably ask why are electrons distributed in a form of cloud? (Its not really a cloud but the concept defines a region where many possible locations of their position could exist). Well Coulombs law gives you a clue because electrons have the same negative charge and therefore repel one another so they would not all sit comfortably in the same point in space and time. Instead they move in regions of the cloud space within the atom where the most probable stable least energy states exist at a given time.

Given the modern model of the atom the electron cloud is distributed at relatively huge distances from the nucleus but that is not the case for those positively charged protons all packed tightly together in the tiny nucleus. So again since the positively charged protons are packed so closely together and their separation is minute then, from Coulombs law, this would imply huge repulsive forces wanting to blow the nucleus apart. So why doesn’t it? Well another force comes into play in these circumstances. In fact the packed protons actually exchange some of their mass for the energy required to counteract this electrostatic repulsive force. This mutation from mass to energy is called the binding energy and gives rise to the so called strong nuclear force which is another one of the fundamental forces of nature.

ELECTRIC FIELD AND VOLTS

If you imagine a static electric field generated say between 2 charged paralel plates A (positively charged) and B (negatively charged) and separated by a distance d.  The Electric field produced would point directly from A to B. Now if we take a point charge Q at B and move it vertically against the force field we have to do work to go from B to A and as we do so we increase the potential of the point charge. This is rather like carrying a brick up a hill. We physically do work against the force of gravity to give the brick an increase in its gravitational potential energy. We convert this work done directly to the brick’s increase in potential energy. If you throw the brick off the top it will release this energy in the form of kinetic energy of motion.

This increase in potential or the potential difference from B to A is known as the Voltage difference and represents the work done in moving  a unit charge from B to A.

The Electric Field E = F / Q ( Electric Force divided by the total charge is the force per unit charge the definition of the electric field E )

so W (work per done per unit charge) = E x d = F x d / Q = (Vb -Va) the potential difference.

The electric field is defined as the force per unit charge E so the work done moving the charge Q from B to A is

W  = Q (Vb -Va) The units of voltage are Joules/Coulomb’s or simply VOLTS

ELECTRIC CURRENT

Atoms are constantly subject to a change of state, influenced by other energy changing factors like radiation, electric fields, heat transfer and chemical bonding or other energy induced effects. In fact in the chemical bonding of everyday life we see materials which are formed from a collection of different atoms and in these compounds the role of electrons is fundamental to their chemistry.

In many compounds it is not possible to say an electron belongs to a given atom in a molecule but in fact is shared in some way with all the constituent atoms forming the molecules but at the same time keeping a stable electrostatic balance. Nature and chemists make things based on the number and distribution of electrons only, to form their compounds. Normal chemistry does not involve the nucleus of atoms to see the normal chemical properties of matter it’s all about electrons.

We can start to think of a pool of shared electrons which act in a way to keep a stable bonding between the chemical elements making up any molecules in any material. Some materials like metals have a much freer cloud of free electrons and have good electrical conductive properties and other materials have much less and are therefore not good electrical conductors.

When a charged particle like an electron is subject to the influence of an electric field it will move towards the positive side of the that field at a rate depending on the local field strength. In fact our first televisions operated exactly on this principle. A heated electrode generated free electrons in a vacuum tube which were accelerated by the influence of an electric field to hit a phosphor coated glass screen which was effectively the other electrode producing the field. The result was scintillation of light as the electrons energy was converted to photons of light in the phosphor.

This flow of electrons is in fact an electrical current. In everyday life current is seen as the flow of electrons carrying their charge under the influence of an electric field. Because electrons carry their special charge, like masses falling in a gravitational field they are in fact transferring their potential energy in their electrical potential field to a motion in a medium that can either absorb this energy or otherwise propagate it in the form of electromagnetic radiation.

It is the rate at which charge passes a given point that defines what the strength of the current is. In fact any charge carrier under the influence of an electric field that freely flows is a current and that charge could be positive! As charge is measured in coulomb’s then current is measured in AMPS  which is in fact defined as the rate of flow of coulomb’s per second.

MAGNETIC FIELDS

When  a current flows in a wire say, another side effect, of this current flow is the production of a magnetic field outside the wire. As a charge passes down the wire under the influence of an electric field it also experiences a magnetic force which acts in a direction perpendicular to the direction of motion of the charges and also perpendicular to the associated magnetic field. So a charge in a current in the wire is subject to two simultaneous forces. The Electric Field and a force from the associated Magnetic Field

If the electric field is constant in time, the resulting magnetic field outside the wire will also be constant at any arbitrary point in local space. If the applied electric field is changing with time then so will the current change with time and the magnetic field changes sympathetically.

This property of  electrically generated magnetic fields is used everyday in so called electro magnets. If we go back to our original televisions the Electric Field provided the accelerated electrons to hit the phosphor screen giving off photons of light while electro magnetic coils outside the vacum tube exerted magnetic force on these electrons to control the motion of the electron beam on the screen

MAGNETIC INDUCTION

The movement of charge relative to a magnetic field has interesting effects. For example a magnetic field that is changing in time near a stationary charge produces forces on that charge that also change with time. If the charge concerned is in a wire say. These forces will induce a current in this wire which will also produce it’s own magnetic field.

Either by changing the applied electric field in time or a physical movement of the charge carrier in a static magnetic field the net effect is the same. If their is relative motion of the charge with the magnetic field then induction takes place.

So suppose we have a wire carrying a current which is changing in time then we know that the magnetic field produced in this wire will apply forces to charges in an adjacent wire. These forces cause charges to move in the adjacent wire and create a so called induced current. This phenomenom was first observed by Michael Faraday.

What we see in these circumstances is an induced voltage in the other wire that produces a current with its own magnetic field which produces a force designed solely to maintain the original state in the wire. So this magnetic field opposes the magnetic field responsible for producing the original current in the second wire. This effect is known as Lenzes Law.

Note it is important that there exists a changing magnetic field for induction to occur. Everyday Items like a Transformer rely on just this principle as normally AC electricity is applied to the primary side of a transformer and the secondary winding picks up an induced voltage and current in the secondary winding transferring power. Passing a unchanging current in the primary winding will not create a changing magnetic field so no induction will take place.