Basic Electric Circuits
When the power goes out in the neighborhood, many homes turn on a light bulb with the inverter. And if you turn off the living room fan at your house, the kitchen light stays on. What does a light bulb need in order to shine, and how are the appliances in your home connected so that one can go off without affecting the others? Let's build circuits step by step.
Elements of a circuit
A light bulb doesn't light up with just a battery: it needs wires and a complete path for the current. The electric current (also called intensity, measured in amperes, A) is the orderly movement of charges through a conducting material. What pushes them is the voltage (V, in volts) provided by the battery: the higher the voltage, the stronger the "push."
In diagrams, each part is drawn with a standard symbol, the same all over the world.
Match each component of a circuit with its function. Components: generator, conducting wire, receiver, and switch. Functions: drives the charges, carries current from…
Match each component of a circuit with its function. Components: generator, conducting wire, receiver, and switch. Functions: drives the charges, carries current from one point to another, makes use of electrical energy, and opens or closes the path of the current.
What is an electrical receiver? Give three examples of electrical receivers in your home and state what the electrical energy is transformed into in each one (for…
What is an electrical receiver? Give three examples of electrical receivers in your home and state what the electrical energy is transformed into in each one (for example, a bulb transforms it into light).
State whether each component in this list is a generator or a receiver: the battery of a remote control, the motor of a fan, the battery of an electric scooter, a…
State whether each component in this list is a generator or a receiver: the battery of a remote control, the motor of a fan, the battery of an electric scooter, a doorbell buzzer and a bulb.
Diagrams and assembly of a simple circuit
To communicate a circuit, you draw its diagram with standard symbols: the battery is two parallel lines (one long and thin, one short and thick), the bulb is a circle with a cross, and the switch is a raised line. The wires are straight lines joining everything.
In circuit diagrams, circuit components are drawn using standardised symbols, which are the same worldwide: a cell is represented by two parallel lines, one long and…
In circuit diagrams, circuit components are drawn using standardised symbols, which are the same worldwide: a cell is represented by two parallel lines, one long and thin and the other short and thick; a bulb, a circle with a cross inside; a switch, a raised line. Here are the three symbols:
Draw the circuit diagram of a circuit with a cell, a switch and a bulb in your notebook.
You can do it on paper and send a photo with your answer.
You want to set up a circuit to light a bulb using a cell. Make a list of all the materials you need and explain what each one is used for.
You want to set up a circuit to light a bulb using a cell. Make a list of all the materials you need and explain what each one is used for.
Inside a torch there is a cell, a bulb, a switch and a metal spring connecting everything, as shown in the diagram. a) Which component drives the charges? b) Which…
Inside a torch there is a cell, a bulb, a switch and a metal spring connecting everything, as shown in the diagram.
- a) Which component drives the charges?
- b) Which component makes use of the energy?
- c) What does the switch do when turning on the torch?
In a single-path circuit there is a cell, a switch and a bulb. If we change the order and place the bulb first and then the switch, will it still work in the same way?…
In a single-path circuit there is a cell, a switch and a bulb. If we change the order and place the bulb first and then the switch, will it still work in the same way? Explain why the order of components does not matter when there is only one path.
Draw a circuit diagram in your notebook, using standardised symbols, of a circuit with a cell, a switch, a bulb and a buzzer, all on the same path. If the switch is left…
Draw a circuit diagram in your notebook, using standardised symbols, of a circuit with a cell, a switch, a bulb and a buzzer, all on the same path. If the switch is left open, which components work? Explain why.
You can do it on paper and send a photo with your answer.
Open circuit and closed circuit
Some switches stay in the position where you leave them (the light switch in your room) and there are push buttons, which close the circuit only while you hold them down and open it when you let go, because a spring separates the contacts. The button of a doorbell is a push button: when you press it, the circuit closes and the bell rings; when you release it, the circuit opens and the ringing stops. The button and the bell are in series, on the same path.
Explain in your own words the difference between an open circuit and a closed circuit. Look at the two setups in the diagram: In which of the two does the bulb light up?…
Explain in your own words the difference between an open circuit and a closed circuit. Look at the two setups in the diagram:
In which of the two does the bulb light up? Why?
What does a switch do in a circuit? Relate it to open and closed circuits: what state does each switch position leave the circuit in?
What does a switch do in a circuit? Relate it to open and closed circuits: what state does each switch position leave the circuit in?
Lucía sets up her circuit with a cell, wires and a bulb, but it does not light up. State three possible causes of the fault and explain how she would test each one to…
Lucía sets up her circuit with a cell, wires and a bulb, but it does not light up. State three possible causes of the fault and explain how she would test each one to find the problem.
A doorbell only rings while the button at the front door is kept pressed. a) What type of control component is that button, and what does it do to the circuit when…
A doorbell only rings while the button at the front door is kept pressed.
- a) What type of control component is that button, and what does it do to the circuit when pressed and released?
- b) Describe the complete circuit of the doorbell when a visitor presses and releases it.
- c) Are the button and the doorbell in series or in parallel? Give reasons for your answer.
Series circuits
In old Christmas light strings, as soon as a single bulb comes loose or burns out, all the others go off. How are they connected so that the failure of one keeps the rest from working?
And if you add bulbs in series to the same battery, each one shines less. Each bulb is one more obstacle on the same path: the current has more difficulty getting through, so it is smaller and the brightness of each bulb drops. With more voltage (for example, two 1.5 V batteries one after the other, which add up to 3 V) the push is greater and the brightness increases.
Two bulbs are connected one after the other, in series, to the same cell, as shown in the diagram. a) If one of the bulbs blows, what happens to the other one? b)…
Two bulbs are connected one after the other, in series, to the same cell, as shown in the diagram.
- a) If one of the bulbs blows, what happens to the other one?
- b) Explain this by considering the path the current follows.
With a $1.5\ \text{V}$ cell, the bulb in the setup shines dimly. The diagram shows the setup with two cells placed one after the other: a) What voltage will two…
With a $1.5\ \text{V}$ cell, the bulb in the setup shines dimly. The diagram shows the setup with two cells placed one after the other:
- a) What voltage will two $1.5\ \text{V}$ cells supply when placed one after the other?
- b) What will happen to the brightness of the bulb when switching to two cells?
A corridor has two bulbs connected in series. Where should the switch be placed to turn off both at the same time: next to the first one, next to the second one, or does…
A corridor has two bulbs connected in series. Where should the switch be placed to turn off both at the same time: next to the first one, next to the second one, or does it not matter where on the path it is? Justify your answer.
A set of Christmas lights has twenty bulbs connected in series. a) Why do all the bulbs go out when just one blows? b) How should they be connected so that this does not…
A set of Christmas lights has twenty bulbs connected in series.
- a) Why do all the bulbs go out when just one blows?
- b) How should they be connected so that this does not happen?
- c) Propose a practical method to locate the blown bulb among the twenty.
We add light bulbs in series to the same battery: first one, then two, and then three. a) What happens to the brightness of each bulb as more are added? b) Explain the…
We add light bulbs in series to the same battery: first one, then two, and then three.
- a) What happens to the brightness of each bulb as more are added?
- b) Explain the cause by considering the difficulty the current faces in passing through.
- c) Predict what would happen to the brightness if we added them in parallel instead of in series.
Parallel circuits
In a house you can turn on the fan and turn off the kitchen light without one appliance interfering with the other. How are the wires laid out so that each receiver works on its own?
When you add bulbs in parallel, each one still receives the full voltage of the battery and keeps its brightness: there are more paths for the current, not more obstacles on a single one. That is why parallel is used for the lights in a house. The battery, however, wears out faster because it delivers more current in total.
Now the two bulbs are connected in parallel: each one has its own path for the current from the cell, as shown in the diagram. a) If one blows, does the other one stay…
Now the two bulbs are connected in parallel: each one has its own path for the current from the cell, as shown in the diagram.
- a) If one blows, does the other one stay lit?
- b) How do you think the lights in your home are connected, in series or in parallel? Give reasons for your answer.
Two bulbs are connected in parallel and we want to turn off both at the same time with a single switch. Where would you place it: in one of the branches or in the main…
Two bulbs are connected in parallel and we want to turn off both at the same time with a single switch. Where would you place it: in one of the branches or in the main path, before the circuit splits? Draw the circuit diagram in your notebook and justify your answer.
You can do it on paper and send a photo with your answer.
Using the same cell, we set up two circuits: one with two bulbs in series and another with two bulbs in parallel. a) In which of the two do the bulbs shine brighter?…
Using the same cell, we set up two circuits: one with two bulbs in series and another with two bulbs in parallel.
- a) In which of the two do the bulbs shine brighter? Give reasons for your answer by considering how difficult the path is for the current.
- b) What happens in each circuit if a bulb blows?
- c) Which of the two setups would you choose for home lighting? Why?
You want to set up two light bulbs in your bedroom that can be turned on and off separately using a single battery. a) How would you connect them, in series or in…
You want to set up two light bulbs in your bedroom that can be turned on and off separately using a single battery.
- a) How would you connect them, in series or in parallel? Why?
- b) How many switches do you need and at which point in the circuit?
- c) Draw the diagram in your notebook.
You can do it on paper and send a photo with your answer.
Conservation of charge and energy
When you turn on a bulb, are the charges that pass through it "used up"? No. Charges are neither consumed nor lost: for every charge that leaves one terminal of the battery, another arrives at the other terminal. That is why the current is the same at every point of a single path. This is the conservation of charge.
What does change is the energy. The battery gives energy to the charges and they deliver it at the receiver, where it is transformed into light and heat. Part of it also becomes heat in the wires. The total energy is neither created nor destroyed: the energy delivered by the battery equals the energy transformed in the receiver and in the wires. This is the conservation of energy.
In a parallel circuit, the current leaving the battery splits among the branches, but when it comes back together it adds up to the same amount: no charge is lost or created.
Short circuits, faults, and electrical safety
If you accidentally connect the two terminals of a small battery with a piece of aluminum foil, the metal heats up right away and burns your fingers. Why does a circuit get so hot when we connect the terminals with a wire and no receiver in between?
Without a receiver to make the passage harder, all the energy of the battery is converted into heat in the wire.
The opposite mistake is leaving the circuit without a complete path. Hugo connects a wire from one terminal of the battery to the bulb, and from there it goes back to the same terminal: the current never reaches the other terminal, so the circuit is not complete and the bulb doesn't light up. The current must leave from one terminal and return to the other.
One last everyday example: the refrigerator light turns on when you open the door and goes off when you close it. A small push button, operated by the door, closes the circuit when the door opens and opens it when the door closes. That way no extra energy is wasted.
If we connect the two terminals of a battery directly with a wire, without any component, as shown in the figure, a short circuit occurs: the battery and the wire get…
If we connect the two terminals of a battery directly with a wire, without any component, as shown in the figure, a short circuit occurs: the battery and the wire get very hot, and the battery gets damaged.
- a) Explain what happens to the current when it finds nothing to use the energy.
- b) Why can a short circuit in a home installation be dangerous?
Hugo sets up his circuit as follows: a wire goes from one terminal of the battery to the bulb, and another wire goes from the bulb back to the same terminal of the…
Hugo sets up his circuit as follows: a wire goes from one terminal of the battery to the bulb, and another wire goes from the bulb back to the same terminal of the battery, as shown in the figure.
- a) Will the bulb light up?
- b) What mistake has he made?
- c) Explain how the circuit should be arranged so that the current completes a full loop.
The light inside the fridge turns on when opening the door and turns off when closing it. a) What circuit component does the door activate without us noticing? b)…
The light inside the fridge turns on when opening the door and turns off when closing it.
- a) What circuit component does the door activate without us noticing?
- b) Explain what happens to the light circuit when opening and closing the door.
- c) Considering household energy consumption, why is it important that the light turns off automatically?
Back to the beginning: the bulb shines when there is a generator and a closed path. In homes the appliances are in parallel: each one has its own branch and goes off without affecting the others. In series, a single fault cuts the only path and everything goes off.