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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
There is hardly any area of present day human activity that does not depend in some way on electrical or
electronic devices. And most of them contain control systems, that is, circuits which control the way the device
operates.
1.- ELECTRIC CHARGE
Matter is made up of atoms. Atoms are made up of other smaller particles:
 Neutrons: they are in the nucleus. They don’t have an electric charge.
 Protons: they are in the nucleus. They have a positive electric charge.
 Electrons: they’re around the nucleus. They have a negative electric charge
Normally, matter is neutral because there is the same amount of positive and negative charges. However,
sometimes electrons can go away from their atoms to different ones so that they can pass from an object to
another material.
For example, if we rub a biro with a
cloth, electrons pass from the cloth to
the biro; the biro will then have a
negative charge. If we bring the biro
close to small pieces of paper, they
will be attracted by the biro.
But the negative charges in the biro are at rest, they do not move through the material. So we say there is
STATIC ELECTRICITY.
Remember: two different electric charges (+ and -) are attracted to each other. Electric charges of the same type
repel each other.
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
2.- ELECTRIC CURRENT
It is a flow of electrons travelling through a conductor.
So electrons are not static, they are moving through the material in a specific
direction, like in a water pipe.
Watch this video (or others) on Youtube:
https://www.youtube.com/watch?v=8gvJzrjwjds&index=1&list=PL253772980E9A0F88
There are two kinds of materials:
Conductors:
They allow electric current to travel through them very easily (low electric resistance).
Good conductors are metals. Copper, aluminium, gold and silver are the most important
Insulators:
They do not allow electric current to travel through them at all (high electric resistance).
Good insulators are plastic, wood and ceramic.
There are two types of current:
Name
Abbreviation
Alternating Current
AC
Direct Current
DC
Features
110 V. or higher. No direction of current. No positive or negative
terminals. Used at home (electrical appliances) and in industry.
48 V. or lower. The current has a specific direction in a circuit.
There is a + and a - terminal. Used in circuits with batteries.
Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=vN9aR2wKv0U
3.- ELECTRIC CIRCUIT
It is a group of connected components that allow an electric current
to pass through them constantly while the circuit is complete.
Watch this video (or others) on Youtube:
https://www.youtube.com/watch?v=VnnpLaKsqGU
To allow this, the circuit has to be complete, that is, there should be a
continuous flow of electricity from the + terminal to the – terminal.
A basic circuit is made up of the following components:
Generator: to provide the electric energy to the circuit. For example a battery or a power source.
Conductors and insulators: both allow current to travel. We use them for connecting all the elements.
Receptors: to transform the electric energy into a different type of energy (buzzers-sound, bulbs-light,
motors-movement, resistors-heat…).
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
Besides, to improve the good working of the circuit, we should add two more components:
Control components: they allow us to control the flow of the current whenever and wherever we want
(switches, relay, etc).
Protection components: they can detect any dangerous voltage or current and break the circuit to avoid
higher risks to people and the circuit itself. (Fuses, automatic switches, etc).
4.- ELECTRICAL SYMBOLS
Instead of drawing the components of a circuit, we can represent them by symbols:
The most used electrical symbols are:
Component
Battery
Light bulb
Electric motor
Drawing
Symbol
Mission
Generates voltage
Transforms electric energy into light
Transforms electric energy into spin movement
Resistor
Transforms electric energy into heat
Switch
Controls the flow of current
Single pole double
throw (SPDT) switch
Push switch
Fuse
Connection
Buzzer
Allows to choose between two circuits
Controls the flow of current for certain time
Protects the circuit against high current
Connects two parts of a circuit
Transforms electric energy into sound (buzz)
Ammeter
Measures the current
Voltmeter
Measures the voltage
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
5.- EFFECTS OF AN ELECTRIC CURRENT
Receptors can transform the energy in an electric current into other type of energy that we can use, such as heat,
light, sound and motion.
 Heat
Electrons crash into the atoms of the material they are flowing through. Part of the energy that they are carrying
is transformed into heat when these collisions happen. This is called the Joule effect.
The higher the resistance of a conductor wire, the higher the electrical energy is transformed into heat.
 : Conductivity (depends on the material)
R
L
S
L: Length
S: Section
The components used to transform electrical energy into heat are called resistors. They are commonly used in
appliances such as toasters, hair dryers, ovens and washing machines.
 Light
Take a look at this website:
http://www.leroymerlin.es/productos/iluminacion/bombillas_y_tubos.html
Electrical energy can be transformed into light using different types of components:

Light bulbs:
o Filament bulb: No longer manufactured due to its low efficiency (20 %) and life (1,000 hours).
o Halogen bulb: Low efficiency.
o Low-energy bulb: A kind of fluorescent tube. Low power consumption and long life (5,000 h).
o LED bulbs: They are expensive, but worth because of their very low consumption and very long
life (20,000 h).

Fluorescent tubes: Some gases emit light when they carry an electric current.
 Motion
Electric motors transform electrical energy into mechanical energy. They are based on the forces of attraction
and repulsion between a magnet and a conductor wire placed in one or more loops inside it.
Watch this video (or others) on Youtube:
https://www.youtube.com/watch?v=d_aTC0iKO68
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
6.- BASIC ELECTRIC QUANTITIES
There are three main electrical magnitudes and they are related to each other:
 Voltage (V)
It is the force that causes the electrons to move in an electrical circuit. Just as water needs some pressure
to force it through a pipe, electrical current needs some force to make it flow. Voltage is typically
supplied by either a generator or a battery.
Voltage is measured in Volts (V).
A Voltmeter is the instrument to measure the difference of voltage between two points of a circuit.
Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=zYS9kdS56l8
 Current (I)
Current is determined by the number of electrons that pass through a cross-section of a conductor in one
second.
The electric current is measured in amperes or amps (A).
An ammeter is the instrument we use to measure the flow of electrons (e-), that is, how many amps of
current are flowing in a specific point of an electrical circuit. A current of one amp means that in one
second about 6.24 x 1018 electrons move through a cross-section of a conductor.
 Resistance (R)
Electrical resistance is defined as the resistance to flow of electricity through a material. Even the best
conductors, such as gold, have some resistance. Talking about conductors, resistance increases with an
increase of length or a decrease of cross-section, but all the components in a circuit (lights, motors,
buzzers…) impede the flow of current
The electrical resistance is measured in ohms (Ω).
Remember: the lower the resistance of a material, the better the material conducts.
Magnitude
Unit
Instrument
Amperage
A
Amperes
A
Ammeter
Voltage
V
Volts
V
Voltmeter
Resistance
R
Ohms
Ω
Ohmmeter
The multimeter is an instrument that can measure all of these electrical quantities
Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=THP4GsECIyw
5
I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=-mHLvtGjum4
7.- OHM’S LAW
The relationship between current, voltage and resistance was studied by the 19th century German
mathematician, George Simon Ohm. Ohm formulated a law which states that current varies directly with
voltage and inversely with resistance. From this law the following formula is derived:
I
V
R
There is an easy way to remember which formula to use. By arranging current, voltage and resistance in a
triangle, one can quickly determine the correct formula:
Using the simple circuit below, assume that the voltage supplied by the battery is 10 volts, and the resistance is
5 W. To find how much current is flowing through the circuit, cover the I in the triangle and use the resulting
equation.
I
V
;
R
I
10volts
; I= 2 A
5
8.- ELECTRIC POWER. ELECTRICAL ENERGY
8.1.- Electric Power
Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=p8JQTLkV5C8
The Electric Power consumed in a resistor depends on the amount of current that passes through the
resistor for a given voltage. This is expressed as voltage times current:
P  V ·I
Power is measured in watts (w) and is represented by the letter “P”. The watt is defined as the rate work
is done in a circuit when 1 amp flows with 1 volt applied.
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
8.2.- Electrical Energy
The electrical energy is one of the items we have to pay in the invoice to the provider company. The
more energy you consume at home, the more you pay. The energy transformed into a different type of
energy by an appliance (resistor) depends on two factors: the power that is being consumed in a resistor
and the time that lasts:
E  P·t
Electric Energy is measured in watts per second (w·s), or kilowatts per hour (Kw·h) and is represented
by the letter “E”.
7.3.- Cost of electricity
The variable part of an invoice (if you do not use electricity at home you will pay anyway a fix amount
of money) depends on how much electric energy has been consumed and the price of it.
Cost  E·price
For instance, imagine that you have a LED light bulb of 10 W on for 5 hours and the price of electricity
is 0.15 €/Kwh:
E  P·t ;
E  10w·5h  50wh ;
Cost  E·price ;
E  0.05Kwh
Cost  0.05Kwh·0.15€ / Kwh ;
Cost  0.0075€
A real invoice:
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
9.-
CIRCUITS
9.1.- Series circuits
A series circuit is formed when any number of electric components is connected end-to-end so that there
is only one path for current to flow.
a)
Voltage sources connected in series:
b)
Resistors connected in series:
It  I1  I 2  I 3...
Rt  R1  R2  R3  ...
The resistors can be actual resistors or other
devices that have resistance (motors, bulbs,
speakers…).
Calculations
1.- Draw the equivalent circuit:
2.- Calculate the Total Resistance (Rt).
Rt = R1 + R2 + R3 + R4
Rt = 5 + 1 + 2 + 2 = 10 Ω
3.- Calculate the total current (It), that is the same every where:
It = Vt / Rt = 12 V / 10 Ω = 1.2 A
4.- Calculate the voltage on every resistor.
V1 = I1 · R1 = 1.2 A · 5 Ω = 6 V
V3 = I3 · R3 = 1.2 A · 2 Ω = 2.4 V
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
V2 = I2 · R2 = 1.2 A · 1 Ω = 1.2 V
V4 = I4 · R4 = 1.2 A · 2 Ω = 2.4 V
9.2.- Parallel circuits
A parallel circuit is formed when two or more resistances are placed in a circuit side-by-side so that
current can flow through more than one path.
a)
Voltage sources connected in parallel:
Vt  V1  V 2  V 3
All of them must have the same voltage and be connected the
same way (positive to positive and negative to negative).
This arrangement is used to provide the voltage to the
resistances for more time.
b)
Resistors connected in parallel:
1
1
1
1



Rt R1 R 2 R3
Calculations
1.- Draw the equivalent circuit:
2.- Calculate the Total Resistance (Rt).
1/Rt = 1/R1 + 1/R2
1/Rt = 1/10 + 1/15
1/Rt = 15 + 10 / 150 = 25 / 150
Rt = 150 / 25 = 6 Ω
3.- Calculate the total current (It) in the equivalent circuit, that is the maximum current in the original one.
It 
Vt 24V

 4A
Rt
6
4.- The total voltage (Vt) is given, and is the same for all the resistors (24 V):
Vt = V 1 = V 2
5.- Calculate the current that flows through every resistor:
I1 
V 1 24V

 2.4 A
R1 10
I2 
V 2 24V

 1.6 A
R 2 15
It = I 1 + I 2
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
9.3.- Series-parallel circuits
Series-parallel circuits are also known as compound circuits. At least three resistors are required to form
a series-parallel circuit. The following illustrations show two ways a series-parallel combination could
be found:
Some examples of compound circuits could be the following:
Calculations
There are two methods to know all the electric magnitudes in a series-parallel circuit:
A)
Using equations:
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I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
B)
Using the equivalent circuit:
1) Draw the equivalent circuit and calculate the total resistance RT
2) Calculate the total intensity of current IT = I1
It 
Vt
9V

 0.0036 A  3.6mA
Rt 2500
3) Calculate the voltage dropped across the first resistor VAB
VAB = I1 · R1 = 0,0036 A · 2000 Ω = 7.2 V
4) The voltage dropped across the other two resistors VBC will be the rest of the voltage
VBC = VT – VAB = 9 V – 7.2 V = 1.8 V
5) Calculate the intensity of current flowing through the two branches, I2 and I3
I2 
VBC
1.8V

 0.0018 A  1.8mA
R2
1000
I3 
VBC
1.8V

 0.0018 A  1.8mA
R3
1000
8.4.- Common circuits
11
I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
9.- DANGERS OF ELECTRICITY
Always . . .
 Use shielded plugs.
 Switch off the socket before inserting or removing a plug.
 Switch off the appliance before inserting or removing a plug.
 Select and use the correct fuses.
 Unplug a kettle whilst filling with water.
 Replace all cut, frayed and damaged cables immediately.
 Take care when using mains electricity.
Never . . .
 Touch mains plugs and sockets with wet hands.
 Remove covers from appliances when they are plugged in.
 Connect appliances with covers removed.
 Connect two many appliances to the same socket.
 "Test" by probing with a screwdriver.
 Be tempted to "play" with mains voltages - it could be fatal!
10.- ENERGY CONSUMPTION. EFFICIENCY
There are several ways to save electricity at home through the careful use of home appliances, lights, electronic
gadgets and heating/cooling systems:
Home appliances:
o
Use the washing machine, dishwasher and tumble dryer only when they are full, and use low
temperature and eco programs if possible.
o
Fridge: place it far away from the oven. Check that the doors close properly. Open the doors just
for the necessary time. Keep the freezer at an average of -15 ºC.
o
Induction and ceramic cookers: turn cookers off a few minutes before you finish cooking to make
use of the residual heat they keep.
Lights:
o
Make the best of natural light.
o
Turn off lights you are not using.
o
Use low-energy or LED bulbs. They consume less energy and last longer.
o
Electronic devices: switch off the devices you are not using, even ones that have a stand-by mode
as they continue using some electricity.
Heating and cooling systems:
o
Keep the temperature between 19 and 21 ºC in winter, and between 22 and 25 ºC in summer.
12
I.E.S. Emilio Alarcos
Technology 3º E.S.O.
Electricity
11.- RELAYS
A relay is a switch which is turned on and off by an
electromagnet.
When a small current flows through the coil it produces a
magnetic field that magnetizes an iron core inside. This
attracts the armature which forces the switch contacts to
touch. When the current is turned off, the switch contacts
open again.
Normally, the small current through the coil controls the high current through the contacts.
Relays allow the design of automated circuits that work on their own, so that nobody has to operate any
switch. Some examples are:
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