# UK Amateur Radio ## Foundation Licence 2002
### Technical Basics Part B
### EXPERIMENT no. 2 Move mouse onto the picture to close the switch NOTE: the electrical circuit is not made until the "switch" makes contact. The positive + end of the "battery is marked with a red strip whilst the negative end of the "battery" - is marked with a black strip. | We learned in the previous section that electrons move from negative to positive. So you might well ask does it matter if they flow the other way through the bulb. If disconnect the wires that were attached to the bulb holder and then connect them up again but the other way round and close the switch what do you think will happen? 1. Nothing 2. The bulb will light. The bulb will light, showing that is does not matter to the light bulb which way the current flows through it.
### Electron flow So, is the direction of the electrons important? The answer is "Sometimes". For a bulb it does not matter which way the electronics flow - it will glow just as brightly. In fact, the bulbs you will be most familiar with in your house have the electrons flowing through them in different directions - the direction changing 50 times every second. When the current is changing direction like this we call it "ALTERNATING CURRENT", usually abbreviated to AC. In electronics when something changes regularly like the current in your house it is said to "cycle" and thus something that changes 50 times per second is said to have a frequency of 50 cycles per second. However, some years ago it become more common to refer to one cycle per second as one Hertz, after a German scientist call Heinrich Hertz who made many discoveries in this area. Thus 50 cycles per second is exactly the same as 50 Hertz, or 50 Hz as it is normally written. You should remember that many electronics components DO care which way the electrons flow through them. Either connecting a battery or other DC source (eg a power supply) the wrong way or using AC current instead of DC current will destroy them. If in doubt ask someone who has more understanding of the dangers of in correct connection to help you.
### So what have we learned here:- 1. A torch bulb can light which ever way the electronics flow so will work with DC or AC. 2. Most electronic components do care which way the electrons flow and can be destroyed by either incorrect connection of a DC supply or by AC. 3. With AC current the electron flow changes direction repeatedly. 4. We measure the rate of change of direction of flow or the frequency in either cycles per second or more commonly Hertz (Hz). 5. The mains supply in your house (in the UK) has a frequency of 50 Hz. and in the previous section :- 6. Electrical potential difference is measured in volts symbol V 7. Electrical current flow is measured in amps symbol I. 8. Electrical resistance is measured in ohms symbol R.
### What is POWER ? The rate of doing work. In our first experiment you learned that a bulb was a special conductor which gets very very hot when electrons flow through it. In fact, the bulb does this because it is opposing or resisting the electron flow. When this happens heat is generated. The more the bulb (or any other conductor) resists or opposes the electron flow then the more heat may be generated. Also, if you continue to 'force' more electrons through a conductor by applying a larger Potential Difference, then more heat will be generated. When we generate heat like this we say that we are 'dissipating power' - dissipating means a bit like 'using'. The symbol for power is P and we measure power in Watts (after a famous scientist called James Watt who was born in Scotland). Watts is usually shortened to just W. There is a formula to you can use to calculate the power (or number of Watts) that are being dissipated in a circuit. ### The formula is: P = V x I This formula shows that the power increases as more electrons or current (I) flows or you 'force' more current to flow by applying a larger voltage (V). If we have say 4 volts (V) and a current of 3 amps (I) then from the formula: POWER (P) = 4 x 3 answer 12 watts. There is another formula which can help calculate the relationship between volts current and resistance ### The formula is: V = I x R There is a graphic of the magic triangle below.
### Relationship Volts Amps Resistance and Power
Fortunately there are two 'magic triangle' which you can use to help you remember various combination of the formulae as you need top know how to make simple calculations when given two items to find the missing one. As we dealt with Volts Amps and Resistance first we will show their Magic Triangle.
### Magic triangle for V I R |
Watch the animation above. It will show you how to use the magic triangle to work out the relationships between V, I, and R.
### Magic triangle for P V I |
Watch the animation above. It will show you how to use the magic triangle to work out the relationships between P, V, and I

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