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Basic Background 001

Electricity

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The definition of the Ampere

The ampere is that constant current which, if 

maintained in two straight parallel conductorsof infinite length, of negligible circular cross-

section, and placed 1 metre apart in vacuum,

would produce between these conductors a

force equal to 2 × 10  7 Newton per metre of length

See http://www.gor donengland.co.uk/conver sion/sidef.htm f or more def initions

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Did that definition help ?

For most of you, probably not

Unf ortunately despite its precision, the 

def inition is not very helpf ul

We need to get a f eel f or  electricity

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Basic Principles

Electricity is dynamic

It moves to do its work

We shall start 

considering water 

Here is a bucket containing 10 litres of 

water 

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A tank of water 

Six 10 litre buckets of water  f ill the tank completely

How much water  is now in the tank ?

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Exercise 01

Please calculate the volume of water  now

in the tank

Check your answer  by clicking above

Move to the next slide when you are 

ready

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Measuring Water Flow

At f ir st the water  is static, it is not f lowing

Get a stopwatch ready Open the tap - what happens to the water ?

Time the water  f lowing out of the tank

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Calculating the flow-rate

The water  in the tank empties in 30 seconds

The volume of water held by the tank is 60 litres What is the f low-r ate ?

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Exercise 02

Please calculate the r ate that water  now

leaves the tank Give your answer  in litres per second

Check your answer  by clicking above

Move to the next slide when you are ready

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Slowing down the flow of water 

How could we halve the r ate of f low of water  

f rom the tank ? An easy method would be to close the tap

partially

Write down any other methods that you think 

could limit the f low of water  f rom the tank

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Electricity, not water 

The next f ew slides depict electrons as small 

balls that we can put in a bucket We cannot do this in reality, but the 

explanation holds true f or  the f low of electricity

Please allow your  thoughts to suspend belief 

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Replacing water with electrons

Now we shall ref ill the tank 

with electrons instead(take the water  out f ir st!)

Use the bucket to f ill the tank

Turn the tap on and allowelectrons to f low through the piping

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A flow of electrons = current

Electrons f low analogously to water 

We observe the f low of electrons, counting as they pass the tap

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Rate of electron flow = amp

As with the water we observe the number  of 

electrons f lowing past the tap We have counted a certain number  of 

electrons per second

Electricity f lows in electrons per second

We call this amperes ( amps )

Amps = number  of electrons per second

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The Electron ± a charge carrier 

An electron is a tiny unit of char ge

It carries a small amount of electricity The unit of electrical char ge is the coulomb

One electron carries 1.6 X 10-19 coulombs

6.25 X 1018

electrons make 1 coulomb That¶s 6 ¼ million million, million electrons

Rather a lot of electrons

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Amps, coulombs, electrons

1 ampere = a f low of 1 coulomb per second

1 ampere = 6.25 X 1018 electrons per second Amps are theref ore a measure of the f low of 

electrons in a circuit per second

The amp is of ten seen in circuit diagr ams using

the letter  I

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Review

Make sure that you under stand the principle of 

the f low of electrons being ref erred to ascurrent

The f low of electrons in any circuit is

responsible f or  the oper ation of that circuit

When you are ready, we will move on to the 

volt

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The Volt

Electrons don¶t like moving around

A f orce is needed to move them The f orce is known as the volt

More volts make more amps (electrons) f lowaround a circuit per second

Think of a battery as an electron pump Twice the number  of volts push twice the number  of electrons per second

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Potential Difference ( p.d. )

Volts create a potential diff erence (voltage) 

across a circuit or  component Here the p.d. is 1.5 volts

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Exercise 03

Please calculate the current so we can 

work out the number  of electrons that the battery moves per second

Check your answer  by clicking above

Move to the next slide when you are ready

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625,000,000,000,000,000 electrons

The volts in the battery push a huge number  of 

electrons around our  circuit per second That number  is so high it becomes unwieldy 

and diff icult to work with

Theref ore we measure the f low of electrons in 

amps, not electrons per second

A f low of electrons is known as current

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Electrons do the work

A bulb lights because electrons f low through it

Volts push the electrons Manufacturer s of ten specif y current i.e. the number  of electrons per second the device requires

We need to choose the volts caref ully to avoidpushing too many electrons into our device at once

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Voltage summary

Volts push electrons around a circuit

More volts means more current (amps) Be sure to under stand this bef ore moving on 

When you are ready, we will move on to 

Resistance

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Resistance

Hit the pipe with a hammer, it narrows

The r ate of f low of water  reduces

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Slowing the flow of water 

Why does the water¶s f low r ate reduce?

We have made it diff icult f or  the water  to f low Narrowing a water pipe is analogous to 

resistance in electrical circuits

We have created resistance to the f low of 

water 

We need to compare this to electricity

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Electrical Resistance

Electrons f low through metal easily

Carbon is har der  f or  electrons to tr avel through Carbon f or ms basis of most resistor s

Unit of resistance is the ohm ;

Amps = volts/ resistance

How can we r aise the amps ?

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Operating an LED

Requires around 35 m A

An LED has little resistance Put an LED across 12v battery - disaster 

We must regulate the f low of electrons

Use a resistor  in series with the LED Resistor will make lif e diff icult f or  electrons

f lowing in the circuit and reduce the f low r ate

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Calculate the Resistance

Use the Ohms Triangle to calculate resistance

Use a f inger  to cover  the R symbol Resistance = V/I

Resistance = volts/ amps OR

;= V/I

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Exercise 04

Calculate the required resistor  value 

12 volts is available f rom the battery 35 milliamps is needed in the LED

Use Ohm¶s Triangle if you cannot remember  

the f or mula f or  resistance

Check your answer  by clicking above

Move to the next slide when you are ready

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Conclusion

Amps measures number  of electrons per  

second that f low in a circuit Each electron carries char ge - coulombs

Coulombs per second = amps

Electrons per second = amps

Volts pump the electrons

Ohms resist the movement of electrons

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The End

Thank YouThank You

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Answer 001

6 X 10 = 60 litres of water 

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Answer 002

60 litres of water  f low through the 

tap in 30 seconds

The r ate of f low of water  is

60 litres/30 seconds

= 2 litres per second

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Exercise 03

Current = V/I

= 1.5 / 15 = 0.1 amp Remember  that 1 amp is 6.25 x 1018 electrons

The number  of electrons f lowing is

0.1 x 6.25 x 1018

= 6.25 x 1017

per second OR 625,000,000,000,000,000 electrons per second

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Exercise 04

12 volts is divided by 35 milliamps

12/ 35 x 10-3 = 342.86 ohms (approximately) Thus we need a resistor  of 343 ; (or more) to 

limit the f low of electrons in the circuit

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References

http://www.bsimotor s.com/resistor .jpg