198198

198 199198

SUBJECT: TECHNOLOGY

TITLE OF THE UNIT: ELECTRICITY

AGE OF STUDENTS AND LEVEL: 14 YEARS/ 3º ESO

STUDENTS´ LANGUAGE LEVEL: A1/A2

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 the 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 identify 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 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 safe way– To be conscious of the environmental impact of 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 on the number of exercises and practices to be done.

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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 have 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 from 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 it is called an insulator. Some materials which are electrical insulators are for example: wood, plastic, glass, pottery…

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.

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 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.

200 201200

2. EFFECTS OF ELECTRICITY

3. ELECTRIC CIRCUITS

3.1. COMPONENTS OF AN ELECTRIC CIRCUIT

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 lamp, for example, the energy of the electrons is used to create heat, and the heat in turn creates light.Fluorescent lamps, LEDs (Light Emitting Diodes), etc. 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.

An electric circuit is an unbroken path along which an electric current exists or is able to flow.

The basic components of a circuit are:

ConductorsThese 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.

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Power supplyTh is is a device that causes energy to fl ow 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. Th ey transform chemical energy into electrical energy. Th ere are diff erent 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. Th ere are special containers for collecting and recycling used batteries.Generators or alternators, which produce electrical energy in electrical power stations.

Output devices: AppliancesTh ese 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 electrical energy into light. – Motors: they transform electrical energy into movement.– Bells: they transform electrical energy into sound.– Heaters: they transform electrical energy into heat.

Control elementsTh ese elements are used to control circuits. You can use them to open, close and switch between diff erent circuits. There are different kinds such us push button switch, on-off switch, 2-way switch, etc.

Circuit protection elementsIf too much electrical current passes through a wire, it heats up and a fi re can start. To avoid this, most electrical circuits, including those in houses, use fuses (as shown in the fi gure on the right). A fuse is a device -normally a wire- that burns up and breaks if too much electricity goes through it. Th e element in the fuse melts, breaking the circuit and preventing other components of the circuit from being damaged by the overcurrent.

202 203202

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. The electric potential difference is the force that causes the current to flow.

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.

conductor

switch

resistor

cell battery

Ggenerator

Vvolmeter

Mmotor

Aammeter

push switch two way switch bulb

9V

13A

+

Here you can see an example of an electrical circuit, containing some of these elements:

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4. ELECTRICAL MAGNITUDES

5. OHM’S LAW

4.1. TENSION OR VOLTAGE (V)

4.2. CURRENT (I)

4.3. RESISTANCE (R)

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.

Current is the number of electrons flowing through a circuit in a second. It’s measured in amperes (A).

Resistance is anything which opposes the flow of current in a circuit. The resistance is measured in ohms (_)

For example:– Longer wires have greater 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 the 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.

The relationship between current, voltage and resistance was discovered by George Ohm. From experiments, he found that:

This expression can also be written in other ways:

Looking at the last equation we can see that if the voltage increases, but the resistance is constant, the current also increases.

RIV =

IVR =

RVI =

RIV =

IVR =

RVI =

204 205204

6. TYPES OF CIRCUITS

6.1. SERIES CIRCUIT

Circuits exist in all kinds of different places. The lighting in school or at 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.

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.

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 a series circuit. Different types of components can be connected in series.

+

+ –

M

10K

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.

+9V M10K

+

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SERIES CIRCUIT PARALLEL CIRCUIT

Scheme of the circuit

Resistance The effect is to add more resistance to the circuit

The effect is to reduce the resistance of the circuit

Voltage The voltage of the supply is shared between the components in series

The voltage across each component in parallel, is the same

Current The current that flows across each component connected in series is the same

The current is shared between each component connected in parallel

R1 R2

V+ –

V R1 R2

21 RRRTotal +=

21

111RRRTotal

+=

21 VVVTotal +=

21 VVVTotal ==

21 IIITotal ==

21 IIITotal +=

21 RRRTotal +=

21

111RRRTotal

+=

21 VVVTotal +=

21 VVVTotal ==

21 IIITotal ==

21 IIITotal +=

21 RRRTotal +=

21

111RRRTotal

+=

21 VVVTotal +=

21 VVVTotal ==

21 IIITotal ==

21 IIITotal +=

21 RRRTotal +=

21

111RRRTotal

+=

21 VVVTotal +=

21 VVVTotal ==

21 IIITotal ==

21 IIITotal +=

21 RRRTotal +=

21

111RRRTotal

+=

21 VVVTotal +=

21 VVVTotal ==

21 IIITotal ==

21 IIITotal +=

21 RRRTotal +=

21

111RRRTotal

+=

21 VVVTotal +=

21 VVVTotal ==

21 IIITotal ==

21 IIITotal +=

206 207206

EXAMPLES OF CIRCUITS: SERIES CIRCUIT

+V=12V

R1=2Ω R2=4Ω R3=6Ω

Ω=++= 12642TotalR

AVRVI

Total

TotalTotal 1

1212

=Ω

==

VARIV TOT 22*1* 11 =Ω==

VARIV TOT 44*1* 22 =Ω==

VARIV TOT 66*1* 33 =Ω==

VVVVVVVVTOT 12642321 =++=++=

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EXAMPLES OF CIRCUITS: PARALLEL CIRCUIT

V=12V R1=2Ω R2=4Ω R3=6Ω+

1211

12236

61

41

211

=++

=++=TotalR

Ω=1112

TotalR

AVRVI

Total

TotalTotal 11

111212

=Ω

==

V1=V2=V3 = 12 V

AVRVI 6

212

1

11 =

Ω== AV

RVI 3

412

2

22 =

Ω== AV

RVI 2

612

3

33 =

Ω==

AAAAIIIITOT 11236321 =++=++=

208 209208

SERIES CIRCUIT PARALLEL CIRCUIT

Resistance rules

The total resistance is greater than every single resistance of the circuit

The total resistance is smaller than the smallest of the resistance connected in parallel.

Current rules

The current is the same in every point of the circuitAs 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 as the one leaving that junctionMore current goes through the easier path (the one with less resistance)

Voltage rules

The total voltage is shared between all the appliances of the circuit.A battery voltage is equal to the sum of the voltages at every appliance of the circuit

Appliances in parallel receive the same voltageBattery voltage is equal to the voltage at every appliance of the circuit.

6.3. MIXED CIRCUIT

6.4. ASSOCIATION OF BATTERIES

If a circuit has components connected both in series and in parallel, we call it a mixed circuit.

M+–

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, the energy is stored in the battery.

We shall look at some ways of connecting batteries.

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Batteries in seriesWhen we connect several batteries in series in a circuit, the total voltage increases and the current gets larger.

In circuits A and B, the batteries give 1.5 volts.

In circuit C the batteries, connected in series, give 3 volts. The lamp will be very bright.

Batteries in parallelWhen batteries are connected in parallel, as we can see in circuit B, 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.

1,5volt

1,5volt

1,5volt

A

B

1,5 Volt lamp

1,5 Volt lamp

1,5volt

1,5volt

C

1,5 Volt lamp

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.

�e ammeter is inseries with the lamp

A

210 211210

7.2. VOLTAGE – VOLTMETER

Diagrams A and B below show a circuit before and after connecting an ammeter.

R1

V

I

R2

R1

VR2

Aammeter

– 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.

A B

A voltmeter is an instrument used for measuring the voltage between two points in an electric circuit.

�e voltimeter is inparallel with the lamp

V

212212

Diagram C shows the same circuit after connecting a voltmeter:

R1

V

I

R2

R1

V

I

R2

R1

R2

voltmeter

I

V

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.

R1

R2

ohmmeter

I

Ω

212 213212

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.

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 the way of electricity because it can harm you. Electricity can be dangerous, we need to take care and keep ourselves safe. If you come into contact with 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 an 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 pedestrian areas where people could trip over them. – Don’t use appliances with a frayed flex or a cracked plug. Tell an adult about it.

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– Never use a hairdryer or play an electrical radio or television in 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 KILLSMAKE SURE IT’S SAFE

Umatilla Electric Cooperative© Copyright 2005 - 2007 Umatilla Electric

9. ELECTRICITY AND ENVIRONMENT

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 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

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10. EXERCISES

As electricity is very important in our daily lives, we demand more electricity everyday and we are polluting and increasing the damage to our environment. We must avoid this 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 also 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 washing machine are full when you run them 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.

The following exercises can be done with our pupils during the lessons:

1. Say which of the following materials are good conductors: rubber, copper, nylon, glass, iron and wood.

2. Complete the following sentences:a. The force that is applied to a conductor to cause a 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 _ is _________

e. When you have three bulbs connected in series, the same______________ will flow through all of them.

f. When a lamp breaks and others continue working, they are connected in ______________

3. A current of 3 A, flows through a resistor of 6_ .What is the voltage across R?

4. What is the resistance of a circuit that is crossed by a current of 2 A with a voltage of 40 V?

5. In a circuit R=2 _. What will the current flowing through it be if the voltage is 60 V?

216216

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.

+ +

a)

b)

c)

d)

+ +

M

+–

+–

+–

+–

+–

M

216 217216

8. What is the difference between these two circuits? Which is right? Why?

9. Which of these are electrical appliances?

10. Can you fill the gaps with the words listed?

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:

+– +

– M

M

batteries safer mains water dangerous

Torch Book Radio Mobile phone Lamp Computer Television Plastic cup

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

PotentialDifference

6) A device for closing and opening a circuit

Current 7) Measures the current flowing through a circuit

Resistance 8) A flow of electric charge

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12. Match the symbols with their names:

Resistor

Motor

Battery

Two way switch

Fuse

Bulb

+–

M

13. What would happen in this circuit if:

a) The motor blows

b) Bulb 1 blows

c) Bulb 2 blows

d) You open or close switch 1

e) You open or close switch 2

+-

M

B2

I 1

I 2

14. In each of the two circuits indicate which bulb will be lit if:a. You close all of the switches except switch Ab. You close all of the switches except switch Bc. You close all of the switches except switch C

+– +

–

AA

B

B

CC

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15. Calculate the amount of current flowing through the following circuits:

+–12V

R1 R2

1,5 Ω 1,5 Ω

+–100V

R1 R2

5 Ω 20 Ω

16. Calculate the total resistance and the total current which flows through the circuit in the following examples:

+

–

12 volts

5 Ω 10 Ω

R1 R2 +

–

24 volts

10 Ω 15 Ω

R1 R2

R3 10 Ω

R1 20 Ω

R2 30 Ω

9 volts+

–

220220

11. SIMULATION PRACTICES

Crocodile program, draw and analyse the following circuits, noting whether it is a series circuit, a parallel circuit or a mixed circuit. Note also the current intensity (I) flowing through each resistor and the voltage (V) across each of them.

60

15040

+6 V

+

1k

6V 1k

2k

1k

2k

1k

+6 V

1k

2k

1k

+9 V

2k

1k

2k

+9 V

220 221220

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.

6 V

-+

6 V

-+

6 V

-+

L1 L2

6 V

-+

L1

L2

6 V

-+

L2

L1

L3

6 V

-+

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13. ASSESSMENT

TYPE 1Name: Date:

NOTE: Don’t forget to use the units when needed.

1. For the circuit below, calculate the current intensity flowing through the resistor (1.5 points)

2. What resistance must we place in this circuit if I=2 mA (1.5 points)

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)

150 Ω+10 V

-

+12 V

-

10 Ω 10 Ω

20 Ω+10 V

-

150 Ω+10 V

-

+12 V

-

10 Ω 10 Ω

20 Ω+10 V

-

150 Ω+10 V

-

+12 V

-

10 Ω 10 Ω

20 Ω+10 V

-

222 223222

4. Note 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.

+–

0,018 A

9 V

Lamp 1 Lamp 2

V

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 timeb. Motors work independently from the lamps (it doesn’t matter if the 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 scheme and complete the following sentences:To have both bulbs on, you should The motor works ifIs it possible to have only one bulb on? Why/Why not? (1.5 points)

+–

9 V

S1

S3

S2

M

224224

TYPE 2Name: Year: Date:

1. Draw the symbol of the following electric components:

Switch

Two way switch

Push button switch

Fuse

Battery

Motor

Bulb

Resistor

Bell or 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 units.

+12 V V1 µV

µV µV

V2 V3Voltmeter Voltage

V1

V2

V3

224 225224

Ammeter Intensity

A1 60mA

A2

A3

5. Write the current intensities that these ammeters indicate. Don’t forget to write the units.

+9 V

0.00

0.00

0.00

µA

µA

µA

A3

A2

A1

6. How much current intensity flows through these circuits when we close the switches?

27 Ω

+9 V

- +9 V

- +9 V

-

7. How can you make sure that a DC motor turns clockwise and then anticlockwise?