Static Electricity
To understand electricity, you must understand static electricity and the movement of charges.
Static electricity is a result of the build up of negative and positive charges.
Protons, neutrons and electrons have properties including electrical charge. Protons have what we call a
"positive" (+) charge. Electrons have a "negative" (-) charge. Neutrons have no charge, they are neutral.
Static Electricity
The protons and neutrons in the nucleus are held together very tightly. But some of the outer electrons
are held very loosely. They can move from one atom to another. An atom that looses electrons has more
positive charges (protons) than negative charges (electrons). It is positively charged. An atom that gains
electrons has more negative than positive particles. It has a negative charge. A charged atom is called
an "ion."
Some materials hold their electrons very tightly. Electrons do not move through them very well. These
things are called insulators. Plastic, cloth, glass and dry air are good insulators. Other materials (mostly
metals) have some loosely held electrons, which move through them very easily. These are called
conductors.
When you use a cloth to rub an insulator such as a balloon or a plastic ruler, electrons are rubbed from
one insulator to the other insulator.
Substances that become negatively charged when they are rubbed gain electrons.
Substances that become positively charged when they are rubbed lose electrons.
Static electricity is the imbalance of positive and negative charges.
Two things with opposite charges (a positive and a negative) will attract, or pull towards each other.
Things with the same charge (two positives or two negatives) will repel, or push away from each other.
If you rub a plastic ruler on a cat, you can build up a charge.
When you touch something earthed, the charge flows from your body and you feel the shock. Vehicles
can build up a charge as they move along the road. Because tyres are made of rubber they are
insulators and so the charge stays on the vehicle.
The Van der Graaf is a useful machine for charging up people to demonstrate like charges repelling.
A charged object will also attract something that is neutral. If you charge a balloon by rubbing it on your
hair, it picks up extra electrons and has a negative charge. Holding it near a neutral object will make the
charges in that object move. If it is a conductor, many electrons move easily to the other side, as far from
the balloon as possible. If it is an insulator, the electrons in the atoms and molecules can only move very
slightly to one side, away from the balloon. In either case, there are more positive charges closer to the
negative balloon. Opposites attract. The balloon sticks.
Static electricity can be dangerous in the workplace. When powders and fuels are poured through pipes,
the rubbing of the chemical on the pipe can build up electric charges.
Refueling a jumbo jet requires the fuel line to be earthed to make sure that any electric charge created
by the rubbing of the fuel on the fuel line cannot form a spark. Clouds can build up charges. Sometimes
there is enough energy to form long sparks called lightning.
Electric Fields
If an object has mass then it will be affected by a gravitational field.
If an object has charge then it will be affected by an electric field.
Big electric fields exist during lightning storms. Charges form at the top and bottom of the cloud which
induces charges in the ground which can cause a lightning strike.
The lightning strike is more complex than it first seems.
Lightning can be prevented from being dangerous by the use of lightning rods.
This process can be usefully used in things such as the painting of car bodies.
Since there is an electric force, the droplets of paint are accelerated – because they all have the same
charge they spread out and if the car body is made positive virtually no paint is wasted since it is
attracted to the car body. A similar process is used in photocopiers.
This movement of charges can be predicted by field lines. Remember the arrows indicate the direction of
the force that a positive charge would experience.
The direction and size of the force depends upon the size and nature of the charge and also on
the strength of the field.
The direction of the field lines indicates the direction of the movement of the charged particles.
The size of the electric field can be estimated by using:
The electric field strength E at a point in a field is defined as the force per unit charge on a positive test
charge placed at that point. The unit of E is NC-1 or Vm-1.
For lightning to occur there must be an electric field of about 1 M Vm-1 in dry air, although a considerably
smaller electric field strength is required in wet conditions.
In an area where an electric field exists, objects which possess charge will have a force exerted on them.
If charged particles are in an electric field – negative charges – such as electrons and beta particles move towards positive plate. Positive particles - including alpha particles – move toward the negative
plate. As the charges move, electrical potential energy is converted to kinetic energy
The conservation of charge
All of the electrons and protons, which are present in the universe, now have been present since the
beginning of time. Protons do decay but they are likely to do so at a rate, which will take longer than the
life of the universe (50000000000 years perhaps). This means that for all real purposes charge cannot
be created or destroyed.
This means that the effects we see are due to charge being moved about NOT being created or
destroyed.
Electric charge is quantized; that is, it comes in multiples of individual small units called the
elementary charge, e, approximately equal to 1.602×10−19 coulombs (the charge on a single
electron).
A coulomb consists of approximately 6 250 000 000 000 000 electrons (6.25 × 1018)
Electric Forces
Since E is the force per unit charge, then the force on a test charge q is equal to qE. In other words,
electric field strength equals force/charge
e.g. a charge of 125 C is placed in a field where it experiences a force of 643 N the field strength is given
by
E = 643 /125 = 5.14 NC-1 (Newtons per Coulomb)
(This is the electrical equivalent of F = ma where a is measured in N Kg-1 for a gravitational field)
Electric Potential
In the same way as gravitational potential energy exists before an object falls to the ground, electric
potential energy exists as charged particle moves in an electric field.
Electric potential energy:
Where Ep is the change in potential energy measured in Joules, q is the charge being moved measured
in Coulombs and V is the potential difference between the beginning and end of the movement in Volts.
(This is the equivalent of Ep = mgh)
Electric potential energy per unit charge is called electric potential.
The task done by batteries or electrical generators is to give electrons potential energy. When we switch
on an electrical appliance by completing a circuit we are providing a path for the electrons to fall back to
their lower energy state and it is their kinetic energy, which we transform into useful work in motors or
other electrical devices.
Electric Voltage
Electric potential is often called voltage.
When a potential difference (voltage) is applied across the ends of a wire an electric field is created in
the wire.
Because of this, 1 volt = 1 joule /1 coulomb
Electric Current
Current is defined as the rate of flow of charge. The charge can be in one of two forms.
Electrons such as in wires, semiconductors and electrical discharges such as lightning strikes
Ions such as in electrolytes e.g. copper sulfate solution and ionised gases
The unit of electrical current is the Ampere (A) and is given the symbol I in equations.
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Remember, each electron only carries 1.6 x 10 Coulombs of charge so 1 Coulomb of charge moving
through a wire involves the movement of 6 250 000 000 000 000 electrons (but electrons move very
slowly – in a typical copper wire they move at about 1 millimetre per second).
A typical AA battery will be able to deliver 10404 Coulombs of charge before it becomes flat.
A rate of flow of charge of 1 Coulomb per second is 1 Ampere.
Often written as Q = It (where Q = charge in Coulombs, I = current in Amperes and t = time in seconds)
A current of 1 Ampere through a wire represents 6.25 × 1018 electrons per second moving through the
wire.