Electricity: Circuits and their Components — Class 7 Science
Curiosity · Chapter 3
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7- Mathematics
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- Charts from the book's own data
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2- Mathematics
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- Space weighted by chapter size
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- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Electric cell
- A portable source of electrical energy with a positive metal-cap terminal and a negative metal-disc terminal.
- Battery
- A combination of two or more cells joined so the positive terminal of one connects to the negative terminal of the next.
- Incandescent lamp
- A lamp whose thin filament heats up and glows to produce light when current passes through it.
- LED
- A Light Emitting Diode with no filament that allows current to flow in one direction only; its longer wire is positive.
- Electrical circuit
- A complete path that allows electric current to flow, making a lamp glow only when the path is unbroken.
- Switch
- A simple device that completes (ON) or breaks (OFF) a circuit to control the flow of current.
- Conductor
- A material such as a metal through which electric current can flow easily.
- Insulator
- A material such as plastic, rubber, or glass through which current cannot pass; also called a poor conductor.
Extra questions & answers
What are the two terminals of an electric cell called and how are they marked?
The terminals are the positive terminal (the protruding metal cap, marked +) and the negative terminal (the flat metal disc, marked -).
How is a battery made from cells?
A battery is made by connecting two or more cells so that the positive terminal of one cell touches the negative terminal of the next.
What makes the filament of an incandescent lamp glow?
When electric current passes through the thin filament, it gets hot and glows to produce light.
How does an LED differ from an incandescent lamp?
An LED has no filament and allows current to flow in only one direction; it glows only when its positive (longer) wire connects to the positive terminal of the battery.
When does a lamp glow in a circuit?
A lamp glows only when its two terminals are connected to the two terminals of the cell so that the circuit is complete and current can flow.
In which direction is current taken to flow in a circuit?
By convention, electric current flows from the positive terminal to the negative terminal of the electric cell.
What is the difference between the ON and OFF positions of a switch?
In the ON position the circuit is closed and current flows; in the OFF position there is a gap, the circuit is open, and no current flows.
Why are electrical wires made of metal?
Metals such as copper are good conductors of electricity, so current can flow through them easily.
Why are wires covered with plastic or rubber?
Plastic and rubber are insulators (poor conductors), so the covering protects people from electric shocks.
What is a circuit diagram?
A circuit diagram is a representation of an electrical circuit using standard symbols for components such as cells, lamps, switches, and wires.
Let Us Enhance Our Learning
1. Choose the incorrect statement.
(i) A switch is the source of electric current in a circuit.
(ii) A switch helps to complete or break the circuit.
(iii) A switch helps us to use electricity as per our requirement.
(iv) When the switch is in ‘OFF’ position, there is an air gap between its terminals.
The incorrect statement is (i). A switch is not the source of electric current; the electric cell or battery is the source. A switch only makes or breaks the continuous path. In OFF position it creates an air gap so current cannot flow, and in ON position it closes the gap so current can flow as needed.
2. Observe Fig. 3.16. With which material connected between the ends A and B, the lamp will not glow?
The lamp will not glow when the material connected between A and B is an electrical insulator. In the tester circuit shown, current can flow only through a conductor. So if a conductor like copper wire, aluminium foil, iron key or steel pin is connected, the lamp glows. If an insulator like rubber, plastic scale, glass bangle, wooden stick, paper, eraser, wax candle or cork is connected, the circuit remains open and the lamp does not glow.
3. In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify your answer.
In Fig. 3.17 the two lamps are connected one after the other in a single continuous path, that is in series. If the filament of one lamp is broken, its filament becomes an air gap and the whole circuit becomes open. No current can flow through the circuit, so the other lamp will also not glow, even though its own filament is intact.
4. A student forgot to remove the insulator covering from the connecting wires while making a circuit. If the lamp and the cell are working properly, will the lamp glow?
No, the lamp will not glow. The plastic or rubber covering of the wire is an insulator. If it is not removed at the ends, the metal of the wire does not touch the terminals of the cell, lamp or switch. Current cannot pass through the insulating layer, so the circuit remains open and the filament does not get current.
5. Draw a circuit diagram for a simple torch using symbols for electric components.
A simple torch is a series circuit. Using standard symbols: draw a short line for the negative terminal and a longer line for the positive terminal of a cell to show one cell, or two cells joined positive to negative to show a battery, then a wire to a switch, then a wire to the electric lamp and back to the cell. When the switch is shown in ON position the path is closed and the lamp glows, when shown in OFF position there is a gap at the switch and the lamp does not glow.
6. In Fig. 3.18: (i) If S2 is in ‘ON’ position, S1 is in ‘OFF’ position, which lamp(s) will glow?
If S1 is OFF the main path from the battery is open, so no current can reach any lamp. Even though S2 is ON, neither L1 nor L2 will glow.
6. In Fig. 3.18: (ii) If S2 is in ‘OFF’ position, S1 is in ‘ON’ position, which lamp(s) will glow?
S1 ON closes the main circuit but S2 OFF keeps the branch of L2 open. So current can flow only through L1. Therefore only lamp L1 will glow and L2 will remain dark.
6. In Fig. 3.18: (iii) If S1 and S2 both are in ‘ON’ position, which lamp(s) will glow?
When both S1 and S2 are ON, both branches have a complete path from the positive to the negative terminal of the battery. Current flows through both lamps, so both L1 and L2 will glow.
6. In Fig. 3.18: (iv) If both S1 and S2 are in ‘OFF’ position, which lamp(s) will glow?
If both switches are OFF there is no closed path for current at all. Neither L1 nor L2 will glow.
7. Vidyut has made the circuit as shown in Fig. 3.19. Even after closing the circuit, the lamp does not glow. What can be the possible reasons? List as many possible reasons as you can for this faulty operation. What will you do to find out why the lamp did not glow?
Possible reasons are: the cell is discharged or dead, the cells in the battery are reversed or not connected positive to negative, the plastic insulation was not removed from wire ends, a wire is broken inside, connections at the cell holder, lamp holder or switch are loose, the switch is not really touching in ON position, the lamp filament is broken or the lamp is fused, and for an LED the long and short wires are reversed. To find the fault test step by step: first replace the cell with a known good cell, then check the lamp by connecting it directly to a good cell with clean bare wires, then check each wire and the switch with a tester circuit which has a working lamp, and finally reconnect everything ensuring positive to positive for LED and tight contacts.
8. In Fig. 3.20, in which case(s) the lamp/LED will not glow when the switch is closed?
Only case (d) will not glow. (a) and (b) are incandescent lamps, which glow regardless of the cell's polarity as long as the circuit is a closed loop. (c) is an LED connected the correct way (forward-biased) in a complete loop, so it glows. (d) is an LED connected with reversed polarity, so it blocks the current and does not glow.
9. Suppose the ‘+’ and ‘–’ symbols cannot be read on a battery. Suggest a method to identify the two terminals of this battery.
Make a tester with a known LED and a battery holder that needs two cells. Take an LED whose longer wire is known to be positive and shorter wire negative. Connect the LED to the unknown battery. If the LED glows, the wire of the battery touching the longer LED wire is positive and the other is negative. If it does not glow, reverse the connections. Only the correct pairing - battery positive to LED longer wire and battery negative to shorter wire - allows current in the allowed direction and makes the LED glow, so the terminals can be identified.
10. You are given six cells marked A, B, C, D, E, and F. Some of these are working and some are not. Design an activity to identify which of them are working. (i) List the items that you require.
Required items are: a working incandescent torch lamp with holder, a cell holder for one cell, two connecting wires with about one centimetre of insulation removed at both ends, and the six cells A to F to be tested. Optionally a working cell for comparison and electrical tape.
10. You are given six cells marked A, B, C, D, E, and F. Some of these are working and some are not. Design an activity to identify which of them are working. (ii) Write the procedure that you will follow.
Procedure: 1. Make a simple tester circuit - connect one wire from the cell holder to one terminal of the lamp holder and another wire to the other terminal, leaving the cell holder empty. 2. Insert cell A into the holder with its flat disc side towards the spring and cap side away. Close the circuit momentarily. 3. Note if the lamp glows. If it glows the cell is working, if not it is weak or dead. 4. Remove cell A and repeat the same steps for B, C, D, E and F one by one using the same lamp and wires without changing anything else.
10. You are given six cells marked A, B, C, D, E, and F. Some of these are working and some are not. Design an activity to identify which of them are working. (iii) With the items, carry out the activity to identify the cells that are working.
On carrying out the test, the lamp glows brightly with the good cells and remains dark with the exhausted cells. For example if B, D and F make the lamp glow and A, C and E do not, then B, D and F are the working cells. The observation should be recorded in a table marking Glow or No Glow against each cell to conclude which cells can still supply current.
11. Using an LED that requires two cells in series to glow, Tanya made the circuit as shown in Fig. 3.21. Will the lamp glow? If not, draw the wires for correct connections.
No, the lamp will not glow as drawn. The two cells are not connected positive to negative and the LED polarity is likely reversed, so there is no proper closed loop and the LED is reverse biased. Correct connection: join the positive terminal of one cell to the negative terminal of the other cell to make a battery of two cells. Then connect the free positive terminal of the battery to the longer wire of the LED and the free negative terminal to the shorter wire of the LED, with a switch in series if needed. When the loop is closed in this way current flows in the allowed direction and the LED glows.
Exploratory Projects
Suppose that due to some problem, the power supply is disrupted in your area for two days. List out which actions from your daily life you would not be able to do.
Without mains supply we could not use many daily services: no electric lights at night, no fans, coolers or air conditioners, no refrigerator so food would spoil, no electric water pump, no charging of phones and laptops, no internet WiFi, no television, no mixer, microwave or induction stove for cooking, lifts and escalators would stop, washing machine would not work, and schools or offices depending on electric lights and computers would be affected. We would need candles, manual hand fans and alternative cooking methods.
Using a solar panel (Fig. 3.22a) as a source of electrical energy, make a circuit to run a toy fan (Fig. 3.22b) as shown in Fig. 3.22c.
Connect the two output wires of the solar panel to the two terminals of the small DC toy motor that drives the fan. No cell is needed because the solar panel acts as the source. Place the panel in bright sunlight, close the switch if provided, and the motor gets direct current from the panel and spins the fan blades. Covering the panel stops the current and the fan stops, showing that light energy is converted to electrical energy and then to mechanical energy.
Visit an electrical items shop. With the help of the shopkeeper, identify the various types of cells available. For each cell, also find out which device(s) it is used for. Prepare a report.
A sample report: Cylindrical zinc-carbon and alkaline cells of size AAA, AA, C and D are used in torchlights, wall clocks, remote controls and toys. Button cells are used in wrist watches, hearing aids and calculators. Rechargeable lithium-ion mobile phone batteries and laptop battery packs are used in phones and computers. Lead-acid and lithium-ion inverters and vehicle batteries are used in cars and home inverters. Each type is chosen based on voltage, capacity and whether it needs to be rechargeable.
Prepare a list of objects in your home under three categories: (i) Objects which are electrical insulators only
Objects made only of insulators: plastic comb, wooden stool, glass cup, rubber eraser, paper notebook, wax candle, cork board, cotton cloth, ceramic cup. They do not allow current to pass and are often used as covers.
Prepare a list of objects in your home under three categories: (ii) Objects which are electrical conductors only
Objects made only of conductors: copper wire without covering, aluminium foil, iron nail, steel spoon, brass key. They allow current to flow easily but are not safe to touch directly when connected to mains.
Prepare a list of objects in your home under three categories: (iii) Objects which are made of both, whose some parts are insulators and some electrical conductors
Objects with both parts: electric plug where metal pins are conductors and plastic body is insulator, screwdriver with metal blade and plastic handle, electric iron with metal base and plastic handle, wire itself with copper inside and plastic outside, switch with metal contacts inside and plastic cover outside. This design lets current flow where needed while protecting the user from shock.