Types of circuits: Series and parallel

As you already know, a circuit is simply a closed loop carrying charges that continuously move through the different elements that compose it. These elements or components can be connected to each other in series or in parallel.

Series circuits

Two components are in series if they share a common node and if the same current flows physically through them. The total resistance of resistors connected in series is the sum of their individual resistance values. Nodes are the elements that connect one component of an electric circuit to another —in this case, the wire.

Resistors are passive devices. A resistor hinders or slows the flow of current running through it. You can use them for controlling the voltage or current in a circuit. The value of their resistance can be set by the user, or be variable.

  • Resistor symbol/-15
  • Resistor symbol

In the following figure you can see an electric circuit with the components connected in series.

Figura Serial circuit

There is only one way for the current to flow in the circuit. Starting from the positive terminal of the battery, current flow will first find R1. From there the current will flow straight to R2, then to R3, and finally back to the negative terminal of the battery. There is only one path for current to follow. So, these components are in series.

Parallel circuits

If components share two common nodes, they are in parallel. In the next figure you have an example schematic of three resistors in parallel.

Figura Parallel circuit

From the positive battery end, current flows simultaneously through R1, R2, and R3, each resistor taking part of this current flow depending on its resistance value. The node that connects the battery to R1 is also connected to the other resistors. The other ends of these resistors are similarly tied together, and then tied back to the negative terminal of the battery. There are three different paths that current can take before returning to the battery. The three resistors are in parallel.

Where series components all have equal currents running through them, parallel components have the same voltage drop across them.

We can connect resistors both in parallel and series combinations: we call them mixed resistor circuits.


Jenny: Bill, look at my Christmas tree!

Bill: Yes, it’s beautiful, but not all lights are on.

Jenny: Oh, you’re right. The blue lights are off. But… I don’t understand why.

Bill: Let me see. Look at the way they are connected.

Jenny: They are connected in series, aren’t they?

Bill: Well, you are right, some of the lights in the string are connected in series, while some are connected in parallel.

Jenny: Really?

Bill: Yes, that’s why all the blue lights are off. The lights of the same colour are connected in series. So, when one of them goes off, the current is cut off, as if you had opened a switch. However, the other lights are still on because they are connected to the circuit in parallel.

Jenny: Wow! I never thought about this. And how can we get the blue lights back on?

Bill: We could look for the light bulb that is blown and replace it. Alternatively, we can connect the rest of blue lights without the blown bulb. In this case, they will be brighter, since the intensity thorugh them will be higher.


  • Series/parallel circuit: circuit en sèrie/paral·lel
  • Resistor: resistència
  • Wire: cable
  • Device: dispositiu
  • Overheating: sobreescalfament
  • Path: ruta


In English, there are three verb tenses (Cat. temps verbals). The tenses indicate when the action happens:

  • Present
  • Past
  • Future

There are also three aspects (Cat. modes). The aspects indicate how the action happens:

  • Simple: the simple tenses express the actions of the verb.
  • Continuous: the continuous tenses express actions that continue for a period of time.
  • Perfect: the perfect tenses express actions that are finished at a particular moment.

The two present tenses are the present simple and the present continuous.

The prepositions are short words that are usually placed in front of a noun. There are few rules to explain the use of the prepositions. The only way of learning them is by reading a lot and learning useful phrases by heart (Cat. de memòria). There are different types of prepositions. The most important are:

  • Prepositions of place
  • Prepositions of time

The present simple and the present continuous

Here are the forms and uses of the present simple and the present continuous.

The present simple:

Taula: Conjugation of the present simple (work)
Affirmative Negative
Long form
Short form
I work do not work don’t work Do I work…?
You work do not work don’t work Do you work…?
He/She/It works does not work doesn’t work Does he work…?
We work do not work don’t work Do we work…?
You work do not work don’t work Do you work…?
They work do not work don’t work Do they work…?

Notice the following:

  • The third person singular adds -s to the base form of the verb.
  • The negative is formed with do not or does not in front of the base form.
  • In the interrogative form, the auxiliary do/does is placed before the subject.

The main uses of the present simple are:

1) To express an habitual, regular action:

  • I get up at seven everyday (Cat. em llevo a les set cada dia).
  • David lives in London (Cat. David viu a Londres).
  • We work in a big company (Cat. Nosaltres treballem a una gran empresa).

2) To express a permanent state, something that is always true:

  • Australia is a very big country (Cat. Austràlia és un país molt gran).
  • The sun rises in the east (Cat. El sol surt per l’est).
  • Water boils at 100ºC (Cat. L’aigua bull als 100ºC).

Adverbs of frequency

The adverbs of frequency express the frequency of an action. They are very common with the present simple because they indicate regular actions. Here is a list of the most important adverbs of frequency:

  • Always (Cat. sempre)
  • Usually (Cat. normalment)
  • Often (Cat. sovint)
  • Sometimes (Cat. algunes vegades)
  • Seldom (Cat. poques vegades)
  • Never (Cat. mai)
  • Everyday (Cat. cada dia)
  • Every week (Cat. cada setmana)
  • Once a week (Cat. una vegada a la setmana)
  • Twice a week (Cat. dues vegades a la setmana)
  • Three times a week (Cat. tres vegades a la setmana)
  • From time to time (Cat. de tant en tant)

The adverbs always, usually, often, seldom and never are placed in the middle of the sentence:

  • He always arrives late (Cat. Sempre arriba tard).
  • We are often tired in the evening (Cat. Estem sovint cansats al vespre).

The adverb sometimes is placed at the beginning, at the end or in the middle of the sentence:

  • He sometimes calls me (Cat. Em truca algunes vegades).
  • Sometimes he calls me.
  • He calls me sometimes.

The expressions everyday, every week, etc. are usually placed at the end of the sentence:

  • We meet everyday (Cat. Ens reunim cada dia).
  • I see David twice a week (Cat. A David el veig dues vegades a la setmana).

The present continuous

Taula: Conjugation of the present continuous (work)
Long form
Short form
Long form
Short form
I am working ‘m working am not working ‘m not working am I working…?
you are working ‘re working are not working aren’t working are you working…?
he/she/it is working ‘s working is not working isn’t working is he working…?
we are working ‘re working are not working aren’t working are we working…?
you are working ‘re working are not working aren’t working are you working…?
they are working ‘re working are not working aren’t working are they working…?

The gerund is an invariable form of the verb. It always ends in -ing. It is mostly used (together with the auxiliary verb be) to form the continuous tenses of the verbs.

The present continuous is composed of two elements: the auxiliary verb be, conjugated in the present tense, and the gerund of the verb (working).

The main uses of the present continuous are:

1) To express an action that is happening at the moment of speaking:

  • Look! David is talking to the director (Cat. Mireu! David està parlant amb el director)
  • I am writing the report now (Cat. Ara estic redactant l’informe)
  • He is flying to London in this moment (Cat. En aquest moment està volant cap a Londres)

2) To express a temporary action, one that is not permanent:

  • I am working at night this week (Cat. Aquesta setmana treballo de nit)
  • She is living with her sister (Cat. (Ella) viu/està vivint amb la seva germana)
  • My car is in the garage so these days I am taking the bus to come to work (Cat. El meu cotxe és al taller així que aquests dies agafo l’autobús per a venir a treballar)

Adverbs of time

Some adverbs and adverbial expressions of time are frequently used with the present continuous tense. For example:

  • Now (Cat. ara)
  • In this moment (Cat. en aquest moment)
  • This week (Cat. aquesta setmana)

We usually place these words at the end of the sentence:

  • He is sleeping now (Cat. Ara està dormint).
  • I’m reading your mail in this moment (Cat. En aquest moment estic llegint el teu mail).
  • We are not working this week (Cat. Aquesta setmana no treballem).

Verbs that do not take the continuous tenses

Some verbs are never expressed in the continuous tenses. Some of these verbs are:

  • Believe (Cat. creure)
  • Doubt (Cat. dubtar)
  • Enjoy (Cat. agradar, gaudir)
  • Hate (Cat. odiar, detestar)
  • Have (Cat. tenir)
  • Know (Cat. saber)
  • Like (Cat. agradar)
  • Love (Cat. estimar)
  • Prefer (Cat. preferir)
  • Remember (Cat. recordar)
  • See (Cat. veure)
  • Think (Cat. opinar)
  • Understand (Cat. entendre)
  • Want (Cat. voler)

It is wrong to say: *I am liking this film. We must say: I like this film.

But we can use the continuous tenses with two of these verbs when they have a different meaning:

  • Have (Cat. prendre): I am having breakfast (Cat. Estic prenent l’esmorzar/Estic esmorzant)
  • Think (Cat. pensar): What are you thinking about? (Cat. En què estàs pensant?)

Prepositions of place (I)

Some prepositions of place express position and others express movement.

After a preposition, we can say:

  • a noun: in the garden
  • an object pronoun: for him
  • a verb in -ing: without saying anything

The most important prepositions that express position are:

  • In (Cat. a, dintre de)
  • On (Cat. a sobre de, damunt de)
  • At (Cat. a)
  • Next to (Cat. al costat de)
  • Over (Cat. per damnunt de)
  • Under (Cat. a sota de)
  • In front of (Cat. al davant de)
  • Opposite (Cat. davant de)
  • Behind (Cat. al darrera)
  • Between (Cat. entre)
  • Across (Cat. a l’altra banda de)

The figure shows a graphical representation of the prepositions of place indicating position:

Figura The prepositions of place (position)

Some common expressions with these prepositions are:

  • In London (Cat. a Londres)
  • In England (Cat. a Anglaterra)
  • In the street (Cat. al carrer)
  • On the corner of (Cat. a la cantonada de)
  • On the wall (Cat. a la paret)
  • On the floor (Cat. a terra)
  • On the train (Cat. al tren)
  • At home (Cat. a casa)
  • At work (Cat. a la feina)
  • At school (Cat. a l’escola)
  • At the table (Cat. a la taula)

The prepositions that express movement are:

  • To (Cat. a, cap a)
  • Into (Cat. cap a dins de)
  • Onto (Cat. cap el damunt de)
  • Along (Cat. per, al llarg de)
  • From (Cat. de, des de)
  • In front of (Cat. cap al davant)
  • Behind (Cat. cap el darrera)
  • Across (Cat. a través de)

The figure below represents the prepositions of place that indicate movement:

Figura The prepositions of place (movement)

We use all these prepositions with verbs that indicate movement, for example: go (Cat. anar), drive (Cat. conduir), walk (Cat. caminar), run (Cat. córrer), fly (Cat. volar), etc. Here are some examples:

  • I am flying to London (Cat. Estic volant cap a Londres).
  • He is running into the room (Cat. Entra corrents a l’habitació).
  • Put the boxes onto the table (Cat. Posa les caixes sobre la taula).
  • I am coming from the office (Cat. Vinc de l’oficina).
  • They are walking behind the house (Cat. Estan caminant cap a darrera de la casa).

But we do not use a preposition with the noun home: I am going home (Cat. Vaig cap a casa). It is wrong to say: *I am going to home.

Other prepositions

Other important prepositions are:

  • With (Cat. amb)
  • Without (Cat. sense)
  • Of (Cat. de)
  • For (Cat. per a)
  • By (Cat. per)


  • I work with very interesting people (Cat. Treballo amb gent molt interessant).
  • He left without saying goodbye (Cat. Va marxar sense dir adéu).
  • February is the second month of the year (Cat. El febrer és el segon mes de l’any).
  • Is this present for me? (Cat. Aquest regal és per a mi?).
  • The job was finished by Peter (Cat. La feina va ser acabada per en Peter).


Reading and writing are the two written skills. Reading is a passive skill. This means that reading only requires to understand a written text. On the other hand, writing is an active skill, which means that we have to produce a text. Some common types of text are:

  • Letters and emails
  • Manuals
  • Articles
  • Fiction

See Unit 2, section 2 ‘Communication’ to learn about translations and summaries.

At work, you will probably have to write letters and emails, and you will probably need to make translations (Cat. traduccions) and/or summaries (Cat. resums) of manuals and articles. For all this, it is very important that you learn the techniques for producing an accurate text.

The reading skills

Reading is very easy to practise because it is an individual activity. If you read in English, you will learn a lot of new words and a lot of grammatical structures, so reading is a very good activity for developing your writing skills.

At work, you will mostly read letters and emails in English, and you will probably need to read manuals or articles, too.

See the ‘Interesting links’ section for links to the recommended websites.

This is a small selection of material for practising your reading skills:

  • Graded readers: if you like fiction, there are many graded readers at different levels, from beginners to advanced learners. These readers offer adaptations of classical works from the English and American literature.
  • Wikipedia: if you prefer reading online, the Internet offers a great quantity of material. The English version of the Wikipedia has more than 5 million articles about any possible topic.
  • Simple English Wikipedia: if you find the articles in the Wikipedia too difficult, there is a version specially designed for learners of English. The Wikipedia in Simple English uses basic words and very simple grammatical structures. It currently has more than 125,000 articles.
  • Howstuffworks: in this website, you can find a great variety of articles divided into different topics. There are a lot of technical manuals that explain how things (stuff) works. It is recommended for intermediate and advanced students.

[Imatge: graded readers (càmera)]

Here are some ideas to practise your reading skills:

  • Read slowly. You will need to read a sentence or a paragraph several times before you understand the meaning.
  • You do not need to know all the words to understand the general meaning of a text.
  • Do not translate the text into your own language: the objective is to practise your reading skills in English.
  • Try to guess the meaning of unknown words. You will notice that many English words are of Latin origin. For example, you do not need a dictionary to understand: family, future, university, excuse, practice, manual, dictionary, etc.
  • Read English as much as you can in your free time. This will help you to improve your reading/writing skills more quickly.

The writing skills

Writing is a very common activity. At work, you will probably need to write emails and letters in English, and perhaps instructions or short notes. As in all the skills, the only way of improving your writing is to practise as much as possible.

When you write a text in English, it is very important to consider the following aspects:

See Unit 1, section 1 ‘Communication’ for the characteristics of the formal and the informal language.

  • Clarity: write short, simple sentences; write different paragraphs and separate them clearly; use connectors and punctuation to link the sentences.
  • Style: choose the appropriate style (formal or informal), depending on the situation.
  • Accuracy: use the correct vocabulary, grammar structures and spelling; take your time to correct mistakes.

The term layout (Cat. distribució, composició) refers to the position of the different elements in a text, normally in letters and emails.

  • Layout: use the appropriate layout and conventions used in formal texts, especially in business letters.

Here is an example of a formal text (a business letter):

Figura Business letter

See the annex ‘Writing: emails and letters’ for more information.

Notice the position and characteristics of the different elements:

  1. The sender’s address (without the name)
  2. The date
  3. The receiver’s address
  4. A standard formal opening phrase
  5. The text of the letter: two paragraphs, formal style (I would be grateful if…, could you…?), use of connectors (firstly, secondly, finally)
  6. A standard formal ending
  7. A standard formal closing phrase
  8. The sender’s signature

Connectors and punctuation

Connectors and punctuation are very important elements because they contribute to the accuracy of the text. They bring clarity and coherence, so it is necessary to learn their use.


Here is a list of some common English connectors. We include their translation into Catalan and examples of their use:

Expressing addition:

  • and (Cat. i): I like reading and listening to music (Cat. M’agrada llegir i escoltar música).
  • or (Cat ni) (in negative sentences): I don’t like reading or listening to music (Cat. No m’agrada llegir ni escoltar música)
  • also (Cat. també) (formal): I also like reading (Cat. A mi també m’agrada llegir).
  • too (Cat. també) (informal): I like reading too (Cat. A mi també m’agrada llegir).

Expressing alternatives:

  • or (Cat. o): You can go or wait a little longer (Cat. Pots marxar o esperar-te una estona més).

Expressing contrast:

  • but (Cat. però): It’s late, but I will wait a little longer (Cat. És tard, però ,'esperaré una estona més).
  • however (Cat. malgrat tot): It’s late. However, I will wait a little longer (Cat. És tard. Malgrat tot, m’esperaré una estona més).
  • although (Cat. encara que, tot i que ): Although it’s late, I will wait a little longer (Cat. Encara que sigui tard, m’esperaré una estona més).
  • on one hand…on the other hand (Cat. per una banda…per una altra banda): On one hand it is raining. On the other hand, it is very cold (Cat. Per una banda, està plovent. Per una altra banda, fa molt de fred).

Explaining ideas:

  • in other words (Cat. amb altres paraules): It’s crystal clear. In other words, it is very clear (Cat. Està clar com el cristall. En altres paraules, està molt clar).
  • for example (Cat. per exemple). There are many things to do. For example, bring water and take out the chairs (Cat. Hi ha moltes coses per fer. Per exemple, portar aigua i treure les cadires).

Listing ideas:

  • firtsly,…;secondly… (Cat. en primer lloc,…en segon lloc, …): Firstly, bring water; secondly, take out the chairs (Cat. En primer lloc, porta aigua; en segon lloc, treu les cadires).
  • and then (Cat. i aleshores): Bring water and then take out the chairs (Cat. Porta aigua i aleshores, treu les cadires).
  • next (Cat. després): Bring water. Next, take out the chairs (Cat. Porta aigua. Després, treu les cadires).
  • finally (Cat. finalment, per acabar): Bring water, take out the chairs and finally check the computer (Cat. Porta aigua, treu les cadires i finalment comprova l’ordinador).

Expressing purpose:

  • to (Cat. per a, per tal de): Use a dictionary to look up the meaning of a word (Cat. Utilitza un diccionari per a consultar el significat d’una paraula).

Expressing the reason:

  • because (Cat. perquè): I study English because it is very important (Cat. Estudio anglès perquè és molt important).
  • because of (Cat. degut a, a causa de): I study English because of its great importance (Cat. Estudio anglès degut a la seva gran importància).

Expressing the result:

  • so (Cat. així que, per tant): I need money so I work extra hours (Cat. Necessito diners així que faig hores extres).

Summarizing ideas:

  • in conclusion (Cat. en conclusió): We have no money and we do not work. In conclusion, we cannot give you anything (Cat. No tenim diners i no treballem. En conclusió, que no et podem donar res).
  • Summing up (Cat. resumint). We have no money and we do not work. Summing up, we cannot give you anything (Cat. No tenim diners i no treballem. Resumint, que no et podem donar res).

Now see the difference between a text with and without connectors:

Text without connectors:

The Internet has some advantages. It is useful to broaden your knowledge and meet new people. It has some disadvantages. Its free access to pages which show violence and crimes. There are people who think that the Internet should be a free space to express ideas. Some people consider that the Internet can be dangerous if it is not controlled. The Internet has both advantages and disadvantages. People must learn to use it for positive aims.

Text with connectors (in bold type):

The Internet has some advantages because it is useful to broaden your knowledge and meet new people, but it also has some disadvantages, as for example, its free access to pages which show violence and crimes. On the one hand, there are people who think that the Internet should be a free space to express ideas. On the other hand, some people consider that the Internet can be dangerous if it is not controlled. In conclusion, the Internet has both advantages and disadvantages, but people must learn to use it for positive aims.


In writing, it is very important to use the punctuation marks correctly because they contribute to give clarity to the text.

Here are the most important uses of the punctuation marks:

  • Do not write a comma (,) between the subject and the verb.
  • Write a full stop (.) at the end of a sentence.
  • The adverbials and connectors are usually separated from the sentence by a comma. For example: in the mornings , I usually check my mail.
  • Write a colon (:) to introduce a list of items or an explanation. For example: the continents are: Europe, Asia, Africa, America and Oceania.
  • Write a comma to separate the elements in a list. For example: I visit customers on Monday , Wednesday , Thursday and Friday.
  • Use a semi-colon (;) to separate the groups of items in a list. For example: in computing, we must distinguish between hardware and software. Hardware refers to the material parts of a computer ; software refers to programming and includes operating systems, compilers, editors, etc.
  • Use brackets () to add extra information. For example: in writing, we must use connectors (also called ‘linking words’).
  • Write a capital letter at the beginning of a sentence or after a full stop.
  • Write a question mark (?) at the end of a sentence. For example: what’s your name ?.
  • Write an exclamation mark (!) at the end of a sentence with the imperative form. For example: Come in !.


Capitalization is the use of capital letters (Cat. lletres majúscules). In English, capitalization is sometimes different from Spanish or Catalan.

We use capital letters:

  • with the pronoun ‘I’
  • with adjectives derived from proper names: English, Londoner, etc.
  • with days of the week and months of the year: Monday, Tuesday, February, April etc.
  • with names of personal and job titles: Mr., Miss, Sales Manager, Director, etc.
  • at the beginning of a sentence.
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