Electricity Introduction

Electricity Introduction

Objective

Identify the beginnings and discovery of electricity.

Identify some basic terms and effects.

Early (18th century) scientists started to wonder why certain otherwise unexplained events occurred. Unbeknown to them, these events were due to electricity but no one had been able to identify the phenomena. The problem with electricity is that since it is unseen and otherwise undetectable, investigation required a high level of brainpower.

Galvanni noticed that frogs’ legs hung by zinc wire on copper pipe would ‘twitch’.

Galvanni had in fact noticed the result of an electrochemical process.

This process occurs when two different metals, in contact with a conductive liquid, create an electric current.

The metals were the Zinc and Copper, the conductive liquid the salty fluids of the frog’s leg. (Salty liquid are good conductors).

This current then caused the muscles in the frog’s legs to contract. This same electrochemical process is in use today in the form of the battery.

Very soon after Galvanni’s experiments, the first basic battery was invented. It consisted of alternate layers of zinc and copper separated by cloth soaked in brine. (Brine is a salt solution)

?Later work by Michael Faraday established a link between magnetic fields and electricity.

He discovered that a wire moving through a magnetic field caused a potential difference (Voltage) to exist at the ends of the wire. This is the principle of the generator.

?Likewise, an electrical current flowing in a wire creates a magnetic field.

Later and almost by accident, it was discovered that a generator which converts mechanical energy into electrical energy can work in reverse. When fed with electrical energy, it produces mechanical energy. The electric motor was invented.

Some basics

Electricity is due to the flow of electrons. Electrons are negatively charged, sub-atomic particles. Sub atomic means that they are actually part of an atom.

They are very, very small.

The electrons in an atom spin around the nucleus, a bit like planets around the sun.

Physicists are not even sure what they are made of. Are they are a particle or a wave form (like light)? Depending on the viewpoint, they can behave as either! A famous physicist (W?H?Bragg) said

“Physicists use the wave theory on Mondays, Wednesdays and Fridays, and the particle theory on Tuesdays, Thursdays and Saturdays."

Electricity can be likened to a liquid flowing in a pipe.

The rate of flow (‘current’) is measured as Amps

The energy level or potential difference at the ends of the ‘pipe’ is measured in Volts.

Electrical power is expressed as Watts and is calculated as Volts x Amps

A rough (but not totally accurate) simile would be that of a water pipe. Amps is the speed of flow, Volts the pressure and Watts the amount of water

Andre Marie Ampere.

He devised the rule governing the mutual interaction of current-carrying wires (Ampere's law) and more importantly produced a definition of the unit of measurement of current flow, now known as the Ampere (Amp).

Volts were named after Alessandro Volta.

He became professor of physics at the University of Pavia, a chair he occupied for 25 years. By 1800 he had developed the so-called voltaic pile, a forerunner of the electric battery, which produced a steady stream of electricity

Resistance

One of the early questions was to investigate the relationship between the current (Amps) and the energy level or potential difference (Volts). Very soon, it became obvious that in any circuit, the current was proportional to the potential difference. This became Ohm’s Law after George Ohm who first established this principle.

George Ohm

Another way of expressing this is to state that the Volts are related to the Amps by a constant. This constant is the ‘resistance’ of the circuit and is measured in Ohms and denoted by the Greek letter Ω in recognition of Ohm’s work. The law is expressed mathematically as:

R = V / I

?

Where

?

R =

Resistance in Ohms

V =

Potential difference in Volts

I =

Current in Amps

Thus if a current of four Amps is flowing in a circuit with a potential difference of 12 V, then there must be a resistance of three Ω. As the resistance will be constant in a specific circuit increasing the potential difference to, for example 24 V will increase the current to 8 A.

I = 4

?

V=12

?

R = V / I

?

R = 12/4

3 Ohms

Insulators have very high resistance.

Conductors have very low resistance.

Power

The power in an electrical circuit is given by multiplying current and the potential difference.

Power is the rate of doing work.?It is measured in watts. Expressed mathematically:

P = V x I

For example in the circuit with a potential difference of 12 volts, and carrying four amps, the power will be 48 watts.

I =4

?

V=12

?

P= V x I

?

P = 12 x 4

48 Watts

A key factor is that the same power can be transmitted by any combination of potential difference and current.

Therefore, 48 watts will be the power in a circuit carrying 40 amps at 1.2 volts

I =40

?

V=1.2

?

P= V x I

?

P = 1.2 x 40

48 Watts

?and also in a circuit carrying 0.4 amps at 120 volts.

I =0.4

?

V=12

?

P= V x I

?

P = 120 x 0.4

48 Watts

Combined effects

Combining the power equation and Ohm’s law gives an expression for the power in a circuit in terms of the current and resistance:

P = I2 x R

The significance of this equation occurs when the question of Power Transmission losses are considered. Since it is a ‘square law’, it is important to minimise current in order to minimise losses. If the amperage (I) is increased then the power required goes up by the square of the value. Thus, we want to keep the amperage in any circuit as low as is practicably possible to keep power required low.

This power loss is known as the ‘I squared R’ loss. We will see later that it is important when considering transmission of electricity over wires.

Conductors

Some materials permit the flow of electricity. These are termed?‘conductors’.

All metals are conductors of electricity. Copper, silver and aluminium are amongst the best.

Carbon, although a non-metallic element, does show some metallic properties. One of them is the ability to conduct electricity.

Electrolytes

Some liquids conduct electricity. Solutions of acids, alkalis or salts are good examples. These are termed ‘electrolytes’.

Water, brine and battery acid (sulphuric acid) are typical electrolytes.

Electrolytes are considered to contain the dissolved substance as ‘charged ions’.

?For example water is considered to contain ‘positively charged’ hydrogen (H) ions and ‘negatively charged’ hydroxyl (OH) ions.

Water molecule: consists of 2 positively charged hydrogen atoms and 1 negatively, double charged Oxygen atom

Salt (sodium Chloride) molecule; consists of 1 positively charged sodium atom and 1 negatively charged Chlorine atom

When salt is dissolved in water, it dissociates (breaks apart) into two ions. It also forces the water molecule to do the same thing. This results in a solution containing electrically charged atoms or “ions”. This ionisation enables an electric current to pass through.

It’s worth noting that pure water will not conduct electricity. It needs the addition of an electrolyte.

Insulators

Materials that do not conduct electricity are termed ‘insulators’. Plastics, ceramics, glass and air are all good insulators.

There is a connection between electrical conductivity and thermal conductivity. Generally, those materials that are good electrical conductors are also good thermal conductors.

The potential difference (energy level) of the current will determine the amount of insulation needed around a conductor.

E.g., an insulation thickness suitable for 12 Volts automotive use is not suitable for 240 Volt domestic systems.

Magnetic effects

One important feature of electricity is its connection with magnetism. A current flowing in a conductor will cause a magnetic field to exist around the conductor.

A magnetic field is an invisible “blanket” of force lines. You can see their arrangement around a permanent magnet by using iron filings. The iron filings line up with the lines of force.

The magnetic field around a conductor can be enhanced by forming the conductor into a coil. Very strong magnetic fields will exist at the centre of the coil due to the flow of current in the coil.

The magnetic field can be made even stronger, if the coil is wound around a magnetic material such as iron or special magnetic alloys of iron.

This can be used to perform tasks such as lifting ferrous objects (the electromagnet), or operating valves, (the solenoid valve). This arrangement of coil around a magnetic core is found in many electrical devices such as motors and transformers.

One of the basic points of magnetism is that the lines of magnetic force are considered to run from one magnetic pole to an equal but opposite magnetic pole. These are often termed North and South poles, since this is the way in which they will align with the earth’s own magnetic field.

A feature of magnetic fields is that similar poles will repel each other whilst dissimilar poles will attract each other. This feature of attraction and repulsion is exploited in the electric motor and several other electromagnetic devices.?