Electric Charge

The History of Electricity 

Since ancient times, humans have been interested in Electricity. For thousands of years, humans have observed lightning, and thought this was the Gods warring with each other. Zeus of Greek mythology and Thor of Norse mythology are just two examples of where a God of Thunder was worshipped across the World. 

The word 'Electricity' is based upon the ancient Latin word 'Electrum' - meaning Amber. The ancient Greeks and Romans had discovered that by rubbing rods of the semi-precious stone Amber, it was possible to pick up small feathers with the rods. This is one of the earliest examples of Static Electricity. 

It wasn't until the 1600's when scientists began to truly explain the fundamentals of Electricity, which facilitated the progress that has given rise to the modern World. 

The video below shows an in-depth journey through the history of Electricity:-

Types of Electricity

All Electrical phenomena can be split into two groups:-

1. Static Electricity 

2. Dynamic Electricity (Current flow) 

Static Electricity

The study of Static Electricity has been going on for thousands of years. As stated above, the ancient Greeks discovered that by rubbing Amber it was possible to pick up small feathers. This example of Static electricity was not understood until more recent times, when the nature of Electric Charge was investigated scientifically. 

There are two types of Electric Charge, Positive charge and Negative charge:-

The above diagram shows the Electric field around point charges. 

Note - The arrows show the direction of the Electric field, which always points from Positive to Negative. 

In the Primary Science course, the properties of magnets were investigated. In magnets it was found that 'Like' poles repelled each other and 'Unlike' poles attract.  This is the same for Electric Charges:-

1. Like Charges -  (+ +) or  (- -) gives Repulsion.

2. Unlike Charges - (+ -) or (- +) gives Attraction.

Atomic Structure and Charge 

In the last 100 years, the structure of the atom has been studied in great detail.

The above diagram shows the Bohr model of an atom. An atom consists of the following particles:-

1. Protons - Have a positive charge, found in nucleus of atom.

2. Neutrons - Have no charge, found in nucleus of atom.

3. Electrons - Have a negative charge, found in orbit of nucleus.

Note - As an atom has equal numbers of Protons and Electrons, the atom has no charge overall, and is said to be neutral (no charge). 

For the rest of this unit, we will focus on the Electron, which forms the basis of all Electrical phenomena. 

Examples of Static Electricity 

There are many different examples of Static Electricity in the World around us, however, this section will focus on several key examples:-

1. Charged Rods.

2. Static Shocks and the Van-de-Graaf Generator.

3. Lightning.

Charged Rods

By rubbing certain materials together, it is possible to transfer Electric Charge. This is the principle behind the charged Amber of ancient Greece and Rome. 

The diagram below shows how charge is transferred  when rods of two different materials are rubbed with a cloth:-

As can be seen above, when polythene is rubbed with a cloth, negative charges move from the cloth onto the rod. As the rod now has more negative charges than positive charges, the rod becomes negatively charged. 

When acetate is rubbed with a cloth, negative charges move from the rod to the cloth. As the the rod now has fewer negative charges than positive charges, the rod becomes positively charged. 

Note - In both the above examples, it is the negative charge that moves. This is because negative charge carrier, the Electron, can be easily transferred. This is unlike the positive charge carrier, the Proton, which is bound tightly as part of the nucleus of the atom. In the National 5 course, we will focus only on the flow of Electrons when transferring charge. 

Van de Graaff Generator

The classic example of Static Electricity in a school is the Van De Graaff generator. The image below shows the table-top version that is used within schools:-

The Van de Graaff generator consists of a hollow metal sphere, at the top of an insulating column. A rubber belt runs from the base to a metal comb inside the dome. As the belt moves, Electrons are transferred  from the belt onto the dome. This means that the dome becomes negatively charged:-

This buildup of Charge can be used for a variety of Physics demonstrations:-

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Moving Charges and Current Flow

In the BGE course, the concept of Conductivity was covered. All materials can be defined within two groups:-

1. Conductors - Materials which allow Charge to flow easily through them.

2. Insulators - Materials in which it is difficult for Charge to flow through them.

Almost all of the conductors looked at were metals (Graphite is the exception). This is due to the structure of the atoms within metals. The diagram below shows the structure of a metal to the atomic scale:-

In a metal, Electrons are free to move from atom to atom within the metal. When there is no Electric field present, these Electrons move at random through the material. This means there is no overall flow in any direction. 

If an Electric field is present across the metal, the negatively charged Electrons are repelled away from the negative end of the metal, and attracted towards the positive end. This causes a flow of charge towards the positive end of the metal. The image below shows this process:-

The speed at which these Electrons move depends on two factors:-

1. The size of the Electric field.

2. The ease at which the metal conducts.

Note - Both of these will be looked at in more detail in later sections.

Voltage

The size of the Electric field is shown by the difference in Voltage between each end of the material. Voltage is the 'push' given to the charge to move it around a circuit. 

The Voltage is defined as the amount of Energy given to each Coulomb of Charge passing a point. 

1 Volt  =  1 JC-1 

Current and Charge

The flow of Electrons through a conductor is called an Electric Current. The faster the flow, the larger the Current, just like in a river. 

In Physics, the Current is defined by the amount of Charge passing a point every second. The Current can be found using the following equation:-

Where:-

Q - Charge in Coulombs (C) 

I - Current in Amperes (A) 

t - Time is Seconds (s) 

Note - Current in formulas can be confusing. As 'C' has already been used to denote the unit of Charge, a different letter must be used to denote Current. In all formula work, Current is denoted  using the letter 'I'. 

Current in Circuits

Current will covered in much greater depth in later sections, but all Current flow can be divided into two groups:-

1. Direct Current - occurs in a circuit using a battery.

2. Alternating Current - occurs in a circuit using Mains Electricity.

Direct Current

When a circuit is attached to a battery, Electrons are repelled by the negative terminal and attracted by the positive terminal. This causes a flow of Electrons from negative to positive. As all Electrons are moving in the same continuous direction, this is a direct Current.  On an oscilloscope, direct Current is shown by a horizontal, flat line as shown below:-

Alternating Current 

When a circuit is attached to Mains Electricity, the direction of Electron flow changes back and forth many times a second, giving an alternating Current. On an oscilloscope, alternating Current is shown by a sinusoidal waveform:-

Note - In the UK, Mains Electricity alternates at a Frequency of 50 Hz and operates at a declared Voltage of 230 V. 

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