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Also see: Symbols and Circuit Diagrams
Electricity is the flow of charge around a circuit carrying energy from the battery (or power supply) to components such as lamps and motors.
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.
The diagram shows a simple circuit of a battery, wires, a switch and a lamp. The switch works by breaking the circuit.
With the switch open the circuit is broken - so electricity cannot flow and the lamp is off.
With the switch closed the circuit is complete - allowing electricity to flow and the lamp is on. The electricity is carrying energy from the battery to the lamp.
We can see, hear or feel the effects of electricity flowing such as a lamp lighting, a bell ringing, or a motor turning - but we cannot see the electricity itself, so which way is it flowing?
We say that electricity flows from the positive (+) terminal of a battery to the negative (-) terminal of the battery. We can imagine particles with positive electric charge flowing in this direction around the circuit, like the red dots in the diagram.
This flow of electricity is called conventional current and it is the direction of flow used throughout electronics.
However this is not the whole answer because the particles that move in fact have negative charge, and they flow in the opposite direction! Please read on...
Imaginary positive particles moving in the
direction of the conventional current
When electricity was discovered scientists tried many experiments to find out which way the electricity was flowing around circuits. In those early days they found it was impossible to determine the direction of flow.
They knew there were two types of electric charge, positive (+) and negative (-), and they decided to say that electricity was a flow of positive charge from positive to negative. They knew this was a guess but a decision had to be made. Everything known at that time could also be explained if electricity was negative charge flowing the other way, from negative to positive.
The electron was discovered in 1897 and it was found to have a negative charge. The guess made in the early days of electricity was wrong! Electricity in almost all conductors is really the flow of electrons (negative charge) from negative to positive.
By the time the electron was discovered the idea of electricity flowing from positive to negative (conventional current) was firmly established. Luckily it is not a problem to think of electricity in this way because positive charge flowing forwards is equivalent to negative charge flowing backwards.
To prevent confusion you should always use conventional current when trying to understand how circuits work, imagine positively charged particles flowing from positive to negative.
I recommend Electronics for Kids as a good introduction to electricity and electronics. Printed in full colour with many illustrations, it introduces common components with simple but interesting projects to build at each stage. The book starts by assuming no previous knowledge then carefully builds up straightforward explanations of how components work, plus practical techniques including wire-stripping, soldering and using a multimeter.
Highlights include lighting an LED with lemons, using a relay to flash an LED, building a musical instrument, making a sunrise alarm, a colour-guessing game, a secret code checker and the final project uses three ICs to make a great game.
The author, Øyvind Nydal Dahl, has done a great job in providing clear step-by-step instructions with breadboard (or stripboard) layouts as well as circuit diagrams for projects. As Technical Reviewer for the book I've built all the projects myself and I'm very happy to recommend it to anyone looking for a fun and educational introduction to electronics.
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