Electricity

Electricity in today's world

Electricity is really important in our world today. Without it there would be no televisions, telephones, ultrasound, jet engines, internal combustion engines, radios, computers, hairdryers, washing machines, satellites, electric lights vacuum cleaners, x-ray machines, power tools as well as other devices. All these devices need electricity to work, even if it's only a tiny spark.

Imagine trains still being powered by steam; printing presses powered by hand; making lots of holes with a hand drill; lifting heavy loads with pulleys and ropes; chopping down trees with axes ....

Let's face it electricity makes out lives so much easier!

But how much do we know about it?

Let's find out.

What is Electricity?

First of all, electricity flows around a circuit.

A circuit is an unbroken path made from objects that conduct electricity, usually wires and other electrical components.

A conductor is a material that has free electrons (charges), free meaning they are able to move freely and are not bound to other atomic or molecular elements in the conductor. Metals usually have lots of these free electrons, it's what makes them look shiny, and that is why they are good at conducting electricity. Copper is especially good at it, and that is the reason most electric cables are made from it.

 

Click the Open / Close button to see what happens when the switch is closed.

Voltage, Current and Resistance

The flow of electrons around a circuit is called the Current or sometimes the Impedance.

The thing that drives the electrons around a circuit is the Voltage. Voltage is a force caused by the difference in electrical charge, the potential difference, between two separate points in a circuit. It is usually thought that Current is driven from a positive (+ve) point to a negative (-ve) in the same way a pump drives a fluid by creating a high pressure (+ve) at one side. But electrons are negative and actually flow from a -ve point to a +ve one. This is a bit like magnets behave when dissimilar poles attract each other.

But not all conductors behave the same. Most don't allow the electricity to pass freely; electrons have to make their way through atoms and molecules and this slows them down. This is called Resistance. There are some special materials called Super conductors that are very efficient when cooled to extremely low temperature and have hardly any resistance at all.

So that's the basis of electricity. Voltage creates a Current of free electrons that is slowed down by Resistance.

To get a bit more technical:

Resistivity

In a wire, resistance is calculated by multiplying the material's resistivity by the length of the wire and dividing it by the wire's cross sectional area.

R = pl/A Where

  • R is the Resistance in ohms
  • p is the material's resistivity in ohms per meter
  • l is the length of the wire in metres
  • A is the cross sectional area of the wire in m2

 

Resistivities for some common materials

Material Resistivity (Ωm)
silver 1.6 × 108
copper 1.7 × 108
aluminium 3.2 × 108
carbon 3.5 × 105
graphite 7.8 × 106

A higher resistivity equals a higher resistance

Increase the area by two and the resistance is reduced by half

Increase the length by two and the resistance increases by two

Voltage, Impedance and Resistance

Voltage and Current are related by the expression:

Voltage / Impedance (current) = Resistance V/I=R

Energy and Power

Charge is a fundamental property of matter. The particles of every atom has a charge, some have +ve charge  and some, like the electrons, have a -ve charge. The charge carried by free electrons moves very slowly through a conductor. In an AC supply the charge doesn't go anywhere at all, it stays in one place constantly changing direction. When an electro motive force (EMF) creates a potential difference (pd) the voltage drop causes the charge to create an energy field around the wire that apparently moves along it at a phenomenal rate near the speed of light

Energy is given to the charge by a power supply, raising the voltage. When the charge goes through a component there is a voltage drop. Whenever there is a voltage drop there is an energy transfer, as the charge gives up energy.

The energy is usually converted into one of four things:

  • Heat as it encounters resistance in the circuit
  • Light, from a light bulb or LED (light emitting diode).
  • Sound, through speakers for example
  • Motion, through a motor for example

Charge (Coulombs) is related to Energy (Joules) and Voltage (Volts) by the relationship One Volt = One Joule per Coulomb, which gives the expression V=E/Q.

This means that there is a larger energy transfer as the change in voltage (the p.d. or potential difference), increases for a specific amount of charge.

Energy = Charge x Voltage drop (pd) or E=Q ×V

Power is the rate at which energy is used in a circuit and it is measured in Watts. More specifically it is the amount of work done in a given amount of time.

P = ΔW / Δt

Most domestic appliances have a Wattage, for example an old fashioned bulb may be rated at 100 watts. This means that the bulb uses 100 watts of power every hour.

Power is related to Current and Voltage

P = V × I

For example a high powered LED has a power rating of 0.5 watts.

Find the resistance if the circuit is powered by a 3v battery (2 × 1.5V AA)

0.5 = 3 × I

0.5 / 3 = I

I = 0.166 ampere

V / I = R

R = 3 / 1.66

R = 1.81Ω

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