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Circuits #1

What is electricity? (electron flow)
 Atoms have electrons orbiting them
 Some electrons on the outer orbits can jump from one atom to the next atom
 When an electron moves, it leaves a ‘hole’ 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

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

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 electricity flowing.
Therefore, the lamp does not turn on

When the switch is on, the circuit is closed, and electricity is allowed to flow, and the
lamp turns on

Many switches have ‘poles’ and ‘throws’

The poles are how many electrical paths flow into the switch. 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
electricity 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

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 ‘pressure’ on both
ends of the wire

Voltage is also known as potential difference

Voltage is measured in volts (V)

Potential difference results in pulling or pushing the 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 movement 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 pressure to push or pull electrons

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 move around a circuit.

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

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 across 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 across it

Conductors are materials that have a very low resistance. They allow electrons to
flow across them very freely. Copper, silver and gold are 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 almost zero 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

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)

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 charges, 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

These components should be kept in static bags, etc

Ground yourself before working
Fig. 1 – Battery Symbol
Fig. 2 – Simple Circuit
Fig. 3a – Simple Circuit with a Switch
Fig. 3b – Single Pole Double Throw (SPDT)
Fig. 3c – Double Pole Double Throw (DPDT)
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
Fig. 5 – Numbering Prefixes