ø

NUMBERS - AHOY!

A note for our younger friends: If you haven't had the chance to learn what numbers can do, you may just read through this Fact Sheet to see what this is about. You may skip this one and come back to it on a later day if you like.

This time let's have some fun with numbers. In courses which I have conducted, I sometimes put the question: 'If you go into a shop and ask: "I want potatoes." What is the obvious question that follows?' "HOW MUCH?" or "HOW MANY?", of course.

We must know what we have to say when we talk about quantities. You know that your height is measured in cm, potatoes in pounds or kg, how fast you travel in km or miles per hour. In electronics, and in electricity generally, you'll remember we have VOLTAGE, measured in VOLTS ("V"); CURRENT, measured in AMPERES ("A"); and RESISTANCE, measured in OHMS (here we use the Greek letter "Omega"). This is important because those three factors are mathematically related and you may wish to work with them one day.

In the simplest case - and that is the way the famous Ohm's Law is expressed - "if a voltage of ONE VOLT is applied to a circuit that has a resistance of ONE OHM, then a current of ONE AMPERE will flow in that circuit". Mathematically we say: V = A times R. Change any one of these values, then one of the others (or both) change as well.

If we have any two values, then we can always calculate the third one. Example: Let us apply ONE VOLT to a circuit that (instead of one ohm, as in the formula above) has TWO OHMS, then the current will be

1 over 2 (or 1 divided by 2) = 0.5 A or half an Ampere.

Or, if we have 10 Volts and a current of 2 Amps flows, the resistance is (ten divided by two) 5 Ohms, and so on.

There is an easy way to remember this important relationship and that is the following triangle.

So, you don't know the voltage? Imagine V wasn't in the triangle now. The two leftovers in the triangle then are A and R. Multiply those and - presto - that's the voltage that will have to be applied for the current to flow. Example: R=40 and A=2. Voltage required: 40X2=80 V.

Or: You don't know the current? The two leftovers this time are voltage and resistance. (V over R) Divide the voltage by the resistance and you will get the current that you can expect to flow in that circuit. Example: Voltage=1 kV (very dangerous!) and R=500 Ohms. Current flow will be 1000 divided by 500 which is again 2 Amps. The third possibility - well, I'll let you figure that one out for yourself.

There is one last point you may wish to remember, though: If a little torch light uses, say, 50 mA (milli Amperes, that is 50 thousands of an Amp) we'll have to express it in Amperes. So, before we put the figures into the formula, 50 mA becomes 0.05. - Likewise, say we have 27 k (27 kilo Ohms, that is, which is 27,000 Ohms) it becomes 27,000, and 1 M-Ohm (one million Ohms that is) becomes 1000,000. In the example above, 1 kV becomes 1000. And so on.

As I mentioned in the beginning of this Fact Sheet, it may not be important for you to fully understand this relationship between the three values at this point in time. It depends how old you are and how much maths you have been working with. The important point to remember is that with the three factors we have learned so far, voltage, current and resistance, we can calculate any values from very little to very large amounts. (Imagine for example the electricity supplied from a power house.) It is a kind of A - B - C in the electronics field. In fact whenever you wish to design your own circuits later, it may be helpful to know how to work with Ohms Law.

In our next Fact Sheet let's get down to some practical work: how to build a useful continuity tester.

See you in Electronics Fact Sheet No 5.

Or if you like go back to Electronics Fact Sheet No 3

Click here to return to Fact Sheet links

Do you have any questions about those topics? Please send them to my mail address.

Disclaimer

Copyright © 1997-2010 Peter Schmedding, Child Development Projects, Canberra, Australia.