1. Draw the symbols to represent the following components of electrical circuits:
- Connecting wires: A straight line.
- Switch in the ‘OFF’ position: A break in the line with an open gap.
- Bulb: A circle with a cross inside (representing the filament).
- Cell: A long line (positive terminal) and a short line (negative terminal).
- Switch in the ‘ON’ position: A line connecting the two parts of the switch.
- Battery: A combination of cells connected end to end.
2. Draw the circuit diagram to represent the circuit shown in Fig.10.21.
Unfortunately, I cannot see the figure, but based on the description, you should draw the appropriate components connected in series or parallel (as per the instructions or figure provided in your book). Typically, the diagram would show a cell, a bulb, and a switch connected in a proper manner.
3. Draw lines to indicate how you will connect the terminals of four cells to make a battery.
To make a battery of four cells, you would connect the positive terminal of one cell to the negative terminal of the next. Repeat this for all four cells.
4. The bulb in the circuit shown in Fig.10.23 does not glow. Can you identify the problem?
The issue could be a broken wire, an open switch, or a burnt-out bulb. Check the connections and ensure that the circuit is complete. You could also check if the battery is providing sufficient voltage.
5. Name any two effects of electric current.
- Heating effect: Electric current generates heat when flowing through a conductor.
- Magnetic effect: Electric current produces a magnetic field around the conductor.
6. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
When an electric current passes through a wire, it generates a magnetic field around the wire. This magnetic field interacts with the magnetic field of the compass, causing the needle to deflect.
7. Will the compass needle show deflection when the switch in the circuit shown in Fig.10.24 is closed?
If the circuit is correctly set up and current flows through the wire near the compass, the compass needle will deflect.
8. Fill in the blanks:
(a) Longer line in the symbol for a cell represents its positive terminal.
(b) The combination of two or more cells is called a battery.
(c) When current is switched ‘on’ in a room heater, it becomes hot.
(d) The safety device based on the heating effect of electric current is called a fuse.
9. Mark ‘T’ if the statement is true and ‘F’ if it is false:
(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (F)
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T)
(c) An electromagnet does not attract a piece of iron. (F)
(d) An electric bell has an electromagnet. (T)
10. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
No, an electromagnet cannot be used to separate plastic bags from a garbage heap, as plastic is not magnetic. Electromagnets can only attract magnetic materials, such as iron.
11. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.
No, you should not replace a fuse with a piece of wire. A fuse is a safety device that melts and breaks the circuit when the current exceeds a safe limit. A wire would not provide the same protection and could cause damage or fire in case of overload.
12. Zubeda made an electric circuit using a cell holder shown in Fig. 10.4, a switch, and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.
Possible defects could be:
- The bulb may be burnt out.
- The connections could be loose or broken.
- The battery may be weak or dead.
- The switch may not be functioning properly.
13. In the circuit shown in Fig. 10.25:
(i) Would any of the bulbs glow when the switch is in the ‘OFF’ position?
No, the bulbs will not glow because the switch is open, meaning the circuit is incomplete.
(ii) What will be the order in which the bulbs A, B, and C will glow when the switch is moved to the ‘ON’ position?
The bulbs will glow in the order they are connected. Without seeing the figure, it is assumed they are in series or parallel, and the current will flow through them in that order.
Extended Learning — Activities and Projects
1. Set up the circuit shown in Fig. 10.17 again. Move the key to ‘ON’ position and watch carefully in which direction the compass needle gets deflected. Reverse the connections at the terminal of the cell and note the direction of deflection.
The deflection of the compass needle will reverse when the connections at the terminals of the cell are reversed. This occurs because the direction of the magnetic field produced by the current depends on the direction of current flow.
2. Make four electromagnets with 20, 40, 60, and 80 turns. Connect them one by one to a battery of 2 cells. Bring the electromagnet near a box of pins. Count the number of pins attracted by it. Compare the strengths of the electromagnets.
The electromagnet with more turns will attract more pins because the strength of the electromagnet increases with the number of turns in the coil.
3. Using an electromagnet, make a working model of a railway signal as shown in Fig. 10.26.
You would need an electromagnet, an iron nail, and some pins. When the electromagnet is activated, it should attract the iron nail and move the signal.
4. Visit an electric shop to learn about different types of fuses and MCBs.
This project requires a visit to an electrician or an electric shop to observe how fuses and Miniature Circuit Breakers (MCBs) work. The MCBs provide a safer and more reliable option compared to fuses for protecting circuits from overload.
Bonus:
- The science behind the magician’s trick: The trick likely involves an electromagnet. The magician may have placed a powerful electromagnet under the box, making it difficult for Boojho to lift the box when the magnet was activated. When the magnet was turned off, Boojho could lift the box easily.
1. What is the symbol for an electric cell?
- Answer: The symbol for an electric cell is a longer line (representing the positive terminal) and a shorter, thicker line (representing the negative terminal).
2. What is a battery?
- Answer: A battery is a combination of two or more electric cells connected together, where the positive terminal of one cell is connected to the negative terminal of the next cell.
3. Why is it called a battery?
- Answer: It is called a battery because it consists of multiple electric cells connected together to provide higher voltage.
4. What does the symbol of a switch in the ‘ON’ position look like?
- Answer: The symbol for a switch in the ‘ON’ position is represented by a closed line, showing that the circuit is complete.
5. What does the symbol of a switch in the ‘OFF’ position look like?
- Answer: The symbol for a switch in the ‘OFF’ position is represented by an open line, showing that the circuit is incomplete.
6. How does a circuit work when the switch is in the ‘ON’ position?
- Answer: When the switch is in the ‘ON’ position, the circuit is closed, and the current flows through the circuit, making the bulb glow.
7. What happens if the filament in a bulb breaks?
- Answer: If the filament in the bulb breaks, the circuit becomes incomplete, and the bulb will not glow.
8. What is the heating effect of electric current?
- Answer: The heating effect of electric current occurs when a current flows through a conductor (like a wire), causing it to heat up. This is used in appliances like electric heaters and irons.
9. Why does the filament in an electric bulb get hot?
- Answer: The filament in an electric bulb gets hot because electric current passes through it, and the resistance of the filament causes it to heat up, which makes it glow.
10. What is an electromagnet?
- Answer: An electromagnet is a coil of wire wound around a core (usually iron), which becomes magnetic when an electric current passes through the wire.
11. How can you create an electromagnet?
- Answer: An electromagnet can be created by winding a piece of insulated wire around a metal object (such as an iron nail) and connecting the ends of the wire to a battery or electric circuit.
12. What happens when the current is turned off in an electromagnet?
- Answer: When the current is turned off, the electromagnet loses its magnetism and no longer attracts magnetic objects.
13. What are some common uses of electromagnets?
- Answer: Electromagnets are used in cranes for lifting heavy loads, in electric bells, and in various medical devices like those for removing small pieces of metal from the body.
14. How does an electric bell work?
- Answer: An electric bell works by using an electromagnet. When current flows through the coil, it pulls an iron strip with a hammer, which strikes a gong, producing sound. The current stops when the iron strip moves, breaking the circuit.
15. What are fuses used for?
- Answer: Fuses are used as safety devices in electric circuits. They prevent overheating and fires by breaking the circuit if the current exceeds a safe limit.
16. What is a short circuit?
- Answer: A short circuit occurs when the electrical current flows through an unintended path, usually due to damaged insulation, causing excessive current and possibly a fire.
17. What is overload in an electrical circuit?
- Answer: An overload occurs when too many devices are connected to a single electrical circuit, exceeding the safe current limit and potentially causing damage or fire.
18. What is a Miniature Circuit Breaker (MCB)?
- Answer: An MCB is a safety device used in place of fuses. It automatically shuts off the circuit when the current exceeds the safe limit, and can be reset to resume normal operation.
19. What does a compass needle do when current flows through a nearby wire?
- Answer: The compass needle gets deflected when an electric current flows through a nearby wire, indicating the magnetic effect of the current.
20. What is the magnetic effect of electric current?
- Answer: The magnetic effect of electric current occurs when a current flowing through a wire generates a magnetic field around it, causing nearby magnetic materials, like a compass needle, to be affected.
21. Why do incandescent bulbs give off heat?
- Answer: Incandescent bulbs give off heat because their filament heats up due to the electric current passing through it, producing both light and heat.
22. How does the heating effect of electric current differ in different appliances?
- Answer: The heating effect of electric current is used to produce heat in appliances like heaters, irons, and hair dryers, where the coil or element becomes hot and transfers the heat to the surrounding environment.
23. What is the difference between an electric current and static electricity?
- Answer: Electric current refers to the flow of electric charge through a conductor, while static electricity involves the accumulation of electric charges on an object without a flow.
24. Why is a fuse important in an electric circuit?
- Answer: A fuse is important because it prevents damage to electrical appliances or circuits by breaking the circuit when the current exceeds a safe level.
25. What is the role of a switch in an electric circuit?
- Answer: The switch controls the flow of current in an electric circuit. When the switch is in the ‘ON’ position, current flows; when it is in the ‘OFF’ position, current does not flow.
26. Why is it dangerous to touch a live electric wire?
- Answer: It is dangerous because the electric current can pass through the body, potentially causing severe electric shocks or even death.
27. What happens when a fuse blows?
- Answer: When a fuse blows, it breaks the circuit, preventing further current flow, which protects the circuit and electrical devices from damage due to excessive current.
28. What is the purpose of insulation on electrical wires?
- Answer: Insulation on electrical wires prevents accidental contact with the conducting wire and protects against electric shocks.
29. How does the material of the wire affect the heating effect of electric current?
- Answer: The material of the wire affects its resistance. Wires with higher resistance, like nichrome, produce more heat when current flows through them.
30. How can you prevent an electrical fire caused by a short circuit?
- Answer: To prevent electrical fires, use properly insulated wires, install fuses, and ensure that electrical circuits are not overloaded.
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