Current Electricity

Electric Current

Electric current is the rate of charge flow(of electrons from negaticely charged end to a positively charged end), measured in Coulombs/second which is named Amperes.


Conventional current and electric flow



An electron flow is the movement of electrons towards the positively charged end.

While,

Conventional current flow is the assumption of scientists that an electric current consisted of positive charges flowing from a positively charged end to a negatively charged end.



(Imaginary positive particles moving in the direction of the conventional current).


How is electric current measured?

Since electric current involves moving electric charges, it can be measured by determining the amount of electric charge that passes through a conductor.

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.



Note that the equation above uses the symbol I to represent the quantity current.

The S.I. 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.

1 ampere = 1 coulomb / 1 second

Electric current is measured using an ammeter.
The ammeter must be connected in series in an electric circuit.

Electric Circuit symbols

A basic closed electrical circuit is made of four basic parts.

1. Electrical Energy Source : The energy source that drives the electric charge round the circuit

2. Electrical conductors or wires : Conductors are used to connect the components together.

3. Electrical load : This enables moving charges to do a useful job. Examples include this computer or even an electric bulb

4. Electrical switch : This enables the users to open and close the circuit.

An open circuit occurs when there is a break in the circuit to stop the current from
flowing.


Electromotive force

In an electric circuit, electromotive force is the work done by a source in driving a unit charge round a complete circuit.

It is more commonly referred to by the initials EMF. EMF is another term for electrical potential, or the difference in charge across a battery or voltage source.

In symbols:

ε = W/Q

where ,

ε - e.m.f. of the power supply,
W - Amount of electrical energy converted from non-electrical forms (work done)
Q - Amount of Charge


The S.I. unit of e.m.f. is joules per coulomb ( J/C) or Volt (V)

How do we measure e.m.f.?

To measure e.m.f, we use a volmeter connected in parellel to the circuit.


What is potential difference?

The potential difference (p.d.) between two points in an electric circuit is defined as the amount of electrical energy converted to other forms of energy when one coulomb of positive charge passes between the two points.

In Symbols:

V = W/Q

where,

V is the potential difference,
W is the electrical energy converted to other forms,
Q is the amount of Charge

The SI unit for the potential difference is the same as that for e.m.f., Volt(V).

Electric Field

An electric charge placed in vacuum, it will experience either an attractive or repulsive force depending on whether they are unlike charge or like charges.This force (experienced without the charges being in contact with each other) is known as an electric force.

An electric field is a region where an electric charge experiences an electric force.




Representing the electric field

A more useful means of visually representing the vector nature of an electric field is through the use of electric field lines of force. It is more useful to draw a pattern of several lines that extend between infinity and the source charge. These pattern of lines are referred to as electric field lines.

The lines are directed away from positively charged source charges and toward negatively charged source charges. To communicate information about the direction of the field, each line must include an arrowhead that points in the appropriate direction. An electric field line pattern could include an infinite number of lines. The presence of a few lines around a charge is typically sufficient to convey the nature of the electric field in the space surrounding the lines.



The strength of an electric field is indicated by how close the field lines are to each other.

Static Electricity

Electrostatics



Electrostatics is the branch of science that deals with the study of the electric forces acting on stationary charges, and of how these forces are modi®ed in the presence of matter.


Electrostatic by friction

Insulators can transfer charge by friction.When the surface of one insulator rubs against another, electrons can be transferred.The insulator which gains electrons will get a negative charge,the insulator which loses electrons will get a positive charge.It is most important to know that it is only the negative electrons which can move.Positive charges (protons) cannot move because they are stuck inside the nuclei of the atoms of the material.

For example, if polythene (a type of plastic) is rubbed with a dry cloth, electrons are transferred from the cloth to the polythene.The polythene gains electrons and becomes negatively charged, the cloth loses electrons and becomes positively charged.

It is not possible to predict in advance which way the electrons will go for a certain material.The same cloth, when rubbed against acetate (a different type of plastic) will gain electrons and become negatively charged,
leaving the acetate with a positive charge.


Electrostatic Charge

Attraction and Repulsion.



Opposite charges attract (pull towards each other),Like charges repel (push away from each other).This means that two positively charged things will repel each other,
two negatively charged things will repel each other.One positively charged thing
and one negatively charged thing will attract each other.The further apart the charged things are, the weaker the forces of attraction and repulsion are.


Measurement of Electric Charge

Electric charge is a physical property of matter which causes it to experience a force when near other electrically charged matter.

The SI unit of quantity of electric charge is the coulomb, which is equivalent to about 6.242×1018 e (e is the charge of a proton).

Hence, the charge of an electron is approximately −1.602×10−19 C. 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.


Insulators and Conductors

When materials are rubbed together, electrons are transferred from one material to another. The electrons that are transferred are unable to move about freely within the new material, and they remain at the surface where the material has been rubbed. Such materials where electrons are not free to move about are known as Electrical Insulators. They do not conduct electricity and are charged by friction.

On the other hand, Materials that allow electrons to move freely within them are known as Electrical Conductors. They are able to conduct electricity and are charged by induction.


Neutralising Charged Insulators and Conductors

When an object is charged, it can be neutralised by removing the excess charge from it.This process is known as discharging.


Discharging a charged insulator

One way to discharge a charged insulator is by heating it.This is because the intense heat causes the air surrounding the glass rod to be ionized. The ions in the surrounding air then neutralises the excess charges in the glass rod.

Humid condition will also neutralise a charged insulator over a period of time. This is because the water vapour in the air helps to remove excess charges on the insulator.


Discharging a charged conductor

One way to discharge a charged conductor is by earthing it. Earthing a charged conductor is to provide a path for the excess electrons to flow away or for electrons to flow to the conductor.This will cause the conductor to lose its charge and become neutral.


Charging conductors by induction

Induction charging is a method used to charge an object without actually touching the object to any other charged object.


Charging a Two-Sphere System Using a Negatively Charged Object

The metal spheres are supported by insulating stands so that any charge acquired by the spheres cannot travel to the ground. The spheres are placed side by side.

If a rubber balloon is charged negatively and brought near the spheres, electrons within the two-sphere system will be induced to move away from the balloon. This is simply the principle that like charges repel. Being charged negatively, the electrons are repelled by the negatively charged balloon. And being present in a conductor, they are free to move about the surface of the conductor.

Subsequently, there is a mass migration of electrons from sphere A to sphere B. This electron migration causes the two-sphere system to be polarized. Overall, the two-sphere system is electrically neutral. Yet the movement of electrons out of sphere A and into sphere B separates the negative charge from the positive charge.

Looking at the spheres individually, it would be accurate to say that sphere A has an overall positive charge and sphere B has an overall negative charge.

Once the two-sphere system is polarized, sphere B is physically separated from sphere A using the insulating stand. Having been pulled further from the balloon, the negative charge likely redistributes itself uniformly about sphere B.

Meanwhile, the excess positive charge on sphere A remains located near the negatively charged balloon, consistent with the principle that opposite charges attract. As the balloon is pulled away, there is a uniform distribution of charge about the surface of both spheres (see diagram iv. below). This distribution occurs as the remaining electrons in sphere A move across the surface of the sphere until the excess positive charge is uniformly distributed.