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First_Draft Reading 57. Saturday, 21 February 2009
Changes in RED. Comments in BLUE, to be Expanded in ITALIC
Circuits #1

What is electricity? (electron flow)
 Atoms have electrons orbiting the nucleus of the atom.
 Some electrons on the outer orbits can jump from one atom to the next atom
 When an electron moves, it leaves a ‘hole’ in the orbit for another electron to jump
into
 That electron leaves another hole, and so on
 When there is a large number electrons jumping from one atom to the next in the
same direction, it can be said that the electrons are flowing in that direction
 The flow of electrons makes up electricity. You should consider Static electricity

Simple Circuit

A circuit has to have a power source, such as a battery and a load, such as a lamp

Figure 1 shows the schematic symbol for a battery. The ‘shorter’ end is negative

Figure 2 shows a simple circuit where a power source (battery) is connected to a
load (lamp)

The lines between the battery and lamp represent connections, such as wires

For electricity to flow from the power source through the load, the circuit must be
complete with no breaks. This is a closed circuit. When there is a break in the circuit,
so that current does not flow, this is known as an open circuit.(JimB)

Switches

A switch is used to open and close a circuit.

Opening and closing a circuit turns it off and on

Figure 3a shows the simple circuit with a switch added. It shows the switch in an
open position

When the switch is off, the circuit is open, and there is no current flowing.
Therefore, the lamp does not Light

When the switch is on, the circuit is closed, and current is allowed to flow, and the
lamp lights

Many switches have ‘poles’ and ‘throws’

The poles are how many electrical paths the switch can control. In figure 3a, the
switch has a single pole

The throw is the amount of positions that the switch can be in, and still allow
current to flow. The switch in figure 3a is a single throw switch


Figure 3b shows a Single-Pole-Double-Throw (SPDT) switch

Figure 3c shows a Double-Pole-Double-Throw (DPDT) switch
Prefix Numbering

An important note is the prefix numbering system.

Rather than refer to something as having a thousand volts (1000v), you could say it
has one Kilo-volt (1Kv)

Kilo means thousand, so whenever something is in the thousands, you can use kilo.
Eg, 2200Ω is the same as 2.2KΩ or 2K2

There are other prefixes which go into the millions and beyond, as well as prefixes
that show tiny fractions… ?????

Figure 5 shows additional numbering prefixes that you might commonly see

This is the same system used by computers (such as kilobyte, megabyte, gigabyte,
terabyte, etc)

Voltage

Voltage is the force that moves electrons around a circuit

To move electrons along a wire, there must be a difference in ‘electrical pressure’ on
both ends of the wire

Voltage is also known as potential difference

Voltage is measured in volts (V)

Potential difference results in a flow of electrons around the circuit

Eg, if one end of a wire has a high voltage, and the other end also has the same high
voltage, there is no difference, therefore no ‘pressure’ and no flow of electrons

If one end of a wire has a high voltage, and the other end has a low voltage, there is
a potential difference. This means there is electrical pressure to cause electrons to
flow in the circuit.

The potential difference is created by connecting the positive terminal of a power
supply (such as a battery) to a wire, and having the other end of the wire connected
to the negative terminal (or ground). The negative / ground is zero volts.


Voltage can be thought of as a pump that pushes water through a pipe
Current

Current is the measurement of the electrons that pass thru a point in the circuit in
one second

Current is measured in amperes, also known as amps (A) or milliamps (mA). A
milliamp is a thousandth of an amp (that is, one thousand milliamps equals one
amp)

Electrons don’t always individually move all the way around the circuit. The flow of
electrons is one electron moving, and another one taking its place, and another one
taking its place, and so on.

The actual electrons move/drift very slowly thru a circuit, in the order of a few
cms/sec.


Voltage is the force that makes current flow

In formulas, current is shown by the letter ‘I’

Current is like the water flowing through a pipe
Resistance

Everything physical in a circuit, whether it is a wire, or a more complicated
component will resist current flow thru it

Resistance is measured in Ohms (and KiloOhms and MegaOhms), which is
represented by the Greek letter Omega (Ω)

The higher the resistance of a material, the harder it is for electrons to flow thru it

Conductors are materials that have a very low resistance. They allow electrons to
flow …… very freely. Copper, silver and gold are good conductors

Insulators are materials that have a very high resistance. They don’t allow electrons
to flow across them. This includes wood, etc

Semi-conductors are part way between conductors and insulators. They allow partial
flow of electrons. Silicon can be a semi-conductor

Some parts (like wires) have a low resistance. Any resistance they have is so small
that they generally considered to have zero resistance

Other components have a higher resistance. Resistors are components that are
designed specifically to resist current flow


In formulas, Resistance is shown by the letter ‘R’

Resistance is like putting a bottleneck in a pipe to limit the water flow
Ohm’s Law
This is the Ohms formula, you should note the actual statement that defines the Ohms
law.

Ohm’s Law defines the relationship between Voltage (V), Current (I) and Resistance
(R)

Ohm’s law is one of the most important and fundamental laws to learn

Ohm’s law shows that voltage, current and resistance all affect each other. It shows
that if there is a bottle neck in the pipe (resistance), the pump (voltage) has to pump
harder to get more water flow (current)… incorrect

Dont get too hung-up on the water analogy, in can be misleading (JimB)

It is V = I x R (I = V / R and R = V / I)

Looking at V = I x R we see that if we change any of the values of V, I or R, the others
will change too. Eg, increasing V means that I and / or R will also increase

If you have a circuit with a 9V battery, and 1KΩ of resistance (1000 Ohms), you can
find how much current will flow with I = V / R. I = 9 / 1000, which equals 0.009A, or
9mA

Conventional and Electron Flow

Originally, Benjamin Franklin decided that electricity flows from positive to negative.
This is conventional current flow

Circuit diagrams follow the idea of conventional flow, with current flowing from the
positive terminal, to the load, and back to the negative terminal

In reality, electrons are negatively charged, so they actually flow from negative to
positive

In figure 4, we see how conventional (red) flow is the opposite to electron (blue)
flow


For schematics, it’s easier to think in terms of conventional flow
Static sensitive parts

Keep in mind that some components in electronics are sensitive to static electricty.

These components should be stored in anti-static bags, etc

Ground yourself before working…………… this needs expanding.!
Fig. 1 – Battery Symbol
I would add the + and - symbols
Fig. 2 – Simple Circuit
Fig. 3a – Simple Circuit with a Switch
Fig. 3b – Single Pole Double Throw (SPDT)
Fig. 3c – Double Pole Single Throw (DPST) (JimB)
Fig. 4 – Conventional (Red) and Electron (Blue) Flow
Number
1000000000000
1000000000
1000000
1000
1
0.001
0.000001
0.000000001
0.000000000001
Prefix
terragigamegakilo-
Symbol
T
G
M
K
millimicronanopico-
m
μ
n
p
Only one R in Tera (JimB)
Fig. 5 – Numbering Prefixes