Download Direct Current (DC) Electricity

Electric currents
Whereas static electricity sparks consist of the sudden
movement of electrons from a negative to positive
surface, Direct current or DC electricity is the
continuous movement of the electrons through a wire.
A DC circuit is necessary to
allow the current or steam
of electrons to flow. Such a
circuit consists of a source
of electrical energy (such as
a battery) and a conducting
wire running from the
positive end of the source
to the negative terminal.
Electrical devices may be
included in the circuit.
Electrical circuit
An electrical circuit consisting of a source of DC power and
a wire making a complete circuit is required for DC
electricity to flow.
A flashlight is a good example of a DC circuit
When the switch is closed we say that we have a closed
circuit. When the switch is open , there is a break and
the current stops flowing. We say that we have an open
circuit. If a wire is connected across the switch or across
bulb it will act as a bypass for the electric current as it will
provide an easier path for it to flow. We say that we have
a short circuit.
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Current shown opposite
Although the negative charged electrons move through
the wire toward the positive (+) terminal of the source of
electricity, the current is indicated as going from positive
to negative. This is an unfortunate and confusing
convention.
Ben Franklin originally named charges positive (+) and
negative (-) when he was studying static electricity. Later,
when scientists were experimenting with electrical
currents, they said that electricity travels from (+) to (-),
and that became the convention.
This was before electrons were discovered. In reality, the
negative charged electrons move toward the positive,
which is the opposite direction that people show current
moving. It is confusing, but once a convention is made, it
is difficult to correct it.
Continuous movement of electrons
DC electricity is the continuous movement of electrons
through a conducting material such as a metal wire. The
electrons move toward a positive (+) potential in the wire.
Voltage, current and resistance
The electricity moving through a wire or other conductor
consists of its voltage (V), current (I) and resistance (R).
Voltage is potential energy, current is the amount of
electrons flowing through the wire, and resistance is the
friction force on the electron flow.
A good way to picture DC electricity and to understand the
relationship between voltage, current and resistance is to
think of the flow of water through a hose, as explained
below.
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Electrical voltage
A potential or pressure builds up at one end of the wire,
due to an excess of negatively charged electrons. It is like
water pressure building up in a hose. The pressure causes
the electrons to move through the wire to the area of
positive charge. This potential energy is called Voltage, its
unit of measurement is the Volt ( V ).
Electrical current
The number of electrons is called current and its unit of
measurement is the Ampere or Amp ( A ). Electrical
current is like the rate that water flows through a hose.
Resistance
An Ohm ( Ω ) is the unit of measurement of the electrical
resistance. A conductor like a piece of metal has its atoms
so arranged that electrons can readily pass around the
atoms with little friction or resistance. In a nonconductor
or poor conductor, the atoms are so arranged as to
greatly resist or impede the travel of the electrons. This
resistance is similar to the friction of the hose against the
water moving through it.
V
R
I
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Comparison with hose
The following chart compares water running in a hose and
DC electricity flowing in a wire:
Water in a Hose DC in a Wire Electrical Units
pressure
potential (V)
Volts - - V
rate of flow
current (I)
Amps - A
friction
resistance (R) Ohms - Ω
Creating DC electricity
Although static electricity can be discharged through a
metal wire, it is not a continuous source of DC electricity.
Instead, batteries and DC generators are used to create
DC.Batteries rely on chemical reactions to create DC
electricity.
The automobile battery consists of lead plates in a sulfuric
acid solution. When the plates are given a change from
the car's generator or alternator, they change chemically
and hold the charge. That source of DC electricity can
then be used to power the car's lights .
Current
As a physical quantity, current is the rate at which
charges flow past a point in a circuit. As depicted in
the diagram below, the current in a circuit can be
determined if the quantity of charge Q passing by a
cross section of a wire in a time t can be measured.
The current is simply the ratio of the quantity of
charge and time.
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Charge is defined as the flow of current in a specific time.
Q=I*t
Electromotive Force (EMF)
Charge can flow in a material under the influence of an
external electric field. To maintain a voltage (and flow of
charge) requires an external energy source, ie. EMF
(battery, power supply ) . EMF is measured in volts.
EMF of a cell is equal to the energy (work done) per unit
charge.
V
=
Work done/ charge
=
W/Q
or
Work W = Q V
We can substitute Q = It in above and obtain
Work W = VIT
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Circuit Symbols
Circuit symbols are used in circuit diagrams which show how a
circuit is connected together.
Wires and connections
Component
Circuit Symbol
Function of Component
To pass current very easily from one part of a
circuit to another.
Wire
A 'blob' should be drawn where wires are
connected (joined), but it is sometimes
omitted. Wires connected at 'crossroads'
should be staggered slightly to form two Tjunctions, as shown on the right.
Wires joined
In complex diagrams it is often necessary to
draw wires crossing even though they are not
connected. I prefer the 'hump' symbol shown
on the right because the simple crossing on
the left may be misread as a join where you
have forgotten to add a 'blob'!
Wires not joined
Power Supplies
Component
Circuit Symbol
Function of Component
Supplies electrical energy.
The larger terminal (on the left) is positive (+).
A single cell is often called a battery, but strictly a
battery is two or more cells joined together.
Cell
Supplies electrical energy. A battery is more than
one cell.
The larger terminal (on the left) is positive (+).
Battery
Supplies electrical energy.
DC = Direct Current, always flowing in one
direction.
DC supply
Supplies electrical energy.
AC = Alternating Current, continually changing
direction.
AC supply
A safety device which will 'blow' (melt) if the current
flowing through it exceeds a specified value.
Fuse
Two coils of wire linked by an iron core.
Transformers are used to step up (increase) and
step down (decrease) AC voltages. Energy is
transferred between the coils by the magnetic field
in the core. There is no electrical connection
between the coils.
Transformer
A connection to earth. For many electronic circuits
this is the 0V (zero volts) of the power supply, but
for mains electricity and some radio circuits it really
means the earth. It is also known as ground.
Earth
(Ground)
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Output Devices: Lamps, Heater, Motor, etc.
Component
Circuit Symbol
Function of Component
Lamp (lighting)
A transducer which converts electrical energy to
light. This symbol is used for a lamp providing
illumination, for example a car headlamp or torch
bulb.
Lamp (indicator)
A transducer which converts electrical energy to
light. This symbol is used for a lamp which is an
indicator, for example a warning light on a car
dashboard.
Heater
A transducer which converts electrical energy to
heat.
Motor
A transducer which converts electrical energy to
kinetic energy (motion).
Switches
Component
Circuit Symbol
Function of Component
Push Switch
(push-tomake)
A push switch allows current to flow only when the button is
pressed. This is the switch used to operate a doorbell.
On-Off Switch
(SPST)
SPST = Single Pole, Single Throw.
An on-off switch allows current to flow only when it is in the
closed (on) position.
2-way Switch
(SPDT)
SPDT = Single Pole, Double Throw.
A 2-way changeover switch directs the flow of current to one
of two routes according to its position. Some SPDT switches
have a central off position and are described as 'on-off-on'.
Resistors
Component
Circuit Symbol
Function of Component
Resistor
A resistor restricts the flow of current, for
example to limit the current passing through an
LED. A resistor is used with a capacitor in a
timing circuit.
Variable Resistor
(Rheostat)
This type of variable resistor with 2 contacts (a
rheostat) is usually used to control current.
Examples include: adjusting lamp brightness,
adjusting motor speed, and adjusting the rate of
flow of charge into a capacitor in a timing circuit.
Variable Resistor
(Potentiometer)
This type of variable resistor with 3 contacts (a
potentiometer) is usually used to control voltage.
It can be used like this as a transducer
converting position (angle of the control spindle)
to an electrical signal.
Capacitors
Component
Capacitor
Circuit Symbol
Function of Component
A capacitor stores electric charge. A capacitor is
used with a resistor in a timing circuit. It can also be
used as a filter, to block DC signals but pass AC
signals.
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Diodes
Component
Circuit Symbol
Function of Component
A device which only allows current to flow in
one direction.
Diode
LED
Light Emitting Diode
A transducer which converts electrical energy
to light.
Meters and Oscilloscope
Component
Circuit Symbol
Function of Component
Voltmeter
A voltmeter is used to measure voltage.
The proper name for voltage is 'potential difference',
but most people prefer to say voltage!
Ammeter
An ammeter is used to measure current.
Galvanometer
A galvanometer is a very sensitive meter which is
used to measure tiny currents, usually 1mA or less.
Ohmmeter
An ohmmeter is used to measure resistance. Most
multimeters have an ohmmeter setting.
An oscilloscope is used to display the shape of
electrical signals and it can be used to measure
their voltage and time period.
Oscilloscope
Sensors (input devices)
Component
LDR
Thermistor
Circuit Symbol
Function of Component
A transducer which converts brightness (light) to
resistance (an electrical property).
LDR = Light Dependent Resistor
A transducer which converts temperature (heat) to
resistance (an electrical property).
Ohm's Law for Electrical Circuits
The most fundamental equation in electrical circuits is
called Ohm's Law. While doing experiments on how well
metals conducted electricity, German physicist George
Ohm discovered the law in 1827. Ohm's Law is the
equation V = I * R and is used in both AC and DC
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circuits. Knowing two items in this equation allows you to
calculate the third.
Ohm's Law states that in a simple electrical circuit, the
voltage (V) equals the electrical current (I) times the
resistance (R).
V=I*R
Using equation
The importance of Ohm's Law is that if you know the
value of two of the variables in the equation, you can
then determine the third.
You can measure any of the parameters with a voltmeter.
Most voltmeters or multi-meters measure voltage, current
and resistance for both AC and DC.
Find voltage
If you know current and resistance, you can find voltage
from V = I * R. For example, if the current I = 0.2A and
the resistance R = 1000 ohms, then
V = 0.2A * 1000 Ω = 200V
Find current
If you know voltage and resistance, you can use algebra
to change the equation to I = V / R to find the current.
For example, if V = 110V and R = 22000 ohms, then
I = 110V / 22000 Ω = 0.005A
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Find resistance
If you know voltage and current, you can use algebra to
change the equation to R = V / I to find the resistance. If
V = 220V and I = 5A, then
R = 220V / 5A = 44 Ω
Conductors
The wire and electrical devices must be able to conduct
electricity. Metal such as copper is a good conductor of
electricity and has a low resistance. The tungsten filament
in a light bulb conducts electricity, but it has high
resistance that causes it to heat up and glow.
SERIES AND PARALLEL CIRCUITS
What are "series" and "parallel" circuits?
There are two basic ways in which to connect more than
two circuit components: series and parallel. First, an
example of a series circuit:
Series DC circuit
In an electrical circuit, several electrical devices such as
light bulbs can be placed in a line or in series in the circuit
between the positive and negative poles of the battery.
This is called a series circuit.
Two light bulbs in a series circuit with a battery
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One problem with such an arrangement is if one light bulb
burns out, then it acts like a switch and turns off the
whole circuit.
Here, we have three resistors (labeled R1, R2, and R3),
connected in a long chain from one terminal of the battery
to the other. The defining characteristic of a series circuit
is that there is only one path for electrons to flow.
Charge flows together through the external circuit at a
rate which is everywhere the same. The current is no
greater at one location as it is at another location. There
is a clear relationship between the resistance of the
individual resistors and the overall resistance of the
collection of resistors.
Total Resistance of a circuit is the amount of resistance
which a single resistor would need in order to equal the
overall effect of the collection of resistors which are
present in the circuit.
RT = R1 + R2 + R3 + ...
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where R1, R2, and R3 are the resistance values of the
individual resistors which are connected in series.
1. The current in a series circuit is everywhere the
same.
2. Charge does NOT accumulate at any given
location.
3. Charge does NOT become used up by resistors.
The charges can be thought of as marching together
through the wires of an electric circuit, everywhere
marching at the same rate. Current - the rate at which
charge flows - is everywhere the same. It is the same at
the first resistor as it is at the last resistor as it is in the
battery. Mathematically, one might write
Ibattery = I1 = I2 = I3 = ...
where I1, I2, and I3 are the current values at the individual
resistor locations.
These current values are easily calculated if the battery
voltage is known and the individual resistance values are
known. Using the individual resistor values and the
equation above, the equivalent resistance can be
calculated. And using Ohm's law (V = I • R), the current
in the battery and thus through every resistor can be
determined.
In series circuits, the resistor with the greatest resistance
has the greatest voltage drop.
Since the current is everywhere the same within a series
circuit, the I value of V = I • R is the same in each of the
resistors of a series circuit. So the voltage drop (V) will
vary with varying resistance. Wherever the resistance is
greatest, the voltage drop will be greatest about that
resistor.
V1 = I • R1
V2 = I • R2
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V3 = I • R3
Parallel DC circuit
Devices can also arranged in a parallel configuration, such
that if any bulbs go out, the circuit is still intact. Not only
is a parallel circuit useful for holiday lighting, the electrical
wiring in homes is also in parallel. In this way lights and
appliances can be turned on and off at will. Otherwise if
you turned one light off--or one burned out--all the other
lights in the house would go off too.
Two light bulbs in a parallel circuit
If either light bulb would go out, the other would still
shine. You could add other bulbs or even appliances such
as electric motors in parallel to this circuit, and they would
remain independent of each other.
Again, we have three resistors, but this time they form
more than one continuous path for electrons to flow. Each
individual path (through R1, R2, and R3) is called a branch.
The defining characteristic of a parallel circuit is that all
components are connected between the same set of
electrically common points.
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In a parallel circuit, charge divides up into separate
branches . Nonetheless, when taken as a whole, the total
amount of current in all the branches when added
together is the same as the amount of current at locations
outside the branches. The rule that current is everywhere
the same still works. The current outside the branches is
the same as the sum of the current in the individual
branches. It is still the same amount of current, only split
up into more than one pathway.
In equation form, this principle can be written as
Itotal = I1 + I2 + I3 + ...
where Itotal is the total amount of current outside the
branches (and through the battery) and I1, I2, and I3
represent the current in the individual branches of the
circuit.
Total Resistance
There is a clear relationship between the resistance of the
individual resistors and the overall resistance of the
collection of resistors. Let's begin with the simplest case
of two resistors placed in parallel branches, each having
the same resistance value of 4 . Since the circuit offers
two equal pathways for charge flow, only one-half the
charge will choose to pass through a given branch. While
each individual branch offers 4 of resistance to any
charge that flows through it, only one-half of all the
charge flowing through the circuit will encounter the 4
resistance of that individual branch. Thus, as far as any
given charge is concerned, the presence of two 414
resistors in parallel would be equivalent to having one 2resistor in the circuit. In the same manner, the presence
of two 6- resistors in parallel would be equivalent to
having one 3- resistor in the circuit. And the presence of
two 12- resistors in parallel would be equivalent to
having one 6- resistor in the circuit.
This method is consistent with the formula
1 / R T = 1 / R1 + 1 / R2 + 1 / R3 + ...
Voltage Drops for Parallel Branches
In a parallel circuit, a charge does not pass through every
resistor; rather, it passes through a single resistor. Thus,
the entire voltage drop across that resistor must match
the battery voltage. It matters not whether the charge
passes through resistor 1, resistor 2, or resistor 3, the
voltage drop across the resistor which it chooses to pass
through must equal the voltage of the battery. Put in
equation form, this principle would be expressed as
Vbattery = V1 = V2 = V3 = ...
If three resistors are placed in parallel branches and
powered by a 12-volt battery, then the voltage drop
across each one of the three resistors is 12 volts. A
charge flowing through the circuit would only encounter
one of these three resistors and thus encounter a single
voltage drop of 12 volts.
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Combination circuits
In this configuration, we'd say that R2 and R3 are in
parallel with each other, while R1 is in series with the
parallel combination of R2 and R3. By applying the total
resistance of parallel branches to a combination circuit,
the combination circuit can be transformed into a series
circuit. Then an understanding of the equivalent
resistance of a series circuit can be used to determine the
total resistance of the circuit.
Series and parallel resistor configurations have very
different electrical properties.

In a series circuit, all components are connected
end-to-end, forming a single path for electrons to
flow. Current I is the same in all resistors while
voltage is different across each resistor.(depends on
the resistance)


In a parallel circuit, all components are connected
across each other, forming exactly two sets of
electrically common points. The voltage drop is the
same across all resistors while the current is different
in each resistor (depends on resistance)


A "branch" in a parallel circuit is a path for electric
current formed by one of the load components (such
as a resistor).
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Alternating Current (AC) Electricity
Alternating current or AC electricity is the type of
electricity commonly used in homes and businesses
throughout the world. While the flow of electrons through
a wire in direct current (DC) electricity is continuous in
one direction, the current in AC electricity alternates in
direction. The back-and-forth motion occurs between 50
and 60 times per second, depending on the electrical
system of the country. AC is created by an AC electric
generator, which determines the frequency. What is
special about AC electricity is that the voltage in can be
readily changed, thus making it more suitable for longdistance transmission than DC electricity. But also, AC can
employ capacitors and inductors in electronic circuitry,
allowing for a wide range of applications.
With an AC generator, a slightly different configuration
alternates the push and pull of each generator terminal.
Thus the electricity in the wire moves in one direction for
a while and then reverses its direction when the generator
armature is in a different position.
The charge at the ends of the wire alternates between
negative (-) and positive (+). If the charge is negative (-),
that pushes the negatively charged electrons away from
that terminal. If the charge is positive (+), the electrons
are attracted in that direction.
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Rate of change
AC electricity alternates
back-and-forth in
direction 50 or 60 times
per second, according to
the electrical system in
the country. The
frequency is designated as
either 50 Hertz (50Hz) or
60 Hertz (60Hz).
Light bulbs
Many electrical devices--like light bulbs--only require that
the electrons move. They don't care if the electrons flow
through the wire or simply move back-and-forth. Thus a
light bulb can be used with either AC or DC electricity.
Transformer
The major advantage that AC electricity has over DC is
that AC voltages can be transformed to higher or lower
voltages. This means that the high voltages used to send
electricity over great distances from the power station
could be reduced to a safer voltage for use in the house.
Changing voltages is done by the use of a transformer.
This device uses properties of AC electromagnets to
change the voltages.
Tuning circuits
AC electricity also allows for the use of a capacitor and
inductor within an electrical or electronic circuit. These
devices can affect the way the alternating current passes
through a circuit. They are only effective with AC
electricity.A combination of a capacitor, inductor and
resistor is used as a tuner in radios and televisions.
Without those devices, tuning to different stations would
be very difficult.
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Electrical Power
The electrical power used in operate an electrical device is
defined as the potential energy or voltage times the
current passing through the device. This could also apply
to a whole electrical system, such as the the power used
in running your household appliances. This is compared
to the mechanical definition of power as the work
done over a period of time. The electric company uses
the power used over a period of time to calculate the
energy used and thus your electric bill.
It is compared with the power required to do some work
over a period of time.
Determining electric power
The electrical power required to operate a device is the
input voltage times the current required.
P=V*I
where P = power, V = voltage and I = current in
amperes.
Electrical power is measured in watts. If the amount of
watts is large, kilowatts are used. 1 kilowatt = 1000
watts, just like 1 kilometer = 1000 meters. The
abbreviation for kilowatt is usually kW.
Current
If you look at the top of a light bulb, you will see its power
rating. One example is a 100 watt light bulb. You can use
the equation for electrical power to determine the amount
of current passing through that light bulb.
If your house voltage is 240 volts, then you can see that:
100W = 240V * I
Thus I = 100 / 240 = ______
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amps.
Resistance
You can also find the resistance of the light bulb, using
Ohm's Law V = I * R.
240V = _______A * R
Thus R = 240 /
= __________ohms.
Comparing with mechanical power
The standard or mechanical definition of power is the work
per unit time. In other words, power equals work divided
by time.
P=W/T
where P = power in watts, W = the work done in joules
and T = the time of measurement. Since energy is often
defined as the ability to do work, let's substitute energy E
for work and rearrange the equation:
E=P*T
Thus, the electrical energy used is the electrical power
times the time. If we measure the electrical power as
kilowatts and the time as hours, we get the energy used
by an electrical system in terms of kilowatt-hours. That is
the unit of measurement the electric company uses when
determining your bill.
Equation 2:
Equation 3:
P= V•I
P= V•I
P = (I • R) • I
P = V • (V / R)
P = I2 • R
P = V2 / R
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Calculating your electric bill
The electric company sends you a bill determined by the
amount of work the electricity has done or amount of
energy expended in kilowatt-hours. Most homes have an
electric meter outside that measures the amount of
electrical energy used by the house over a period of time.
Many electric companies charges about 3.5 c per kilowatthour. Thus, you multiply the number of kilowatts of
electricity you use times the amount of time you use it
and multiply that by 3.5 c to get your electric bill.
For example, if you used a 1500-watt hair dryer for 100
hours in a month at a cost of 3.5 c per kilowatt-hour, the
electric company would bill you for:
1500 watt * 100 hours = 150,000 watt-hours = 150
kilowatt-hours.
Thus your bill would amount to:
150 kilowatt-hours * 3.5 c / kW-hr = Lm 5.25
Alternating Current (AC) Home
Wiring
Home wiring
Typically, homes receive 240 volts of AC electricity.
Certain high-power devices, such as an electric heater,
use the full 240 volts.
Wires into home - the Flex
A length of flex will usually consist of three insulated
conductors, encased in an insulating sheath. Each of the
conductors will have a different colour insulation,
according to the terminal it should be connected to:
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


Brown - Live
Blue - Neutral
Yellow and green - Earth
If you have any old style appliances, they may well have
different colours:



Red - Live
Black - Neutral
Green – Earth
The reason for the colour change has to do with red-green
colour-blindness. Under the old system, red-green colourblind people were unable to distinguish between the Live
wire and the Earth wire. The colours were changed to
avoid the potentially deadly consequences of this
situation.
Note that appliances which are double-insulated may not
have an Earth wire, and some appliances (such as
doorbells or fairy lights) may have flex consisting of two
uncoloured wires.
Copper wire is used because it is a good conductor of
electricity. Materials that do not conduct electricity as
good usually have a higher resistance. This results in
wasted energy and the tendency to get hot, which could
be a safety hazard.
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Wall outlets
(These types are mainly used in the United Kingdom,
Ireland, Cyprus, Malta, Malaysia and Singapore).
Power outlets incorporate shutters on line and neutral
contacts to prevent someone from pushing a foreign
object into the socket.
The domestic electrical system uses a ring circuit in the
building which is rated for 32 amps (6 amps for lighting
circuits). Moreover, there is also a fusing in the plug; a
cartridge fuse, usually of 3 amps for small appliances like
radios etc. and 13 amps for heavy duty appliances such as
heaters. Almost everywhere else in the world a spur main
system is used. In this system each wall socket, or group
of sockets, has a fuse at the main switchboard whereas
the plug has none
Safety
Proper grounding and the use of fuses are important for
protection against shock, as well as to prevent electrical
overheating and fire hazards. Correct grounding is very
important. Often ground wires are connected to water
pipes that normally go into the ground or to an earth
electrode.
Another consideration in using water pipes to ground the
circuit is that plastic piping is often being used in
plumbing. You must make sure there are no plastic pipes
between your connection and the outside earth or ground.
Fuses
Fuses and circuit breakers are used as a safety measure
in case of short circuits. A fuse or circuit breaker will
break the connection if more current is passing through
the wire than is considered safe. This will prevent the
house wiring to overheat and start a fire.
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Most homes now use circuit breakers instead of fuses.
One reason is because people with bad wiring in their
homes that constantly blow out fuses, would then force
pennies in the fuse receptacles, thus bypassing the
requirement for a fuse. This removed the aggravation, as
well as the expense of buying new fuses, but it also often
resulted in serious electrical fires in the house.
Measurement of Voltage, Current, and Resistance.
You will be familiar with the use of voltmeters and
ammeters in circuits, in that the ammeter is wired in
series with the component, while the voltmeter is wired
in parallel. We have always treated ammeters and
voltmeters as perfect.
A perfect voltmeter has an infinite resistance
so takes no current.
A perfect ammeter has zero resistance.
Therefore there is no voltage drop across it.
However, real voltmeters and ammeters are not perfect
by any means.
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Voltmeters
 Voltmeters should have a high value of resistance,
but this is not always the case. Some moving iron
meters (the kind with a needle and a scale) have
quite a low resistance, about 10 000 . This can lead
to serious reading errors when we measure high
resistance circuits.
 Digital voltmeters have a very high resistance, about
107 , which makes them almost perfect.
Ammeters

Ammeters have a very low value, but quite definite
resistance.
Multimeters
Electrical and electronic engineers do not carry separate
voltmeters and ammeters with them to where they are
working. Instead they will carry about a multimeter
which is a combined instrument that can:

Measure voltage

Measure current

Measure resistance

Measure frequency in some instruments.

Test diodes and transistors in some instruments.
A typical digital multimeter looks like this.
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An analogue meter is shown below:
For AC use, a diode is placed in the circuit to rectify the AC to DC to which
the meter responds.
Impedance
In AC, we do not use the term resistance. Instead we
use the term impedance.
A simpler way of looking at it is the “effective resistance”
of an AC circuit, but be careful when using this. It is
given the symbol Z, and its units are ohms
Ohm’s Law
Resistance is the ratio of the voltage to the current,
described in the simple equation R = V/I. In a metallic
conductor, we find that if we alter the voltage or the
current, the other variable changes in such a way that the
ratio remains constant.
R = V/I
This is Ohm’s Law, which states:
The current in a metallic conductor is directly
proportional to the potential difference between its
ends provided that the temperature and other
physical conditions are the same.
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Since only metals have constant resistance as stipulated
by Ohm’s law, they are called ohmic conductors. All other
materials which conduct current such as electrolytes and
semiconductors etc. do have resistance. But they are
known as non-ohmic conductors since their resistance is
not constant. Hence for non-ohmic conductors the V-I
graph is a curve.
Voltage Current Characteristics
We can easily measure voltage and current, using the
data to plot voltage current graphs. We use the
following circuit.
From this circuit we take readings of voltage and current
plotting them as a graph called a VI characteristic.
We normally put the voltage on the y-axis and current on
the x-axis. This allows us to determine the resistance
from the gradient. This is a voltage current graph for
an ohmic conductor:
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The straight line shows a constant ratio between voltage
and current, for both positive and negative values. So
when the voltage is negative, the current is negative, i.e.
flowing in the opposite direction. Ohm’s Law is obeyed.
For a filament lamp we see:
The resistance rises as the filament gets hotter, which is
shown by the gradient getting steeper.
Can you explain why the shape of this graph suggests
that a light bulb does not obey Ohm’s Law?
A thermistor
(a heat sensitive resistor) behaves in the opposite way.
Its resistance goes down as it gets hotter. This is
because the material releases more electrons to be able to
conduct.
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Although it looks similar to the graph above, notice how
the gradient is decreasing, indicating a lower resistance.
There is, however, a health warning:



As the current goes up, the thermistor gets hotter.
As it gets hotter, it allows more current to flow;
Therefore it gets hotter and so on.
This is called thermal runaway, and is a feature of many
semi-conductor components. At the extreme the
component will glow red-hot, then split apart. The
thermistor is used wherever any electronic circuit detects
temperature:
Here we see a thermistor protecting a power supply from
too high a temperature.
Why does a thermistor not obey Ohm's Law?
Diodes are semi-conductor devices that allow electric
current to flow one way only.
The diode characteristic graph looks like this:
Note the way it is presented. However many text books
show the graph with the voltage on the horizontal axis
and current on the vertical. Watch out for this bear-trap
in the exam.
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The diode starts to conduct at a voltage of about +0.6 V.
We call this forward bias. Then the current rises rapidly
for a small rise in voltage. If the current is reversed
(reverse bias) almost no current flows until the
breakdown voltage is reached. This usually results in
destruction of the diode.
Can you use the graph to explain why a diodes allows a
current to flow one way only?
Resistivity
The resistance of a wire depends on three factors:



the length; double the length, the resistance
doubles.
the area; double the area, the resistance halves.
the material that the wire is made of.
Resistivity is a property of the material. It is defined as
the resistance of a wire of the material of unit area
and unit length.
First, the total length of the wires will effect the amount of
resistance. The longer the wire, the more resistance that
there will be. There is a direct relationship between the
amount of resistance encountered by charge and the
length of wire it must traverse. After all, if resistance
occurs as the result of collisions between charge carriers
and the atoms of the wire, then there is likely to be more
collisions in a longer wire. More collisions means more
resistance.
Second, the cross-sectional area of the wires will effect
the amount of resistance. Wider wires have a greater
cross-sectional area. This can be attributed to the lower
amount of resistance which is present in the wider pipe. In
the same manner, the wider the wire, the less resistance
that there will be to the flow of electric charge. When all
other variables are the same, charge will flow at higher
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rates through wider wires with greater cross-sectional
areas than through thinner wires.
A third variable which is known to effect the resistance to
charge flow is the material that a wire is made of. Not all
materials are created equal in terms of their conductive
ability. Some materials are better conductors than others
and offer less resistance to the flow of charge. Silver is
one of the best conductors, but is never used in wires of
household circuits due to its cost. Copper and aluminum
are among the least expensive materials with suitable
conducting ability to permit their use in wires of household
circuits. The conducting ability of a material is often
indicated by its resistivity. The resistivity of a material is
dependent upon the material's electronic structure and its
temperature. For most (but not all) materials, resistivity
increases with increasing temperature.
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