Senior School - 10-12
We have all been exposed to electricity in one form or another. We see it produce lightning in a summer thunder storm or we get an electrical shock as we step out of the car on a cold dry day. We see electricity light our cities, heat or cool our homes, cook our meals and run our computers.
As long ago as 600 BC, the Greeks discovered that by rubbing amber with wool it was able to attract lighter objects. In today's terms we would say that the amber was electrified or had become electrically charged.
There is no easy definition for the term electricity, which requires an understanding of the structure of matter, forces, work and energy.
To be better able to answer the question "What is electricity?" we need to examine some fundamental and sometimes complex concepts.
- Electrostatics
- How do objects become electrically charged?
- Conservation of charge
- What is electrical charge measured in?
- Coulomb's Law - the force between charged objects
- How do objects become electrically charged?
- Charging by conduction
- Charging by induction
- Electric Fields
- Electric Potential Energy
- So what is voltage?
Electrostatics
Electrification is the process by which an object becomes electrically charged. The two forms of electrical charge are positive and negative.
When two electrically charged objects are placed near each other they exert a force on each other.
If the objects have the same charge then they will repel and if they carry different charges, they will attract.
How
do objects become electrically charged?
In order to answer this question, we will first examine the atomic nature of matter.
All matter is made up of atoms. An atom is a particle that has a positively charged nucleus that is made up of protons and neutrons1 and is surrounded by a negatively charged electron cloud. Protons (p+) have a positive charge, electrons (e-) a negative charge and neutrons (no) no charge.
An atom is electrically neutral when the number of protons equals the number of electrons. For example, a neutral lithium atom has three protons in its nucleus and three electrons in the electron cloud. This gives the atom a net charge of zero.
1 The exception to this is the hydrogen atom where its nucleus is made up of a single proton. Isotopes of hydrogen can have 1 or 2 neutrons.
An
object is electrically neutral when it has an equal
number of positive charges and electric charges. The
object becomes electrically charged when it either
loses or gains electrons. This can occur when a plastic
rod is rubbed with a nylon cloth.
An object is electrically charged when there is a difference between the number of positive and negative charges. If there is a difference, then we say that there is a net charge on the object.
For example, if an object had 10 positive charges and 8 negative charges then we would say that the object carries a net positive charge.
An object can never lose or gain positive charges: only electrons can be transferred between objects.
Conservation
of charge
The conservation of charge law states that the amount of charge in a closed system always remains the same (a closed system is one that cannot be externally influenced). On a grander scale, the conservation law means that the amount of charge in the universe always remains the same.
What
is electrical charge measured in?
The net charge of objects can vary. This means that an electrically charged object has a quantity of net charge that can be measured. The unit that we measure electrical charge in is the Coulomb (C). The symbol for charge is q (just as we use m as the symbol for mass with the unit of measurement being the kilogram (kg).
The charge on a single electron is -1.6 x 10-19 C and the charge on a single proton is +1.6 x 10-19 C.
Note: the charge on the proton and electron have the same value but are opposite in sign. The charge on a neutron is zero.
Coulomb's Law - the force between charged objects
When two objects carrying a net charge are placed near each other they experience a force of attraction or repulsion. In 1785 Charles Augustine Coulomb conducted experiments that examined the force that existed between charged objects.
From his experiments he determined that the force between two charged objects was proportional to the product of the charges and inversely proportional to the square of the distance between their centres. This is called Coulomb's Law.
Coulomb's Law - the formula for the force between two electrically charged objects
where:
F = force measured in Newtons (N);
r = the distance between the centres of the
two objects measured in metres (m);
q = the charge on the two objects measured
in Coulombs (C);
k = constant of proportionality which has a
value of 8.9874 x 109 Nm2C-2.
Remember this force can cause the objects to repel or attract depending on the charge of the objects.
Note: this formula refers to the force exerted by point charges and varies with distance.
The term "point charge" refers to a charge that has no mass so that the influence of gravitational forces can be neglected.
How
do objects become electrically charged?
If you drive in a car on a cold dry day you might experience an electrical shock as you step out of the vehicle. This occurs because the car builds up an electrical charge and "earths" or flows through you when you step on the ground.
In other words you act as an electrical conductor, in the process giving you an electrical shock.
So how do objects become electrically charged? Objects become electrically charge through a process of conduction or induction.
Charging
by conduction
An electrically neutral object is charged by conduction when a charged object comes into contact with it.
For example when a rod (that has an excess of electrons) touches a neutral ball the charge distributes itself over both objects.
When they are separated, the ball will now be electrically charged.
Charging
by induction
Charging an electrically neutral object by induction is a three-step process.
1. Let's suppose a negatively charged rod is
brought close to an electrically neutral ball. The
electrons on the ball are repelled and move to the
opposite side of the ball.
2.
By touching the negative side of the ball, the
electrons are then "earthed" off. In other
words the electrons flow from the ball leaving it
with a net positive charge.
3. The rod is then removed leaving a positively charged ball.
Electric
fields
Any object carrying electrical charge, whether it is static or current, will generate an electric field.
So what is an electric field? Put simply, if an electrically charged object is placed in an electrical field it will experience a force.
Electrical fields come in two varieties: uniform and non-uniform. If a charged object is placed in a uniform field then it will experience the same force no matter where it is placed in the field.
If
it is placed in a non-uniform field then the force
may vary depending on the position of the charge.
Lines represent electric fields diagrammatically, with arrows indicating the direction of the field.
An important point to remember about an electric field is that it is a vector quantity so it has magnitude and direction.
Electric
field strength
In discussing this topic we will talk about point charges, which have no mass but carry an electrical charge.
By convention (in other words everyone has agreed) that the direction of electric fields is determined with respect to a positive charge.
Note that the field lines move away from the positive
charge and toward a negative charge. This means is
that if a positive test charge is placed in these
fields then it will move away from the positive charge
and toward the negative charge. 
The arrows on the field lines indicate the direction a positive charge will move if it is placed in the electric field. If a negative charge is placed in an electric field it will move in the opposite direction of the field lines.
The strength of the electric field is defined as the force per Coulomb. For example if a 10 C charge were placed in an electric field of strength 10 NC-1, then it would experience a force of 100 N. The strength of an electric field can be calculated by the following:
Formula to calculate electric field strength
where:
F = force measured in Newtons (N);
qt = charge in Coulombs (C);
E = electric field strength in Newtons per
Coulomb (NC-1).
Non-uniform
field - electric field strength of a point charge
Recall that there are two types of electric fields: uniform and non-uniform. A point charge will produce a non-uniform field. This means that the electric field strength will change depending on the distance from the charge.
Formula for Electric Field Strength
Now remember that
and then:
where:
F = electric field strength of a point charge
(q);
r = the distance between the centres of the two
objects measured in metres (m);
q = the charge on the two objects measured
in Coulombs (C);
k = constant of proportionality which has a
value of 8.9874 x 109 Nm2C-2.
Uniform
electric fields - the field between two parallel plates
When two metal plates are connected to the terminals of a battery then they each gain a net positive and negative charge.
This results in a uniform electric field being generated between the two plates.
It is uniform because its strength does not vary with position. That is if a positive charge were placed within the field it would experience the same force irrespective of its position.
The electric field strength is still determined by:
where:
E = electric field strength,
F = force and
qt = charge
This concept is important in understanding how an electrical current is generated.
Electric
Potential Energy
If you have studied mechanics then you would have come across the concept of gravitational potential energy.
You may remember that a body gains gravitational potential energy as it is lifted above the ground - work is done against the force due to gravity.
If the object is allowed to fall then it will gain kinetic energy or energy of movement.
Charged objects can experience the same phenomenon. That is, the electric potential energy of a charged object can be converted to kinetic energy.
If
you want to move the charge at point A to point B
then you would have to work against the electric field.
When the charge reaches point B, then we say that it has gained electrical potential energy.
If the charge is allowed to move from B to A then it will gain kinetic energy. When this happens an electric current is produced.
So
what is voltage?
If you look at the previous diagram then at point A the charge has a certain electrical potential. At point B the charge would have a different potential. The difference between these two points is called the potential difference - this is commonly called the voltage.
The potential difference (or voltage) is the work required to move a unit of positive test charge from A to B and can be measured in volts (V).
Another way of describing potential difference is that it is the energy per unit charge required to move from A to B.
To calculate potential difference between points A and B we need to know the work done on the charge.
i.e. Work done on a charge q = force x distance
where:
E = Electric field strength
q = charge
d = distance charge is being moved
So
what does it all mean?
The human race is no different from the animals and plants on this planet - it needs energy to survive. The human body gets its energy from the food it consumes which ultimately comes from the sun.
What makes us different is that we have learnt how to harness energy and use it as a tool that powers our society.
One of the biggest advances in our technology was to find a way of easily transporting energy from one point to another and being able to easily transform energy from one form to another. We call this electricity. The keyword is move - we must get those electrons moving in order to produce electricity.
Recall
that if a charged particle, such as an electron, is
placed in an electric field it experiences a force.
This force causes it to move. Any moving charge creates
an electric current.
Metal conductors (as found in a piece of electrical wire) have what is called "free electrons". If the wire is connected to the terminals of a battery then a uniform electric field is created in the wire. The free electrons then move from the negative terminal to the positive terminal creating an electric current.
If you imagine a city connected to the terminals of a power station, an electric field is created in all the electrical conductors, which in turn causes the free electrons to move, creating an electric current.
If we are to harness electricity as a tool we need to have an understanding of the fundamentals in order to use it effectively.
More formulas
