Circuits

You probably already know that unlike a hose pipe with water electricity always flows in a circle. A better model of what is happening is more like a hose pipe connected to the input and output of a pump. The pump simply circulates the water. In the case of electricity the pump is usually a battery or something that can generate a voltage. 

Lets look at a real circuit for a moment.

 

The zig zag is the US symbol for a resistor - Europe uses a rectangle to avoid having to draw the difficult line. The Symbol to the right is a battery. You can see that the resistor is connected across the terminals of the battery. In real life there would be wires and connections and perhaps even some soldering.

 

 

This form of diagram, a schematic, only shows you the connections that you have to make. How you make them is up to you. For much electronics a schematic makes the intention of how things are connected clear where say a photo of the real circuit would just look like a mess of wires. 

Although many introductory books use drawings of real life and minimize the use of schematics it is important that you learn how to read one - so we will be using them from the word go.

You can think of the battery as a pump that pumps electricity around the circuit. The pump provides 3V of pressure and the resistor in the circuit provides 50 Ohms of resistance.

Notice that in a real circuit the wires used to connect the resistor would themselves have resistance but in practice this is so small that we ignore it and concentrate only on the resistance in the components we use in the circuit. 

Now you should have no problem in calculating the current flowing in the resistor. 

I=V/R=3/50=0.06 A

or 60mA.

Working With Units

In practice the Ohm is too small a unit for everyday use and the Amp is too big. On the other hand the Volt is about right. What this means is that a 1 Ohm resistor is a very small resistance and not often used. A 1 Amp current on the other hand is fairly large and not often encountered in digital circuits. On the other hand we tend to work with 9 Volt batteries and digital circuits work at anything from 3.3V to 5V. 

in practice we tend to work with resistance in at least 1000s of Ohms and so the Kilo Ohm or KOhm i.e. 1000 Ohms is more commonly encountered. 

If you connect a resistance in KOhms to a small number of volts then the current that flows is in the 1/1000ths of an Amp or a milli-Amp. 

You will also encounter Mega Ohms - 1000,000 Ohms or 1000KOhms and micro-Amps 1/1000,000 Amp or 1/1000 of a milli-amp. 

Again these two units tend to go together because it you put a few volts across a resistance in the Mega-Ohm range you tend to get currents in the micro-Amp range. 

Units:
1mA or 1 milli-Amp is 1/1000th of an amp. To convert Amps to milli-Amps multiply by 1000. 
1uA or 1 micro-Amp is 1/1000,000 of an amp or
1/100th of a milli-Amp. To convert milli-Amps to micro-Amps multiply by 1000

1KOhm is 1000 Ohms
1MOhm is 1000 KOhms or 1000,000 Ohms. 

If you use Ohm's law with resistance in KOhms then the current will be in mA. For example 9V applied across 20KOhms gives a current of:

9/20=0.45 mA

Similarly if you use Ohm's law with resistances in MOhms the current will be in micro-Amps. For example 9V applied across 20MOhms gives a current of:

9/20=0.45 uA