Download electricity - recursostecno

SUBJECT: Technology
TITLE OF THE UNIT: Electricity
AGE OF STUDENTS AND LEVEL: 14 years/ 3º ESO
STUDENTS´ LANGUAGE LEVEL: A1
BRIEF DESCRIPTION OF UNIT AND NUMBER OF SESSIONS:
Electricity is the main unit of 3rd ESO. The objective of the unit is that students know
what electricity is, its effects and its environmental impact. They should learn to mount
and analyse some basic circuits, both in the workshop and using some simulation
program.
Students have never seen electricity before in secondary school. Due to the importance
of electricity nowadays, we think that teacher should dedicate a whole term to this unit.
AIMS:
 To introduce electricity to the students.
 To understand the difference between conductors and insulators.
 To distinguish the components of an electric circuit and its symbols.
 To acquire the concept of electrical current as well as the other electrical
magnitudes.
 To measure the electrical magnitudes with the help of a tester/multimeter
 To know the relationship between resistance and current.
 To distinguish the different types of electrical circuits
 To use electricity in a safety way
 To be conscious of the environmental impact of the electricity
 To draw and analyse some circuits using a simulation program
 To mount some circuits in the workshop
ASSESSMENT: It’s built into the unit
COMMENTS: Depending on the available time and resources in the workshop, the
teacher will decide the number of exercises to be done as well as the practices.
ELECTRICITY
1.
INTRODUCTION .................................................................................................. 3
2.
EFFECTS OF ELECTRICITY............................................................................. 4
3.
ELECTRIC CIRCUITS ......................................................................................... 4
3.1. Components of an electric circuit ..................................................................... 5
3.2. Symbols of electric components ....................................................................... 6
4.
ELECTRICAL MAGNITUDES ........................................................................... 7
4.1. Tension or Voltage (V) ..................................................................................... 7
4.2. Current (I) ......................................................................................................... 7
4.3. Resistance (R) ................................................................................................... 8
5.
OHM’S LAW .......................................................................................................... 8
6.
TYPES OF CIRCUITS .......................................................................................... 8
6.1. Series circuit ..................................................................................................... 8
6.2. Parallel circuit ................................................................................................... 9
6.3. Mixed circuit .................................................................................................. 11
6.4. Association of batteries .................................................................................. 12
7.
MEASUREMENT OF ELECTRICAL MAGNITUDES.................................. 13
7.1. Current intensity – ammeter ........................................................................... 13
7.2. Voltage – voltmeter ........................................................................................ 13
7.3. Resistance – ohmmeter ................................................................................... 14
8.
ELECTRICAL SAFETY ..................................................................................... 16
9.
ELECTRICITY AND ENVIROMENT ............................................................. 18
10. EXERCISES ......................................................................................................... 19
11. SIMULATION PRACTICES .............................................................................. 25
12. WORKSHOP PRACTICES ................................................................................ 26
13. ASSESSMENT ...................................................................................................... 27
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1. INTRODUCTION
Electricity completely surrounds us. For most of us, modern life would be impossible
without it. Here are just a few examples:
Throughout your house, you probably find electrical outlets where
you can plug in all sorts of electrical appliances.
Most portable devices contain batteries, which produce varying
amounts of electricity depending on their size.
During a thunderstorm, there are huge bolts of electricity called
lightning that shoot down from the sky.
It is easy to create electricity from sunlight using a solar cell; or you
can create electricity from the chemical energy in hydrogen and
oxygen using a fuel cell.
It is hard to imagine modern people living without electricity.
Matter is made up of atoms. Atoms contain particles called: protons, electrons and
neutrons.
Protons have a positive charge
Electrons have a negative charge
Neutrons have no charge
Electrons are a fundamental part of electricity. In many materials, electrons can move
form one atom to another. This movement of electrons is the origin of electricity, in fact
the movement of electrons is an electric current.
If the electrons don’t move, the material doesn’t conduct electricity and is called an
insulator. Some materials which are electrical insulators are for example: wood, plastic,
glass, ceramic…
If the electrons can move through the material, it is an electrical conductor. For
example water and metals such us gold, silver, copper, iron:
Copper is used for connecting wires.
Nichrome has more resistance and is used in the heating elements of electric
fires.
Electrical circuits can be complex. But at the simplest level, you always have the source
of electricity (a battery, etc.), a device that uses it (a light bulb, motor, etc.), and two
wires to carry electricity between the battery and the load.
However, electricity can be very dangerous, too. Accidental contact with electrical
currents can cause injury, fire, extensive damage and even death. It is important to
remember that working with and around electricity requires your full attention and
respect.
Most electricity comes from power stations that burn "fossil fuels" such as coal. The
heat boils water into steam, and the steam drives big turbines that generate electricity.
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But there are big problems with this way of generating electricity. Burning fossil fuels is
very bad for the environment, as it creates pollution and also affects the climate of the
Earth. Fossil fuels are also running out. We all need to cut down on the amount of
electricity we use at the moment.
2. EFFECTS OF ELECTRICITY
Electricity is a strong invisible force that gives power to machinery, lights, heaters and
many other forms of equipment.
Moving electrons have energy. As the electrons move from one point to another, this
energy can provide:
Heat: when the electrical current flows through a material, it heats.

Toasters, hair dryers and space heaters turn electricity into
heat.
Light: an electric current flowing through a material increases its
temperature and produces light.

In an incandescent bulb, for example, the energy of the
electrons is used to create heat, and the heat in turn creates
light.

Fluorescent lamps, LEDs … turn electricity into light.
Movement:

Electric motors turn electricity into motion.
Sound:

Speakers turn electricity into sound waves.
Information:

Computers turn electricity into information.

Telephones turn electricity into communication.

TVs turn electricity into moving pictures.

Radios turn electricity into electromagnetic waves that can
travel millions of miles.
3. ELECTRIC CIRCUITS
An electric circuit is an unbroken path along which
an electric current exists or is able to flow.
4
3.1.Components of an electric circuit
The basic components of a circuit are:
Conductors
These are the wires that connect the
different elements of the circuit
which allow the flow of electricity.
The wires are usually made of
copper and covered with plastic in
order to isolate them.
Power supply
This is a device that causes energy to flow through the circuit. A battery, a
generator or an alternator are examples of power supplies. All of them transform
a type of energy into electrical energy.
Batteries are used in small devices. They transform chemical energy into
electrical energy. There are different kinds, for example:




Normal batteries, which are the
most commonly used.
Alkaline batteries, which last
longer.
Rechargeable batteries, which
can be recharged several times.
Button batteries, which are
smaller but pollute the most.
All batteries contain toxic materials so they can not be thrown into the rubbish.
There are special containers for collecting and recycling used batteries.
Generators or alternators, which produces electrical energy in electrical power
stations.
Output devices: Appliances
These elements receive electrical energy and transform it into other types of
useful energy (light, heat, movement, sound, etc.). A bulb, a heater, a motor or a
bell are some examples of appliances.




Bulbs: they transform
energy into light.
Motors: they transform
energy into movement.
Bells: they transform
energy into sound.
Heaters: they transform
energy into heat.
electrical
electrical
electrical
electrical
5
Control elements
These elements are used to control
circuits. You can use them to open,
close and switch between different
circuits. There are different kinds such
us push button switch, on-off switch, 2way switch, etc.
Circuit protection elements
If too much electrical current passes
through a wire, it heats up and a fire
can start. To avoid this, most electrical
circuits, including those in houses, use
fuses (as it’s shown in the figure on the
left). A fuse is a device -normally a
wire- that burns up and breaks if too
much electricity goes through it. The
element in the fuse melts, opening the
circuit
and
preventing
other
components of the circuit from being
damaged by the over current.
To establish an electric circuit there must be two requirements:
1. A closed path for the electrons to flow.
2. An energy supply which creates an electric potential difference across the two
ends of the electrical circuit.
With only these two requirements we get a short circuit. The charge flows between
the terminals and a lot of energy is consumed. The circuit heats the wire to a high
temperature and a fire might break out.
In practice, electrical circuits are designed to serve a useful function. So the circuit
must have a bulb, a motor, a bell or a similar device that uses electricity for a specific
function.
3.2.Symbols of electric components
Schemes are used to represent electrical circuits in which each element is identified by a
symbol.
6
Here you can see an example of an electrical circuit, containing some of these elements:
4. ELECTRICAL MAGNITUDES
4.1.Tension or Voltage (V)
Voltage is the force that causes a current to flow. It is a measurement of the level of the
electrical energy. Voltage is measured in volts (V).
For example:
A small battery used in an alarm clock has 1.5 volts.
A button battery, as in a watch, can have 3 volts.
Electricity at home is supplied at 230 volts.
For a flow of current there must be a different energy level between two points of the
circuit (voltage). A generator is the device that produces the voltage needed in a
particular circuit.
4.2. Current (I)
Current is the number of electrons flowing through a circuit in a second. It’s measured
in amperes (A).
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4.3. Resistance (R)
Resistance is anything which opposes the flow of current in a circuit. The resistance is
measured in ohms (Ω)
For example:
Longer wires have higher resistance.
Resistance increases as the cross-sectional area of the wire decreases.
Electrical insulators offer a very high resistance.
Electrical conductors offer very low resistance.
As temperature increases the resistance of a wire increases (this is used in a
resistance thermometer).
Resistors can be used to control the current in a circuit.
A variable resistor is used to adjust the flow of current in a circuit.
5. OHM’S LAW
The relationship between current, voltage and resistance was discovered by George
Ohm. From experiments, he found that:
V  I R
This expression can also be written in other ways:
R
V
I
I
V
R
Looking at the last equation we can see that if the voltage increases, but the resistance is
constant, the current also increases.
6. TYPES OF CIRCUITS
Circuits exist in all kinds of different places. The lighting in school or home, street
lights, torches and alarm systems all have circuits that are fairly simple.
Circuits are found in calculators, computers, televisions, radios, cars, medical
equipment, aircraft, the Internet, factories, telephones and many more places.
We shall look at some of these connections.
6.1.Series circuit
Components that are placed one after another in the circuit are connected in series. The
current that flows across each component has the same value.
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The diagram on the right shows a circuit with two
lamps connected in series. If one lamp breaks,
the other lamp will not light.
This circuit is another example of series circuit.
Different types of components can be connected in
series.
6.2.Parallel circuit
If we connect several electric components
as shown in the figure on the right, we say
that they are connected in parallel.
The scheme on the right shows a circuit
with two lamps connected in parallel. If
one lamp breaks, the other lamp will
still light.
Series circuit
Parallel circuit
Scheme of
the circuit
9
Resistance
The effect is to reduce the resistance of the
circuit
The effect is to add more
resistance to the circuit
1
RTotal  R1  R2
Voltage
Current
RTotal
The voltage of the supply
is shared between the
components in series

1
1

R1 R2
The voltage across each component in
parallel, is the same
VTotal  V1  V2
VTotal  V1  V2
The current that flows
across each component The current is shared between each component
connected in series is the
connected in parallel
same
I Total  I1  I 2
I Total  I1  I 2
Examples of circuits:
Series circuit
R1= 2 
R2= 4 
R3= 6 
V = 12 V
RTotal  2  4  6  12
I Total 
VTotal 12V

 1A
RTotal 12
V1  I TOT * R1  1A * 2  2V
V2  I TOT * R2  1A * 4  4V
V3  I TOT * R3  1A * 6  6V
VTOT  V1  V2  V3  2V  4V  6V  12V
Parallel circuit
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R1= 2 
V =12 V
1
R Total

R 2= 4 
R 3= 6 
1 1 1 6  3  2 11
  

2 4 6
12
12
RTotal 
I Total 
12

11
VTotal 12V

 11A
RTotal 12

11
V1=V2=V3 = 12 V
I1 
V1 12V

 6A
R1 2
I2 
V2 12V

 3A
R2
4
I3 
V3 12V

 2A
R3 6
I TOT  I1  I 2  I 3  6 A  3 A  2 A  11A
Series Circuit
Parallel Circuit
Resistance
rules
Total resistance is greater than every
single resistance of the circuit
Total resistance is smaller than the
smallest of the resistance connected in
parallel.
Current
rules
Current is the same in every point of
the circuit
As the number of appliances increases
in the circuit, the resistance gets larger
and the current gets smaller
The sum of currents approaching a
junction is the same than the one
leaving that junction
More current goes through the easier
path (the one with less resistance)
Voltage
rules
Total voltage is shared between all the
appliances of the circuit.
Battery voltage is equal to the sum of
voltages in every appliance of the
circuit
Appliances in parallel get the same
voltage
Battery voltage is equal to the voltage
in every appliance of the circuit.
6.3.Mixed circuit
If a circuit has components connected both in series and in parallel, we call it a mixed
circuit.
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6.4.Association of batteries
Cells and batteries are useful sources of electricity. They transfer chemical energy to
electrical energy.
A zinc-carbon cell (dry Leclaché cell): This is the common cell used in torches. The
voltage across the cell is 1.5V. Once the chemicals are used up you throw it away.
There are also re-chargeable batteries. For example, a lead-acid battery in a car turns the
starter motor and is then re-charged when the engine is running. During re-charging,
energy is store in the battery.
We shall look at some ways of connecting batteries.
Batteries in series
When we connect in a circuit several batteries in series, the total voltage increases and
the current gets larger.
In circuit C, the batteries
give 3 volts. The lamp will
be very bright.
As we can see, in circuit C,
the batteries give 3 volts.
The lamp will be very
bright.
Batteries in parallel
When batteries are connected in parallel their voltages don’t add up. The reason for
connecting batteries in parallel is that two batteries last longer than one and can supply a
higher current if needed.
As we can see, in circuit B, the batteries give 1.5 volts. The lamp will have normal
brightness (as in circuit A) but the batteries will last twice as long as those in circuit A.
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7. MEASUREMENT OF ELECTRICAL MAGNITUDES
7.1.Current intensity – ammeter
An ammeter is an instrument which is used to measure the flow of an electric current in
a circuit.
Diagrams A and B below show a circuit before and after connecting an ammeter.
A
B
The ammeter must be connected in series with the resistor.
Ammeters must have a LOW resistance.
(All the current flowing in the circuit must pass through the ammeter. As meters are not
supposed to alter the behaviour of the circuit, or at least not significantly, the ammeter
must have a very LOW resistance).
7.2.Voltage – voltmeter
13
A voltmeter is an instrument used for measuring the voltage between two points in an
electric circuit.
Diagram C shows the same circuit after connecting a voltmeter:
A
C
To measure the voltage, the circuit is not changed: the voltmeter is connected in
parallel.
Voltmeters must have a HIGH resistance.
(This time, you do not need to break the circuit. The voltmeter is connected in parallel,
so it should take as little current from the circuit as possible. In other words, a
voltmeter should have a very HIGH resistance).
Voltage measurements are used much more often than current measurements.
7.3.Resistance – ohmmeter
An ohmmeter is an instrument used to measure the resistance of a particular component
of an electric circuit.
A
D
14
To measure resistance, the component must be removed from the circuit.
If you want to measure the resistance of a particular component, you must take it out of
the circuit and test it separately, as shown in diagram D.
A multimeter or a multitester is an electronic measuring
instrument that combines several functions in one unit. The
most basic instruments include an ammeter, voltmeter, and
ohmmeter.
15
8. ELECTRICAL SAFETY
Electricity can light lamps, can work televisions and many other things we use in our
everyday life but it is important not to get in electricity's way because it can harm you.
Electricity can be dangerous, we need to take care and keep ourselves safe. If you
contact the electricity in an appliance, power cord, or power line, you could be seriously
injured or killed.
Here are some safety rules to help us:
Respect the power of electricity.
Never put anything into sockets.
If you use and adaptor be careful not to overload the socket.
Never poke anything into electrical machines.
Never play with electrical cords, wires, switches, or plugs.
If a plug or switch becomes hot in use turn it off.
If an appliance works improperly or gives the slightest warning of a problem, such
as shocks or sparks, disconnect it and have it serviced.
Never repair a break in an outdoor extension cord. Throw the cord away and buy a
new one.
Protect cords from heat, chemicals and oil. Coil cords loosely when storing tools. If
cords are broken, get new ones or shorten them.
Keep cords out of walking areas where people could trip over them.
Don’t use appliances with a frayed flex or a cracked plug. Tell an adult about it
Never use a hairdryer or play an electrical radio or television into the bathroom or
use them near any water.
Make sure your hands are dry before you touch anything electrical.
Never throw water on an electrical fire.
Stay away from power equipment.
Stay away from broken power lines. Tell an adult.
Take extra precautions when using power tools. Your electrical wiring should be
adequate for the job. Circuits should be grounded. The tools should be insulated.
If you encounter an electrical-injured victim, do not touch the person until they have
been freed from the source of electricity. You can use a non-conductor, such dry
rope or wood to push or pull the victim away from the power source.
Stay away from substations and transformers.
Inform your teacher of faulty equipment in the workshop
ELECTRICITY KILLS
MAKE SURE I’TS SAFE
16
Umatilla Electric Cooperative
© Copyright 2005 - 2007 Umatilla Electric
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9. ELECTRICITY AND ENVIROMENT
The generation and use of electricity affects the environment. Engineers and scientists
are developing new environmentally-friendly ways of generating electricity. There are lots
of ideas such as: nuclear power, wind power, solar power, wave power, tidal power, and
biomass energy.
Below we can see some examples of the environmental impact of the electricity, both of
its generation and its use:
The variety of fuels - coal, oil or natural gas - used to generate electricity has
some impact on the environment. The fossil fuels are burned to create steam.
This steam is used to turn a turbine which will move the electric generator. The
smoke and gas from burning fossil fuels pollutes the air. For example: sulphur
dioxide and NOx emissions contribute to acid rain and carbon emissions
contribute to global climate change.
The force of water -used in a hydro-electric facility- to turn the turbines and the
generator disrupts the natural flow of a river with negative effects on fish and
water plants.
Nuclear power plants are generating and accumulating radioactive waste.
Some of the renewable energy facilities can affect wildlife (fish and birds) and
the disruption of land uses
Since electricity is very important in our daily lives, we are demanding more electricity
service everyday and we are polluting and increasing the damage to our environment.
We must avoid it by using electricity efficiently and obtaining it from the cleanest
sources available.
There are also, some things we can do to save energy in order not only to preserve the
world but either to save money, such us:
Insulate your walls, windows and ceilings.
Set your clothes washer to the warm or cold water setting, not hot.
Make sure your dishwasher and wash machine is full when you run it and use
the energy saving setting, if available.
Select the most energy-efficient models when you replace your old appliances.
Be careful not to overheat or overcool rooms.
Buy energy-efficient compact fluorescent bulbs for your most-used lights.
Whenever possible, walk, bike, car pool, or use mass transit.
Reduce the amount of waste you produce by buying minimally packaged goods,
choosing reusable products over disposable ones, and recycling.
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10.EXERCISES
The following exercises can be used to do with our pupils all around the lesson:
1. Tell which of the following materials good conductors are: rubber, copper, nylon,
glass, iron and wood.
2. Complete the following sentences:
a. The force that is applied to a conductor to cause current to flow is
_______________
b. With an increase of length or a decrease of cross-section of a conductor the
resistance ________________
c. Current must always be expressed in _________________
d. The total current of a simple circuit with a voltage supply of 12 volts and a
resistance of 24 Ω is _________
e. When you have three bulbs connected in series through all of them will flow the
same _________________
f. When a lamp breaks and others continue working, they are connected in
______________
3. A current of 3 A, flows through a resistor of 6Ω .What is the voltage across R?
4. What is the resistance of a circuit that is crossed by a current of 2 A and the voltage
is 40 V?
5. In a circuit R=2 Ω. What will be the current flowing through it if the voltage is 60
V?
6. How are the bulbs connected in each of these circuits?
7. In these drawings there are various connections between bulbs and motors and one
or more batteries. Say which circuits will work and describe what will happen in
each case.
19
M
M
8. What is the difference between these two circuits? Which is right? Why?
A)
B)
M
M
9. Which of these are electrical appliances?
Torch, book, radio, mobile phone, lamp, computer, television and plastic cup.
10. Can you fill the gaps with the words listed?
Batteries, safer, mains, water, dangerous.
Electrical appliances run on mains or………………………or sometimes both.
The mains supply of electricity is very ………………………Batteries are
usually……………than the ……………Never let……………near electrical
appliances.
11. Match the words in the first column to the best available answer in the second
column:
_____ switch
1) energy source for a circuit
_____ battery
2) measures the potential difference across any two points
of a circuit
_____ resistor
3) ratio of potential energy to current
_____ voltmeter
4) potential energy per unit of charge
_____ ammeter
5) converts electric energy into other forms
20
_____
potential
difference
6) a device for closing and opening a circuit
_____ current
7) measure the current flowing through a circuit
_____ resistance
8) a flow of electric charge
12. Match the symbols with their names:
1. Resistor
2. Motor
3. Battery
M
4. two way switch
5. Fuse
6. Bulb
13. What would happen in this circuit if:
a.
b.
c.
d.
The motor blows
Bulb 1 blows
Bulb 2 blows
You open or close
switch 1
e. You open or close
switch 2
I1
M
I2
B2
B1
14. In each of the two circuits indicate which bulb will be lit if:
a. You close all of the switches except switch A
b. You close all of the switches except switch B
c. You close all of the switches except switch C
A
A
B
C
C
B
15. Calculate the amount of current flowing through the following circuits:
21
16. Calculate the total resistance and the total current which flows the circuit in the
following examples:
22
17. Calculate the electrical magnitudes in the table in each one of the following circuits.
R total
I total
R total
I total
R total
I total
R total
I total
R total
V
23
R total
I total
R1
R total
I total
24
11. SIMULATION PRACTICES
Crocodile program, draw and analyse the following circuits, writing whether it is a
series circuit, parallel circuit or mixed circuit. Write also the current intensity (I)
flowing through each resistor and the voltage (V) across each of them.
25
12.WORKSHOP PRACTICES
Here there are some circuits that can be mounted in the workshop. The teacher will
prepare some questions for each circuit that the students will have to answer.
6V
6V
L1
6V
L2
6V
L1
L1
L2
L2
L3
6V
6V
26
13.ASSESSMENT
TYPE 1
Name: ________________________________________ Date:__________________
NOTE: Don’t forget the units when needed.
1. For the circuit below, calculate the current intensity flowing through the resistor (1.5
points)
10 V
150Ω
2. What resistance must we place in this circuit if I= 2 mA (1.5 points)
12 V
3. Write what kind of circuit is the one below. Calculate the current intensity flowing
through each resistor. How much voltage is there in the first resistor? (1.5 points)
10Ω
10Ω
10 V
20Ω
4. Write what kind of circuit is the one below. Write the voltage and current intensity
for each lamp. (1.5 points)
Note: You can assume that both lamps similar.
27
Lamp 1
0.018 A
Lamp 2
V
9V
I
5. Draw the scheme of a circuit with 2 lamps and 2 motors, so that: (1.5 points)
a. Both lamps turn on and off at the same time
b. Motors work independently from the lamps (it doesn’t matter if lamps are on or
off)
c. The complete circuit can be turned off with a switch
6. Write the units of the following electrical magnitudes: (1 point)
a. Voltage, V:
b. Current intensity, I:
c. Resistance, R:
7.
Look at the squeme and complete the following sentences:
To have both bulbs on you should ________________________________
The motor works if _____________________________________________
¿Is it possible to have only one bulb on? Why? (1.5 points)
S1
9V
S2
M
S3
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TYPE 2
Name: _____________________________ Year: ______________ Date: _________
1. Draw the symbol of the following electric components:
Switch
Two way switch
Push bottom switch
Fuse
Battery
Motor
Bulb
Resistor
Bell o buzzer
Voltmeter
2. ¿How can you join 3 batteries of 1,5 volts to have a 4,5 V battery? Draw the
scheme.
3. How can we join three 1,5 V batteries so that they together last 3 times more than
only one?
4. Write the voltage that these voltmeters indicate. Don’t forget to write the unit.
V2
V3
Voltmeter
V1
V2
V3
Voltage
V1
5. Write the current intensities that these ammeters indicate. Don’t forget to write the
unit.
A2
A1
A3
29
Ammeter
A1
A2
A3
Intensity
60 mA
6. How much current intensity flows through these circuits when we close the
switches?
9V
9V
9V
27 Ω
7. How can you make that a DC motor turns clockwise and then anticlockwise?
30